CHAPTER 2 Cells and Tissues of the Immune System
The cells of the innate and adaptive immune system are normally present as circulating cells in the blood and lymph, as anatomically defined collections in lymphoid organs, and as scattered cells in virtually all tissues. The anatomic organization of these cells and their ability to circulate and exchange among blood, lymph, and tissues are of critical importance for the generation of immune responses. The immune system faces numerous challenges to generate effective protective responses against infectious pathogens. First, the system must be able to respond rapidly to small numbers of many different microbes that may be introduced at any site in the body. Second, in the adaptive immune response, very few naive lymphocytes specifically recognize and respond to any one antigen. Third, the effector mechanisms of the adaptive immune system (antibodies and effector T cells) may have to locate and destroy microbes at sites that are distant from the site where the immune response was induced. The ability of the immune system to meet these challenges and to perform its protective functions optimally is dependent on several properties of its cells and tissues. The major cells and tissues of the immune system and their important roles are the following:
Immune responses develop through a series of steps, in each of which the special properties of immune cells and tissues play critical roles. This chapter describes the cells, tissues, and organs that compose the immune system. In Chapter 3, we describe the traffic patterns of lymphocytes throughout the body and the mechanisms of migration of lymphocytes and other leukocytes.
Cells of the Immune System
The cells that serve specialized roles in innate and adaptive immune responses are phagocytes, dendritic cells, antigen-specific lymphocytes, and various other leukocytes that function to eliminate antigens. The cells of the immune system were introduced briefly in Chapter 1. Here we describe the morphology and functional characteristics of phagocytes, other leukocytes, APCs, and lymphocytes and how these cells are organized in lymphoid tissues. The numbers of some of these cell types in the blood are listed in Table 2-1. Although most of these cells are found in the blood, their responses to microbes are usually localized to tissues and are generally not reflected in changes in the total numbers of circulating leukocytes.
TABLE 2–1 Normal Blood Cell Counts
| Mean Number per Microliter | Normal Range | |
|---|---|---|
| White blood cells (leukocytes) | 7400 | 4500-11,000 |
| Neutrophils | 4400 | 1800-7700 |
| Eosinophils | 200 | 0-450 |
| Basophils | 40 | 0-200 |
| Lymphocytes | 2500 | 1000-4800 |
| Monocytes | 300 | 0-800 |
Phagocytes
Phagocytes, including neutrophils and macrophages, are cells whose primary function is to identify, ingest, and destroy microbes. The functional responses of phagocytes in host defense consist of sequential steps: recruitment of the cells to the sites of infection, recognition of and activation by microbes, ingestion of the microbes by the process of phagocytosis, and destruction of ingested microbes. In addition, through direct contact and by secreting proteins, phagocytes communicate with other cells in ways that promote or regulate immune responses. The effector functions of phagocytes are important in innate immunity, discussed in Chapter 4, and also in the effector phase of some adaptive immune responses, as we will discuss in Chapter 10. As a prelude to more detailed discussions of the role of phagocytes in immune responses in later chapters, we will now describe their morphologic features and briefly introduce the functional responses of neutrophils and macrophages.
Neutrophils
Neutrophils, also called polymorphonuclear leukocytes, are the most abundant population of circulating white blood cells and mediate the earliest phases of inflammatory reactions. Neutrophils circulate as spherical cells about 12 to 15 µm in diameter with numerous membranous projections. The nucleus of a neutrophil is segmented into three to five connected lobules, hence the synonym polymorphonuclear leukocyte (Fig. 2-1A). The cytoplasm contains granules of two types. The majority, called specific granules, are filled with enzymes such as lysozyme, collagenase, and elastase. These granules do not stain strongly with either basic or acidic dyes (hematoxylin and eosin, respectively), which distinguishes neutrophil granules from those of two other types of circulating granulocytes, called basophils and eosinophils. The remainder of the granules of neutrophils, called azurophilic granules, are lysosomes containing enzymes and other microbicidal substances, including defensins and cathelicidins, which we will discuss in Chapter 4. Neutrophils are produced in the bone marrow and arise from a common lineage with mononuclear phagocytes. Production of neutrophils is stimulated by granulocyte colony-stimulating factor (G-CSF). An adult human produces more than 1 × 1011 neutrophils per day, each of which circulates in the blood for only about 6 hours. Neutrophils may migrate to sites of infection within a few hours after the entry of microbes. If a circulating neutrophil is not recruited into a site of inflammation within this period, it undergoes apoptosis and is usually phagocytosed by resident macrophages in the liver or spleen. After entering tissues, neutrophils function for a few hours and then die.
FIGURE 2–1 Morphology of neutrophils, mast cells, basophils, and eosinophils.
A, The light micrograph of a Wright-Giemsa–stained blood neutrophil shows the multilobed nucleus, because of which these cells are also called polymorphonuclear leukocytes, and the faint cytoplasmic granules. B, The light micrograph of a Wright-Giemsa–stained section of skin shows a mast cell (arrow) adjacent to a small blood vessel, identifiable by the red blood cell in the lumen. The cytoplasmic granules in the mast cell, which are stained purple, are filled with histamine and other mediators that act on adjacent blood vessels to promote increased blood flow and delivery of plasma proteins and leukocytes into the tissue.
(Courtesy of Dr. George Murphy, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.)
C, The light micrograph of a Wright-Giemsa–stained blood basophil shows the characteristic blue-staining cytoplasmic granules.
(Courtesy of Dr. Jonathan Hecht, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.)
D, The light micrograph of a Wright-Giemsa–stained blood eosinophil shows the characteristic segmented nucleus and red staining of the cytoplasmic granules.
Mononuclear Phagocytes
The mononuclear phagocyte system consists of cells whose primary function is phagocytosis and that play central roles in innate and adaptive immunity. The cells of the mononuclear phagocyte system originate from a common precursor in the bone marrow, circulate in the blood, and mature and become activated in various tissues (Fig. 2-2). The cell type in this lineage that enters the peripheral blood from the marrow is incompletely differentiated and is called the monocyte. Monocytes are 10 to 15 µm in diameter, and they have bean-shaped nuclei and finely granular cytoplasm containing lysosomes, phagocytic vacuoles, and cytoskeletal filaments (Fig. 2-3). Monocytes are heterogeneous and consist of at least two subsets, which are distinguishable by cell surface proteins and kinetics of migration into tissues. One population is called inflammatory because it is rapidly recruited from the blood into sites of tissue inflammation. The other type may be the source of tissue resident macrophages and some dendritic cells.
FIGURE 2–2 Maturation of mononuclear phagocytes and dendritic cells.
Both dendritic cells and monocytes arise from a common precursor cell of the myeloid lineage in the bone marrow, and differentiation into monocytes or dendritic cells is driven by the cytokines monocyte colony-stimulating factor and Flt3 ligand, respectively (not shown). Dendritic cells further differentiate into subsets, the two major being conventional dendritic cells and plasmacytoid dendritic cells. Some dendritic cells may arise from monocytes in inflamed tissues. When blood monocytes are recruited into tissues, they become macrophages. Long-lived resident macrophages are present in all tissues of the body. At least two populations of blood monocytes exist (not shown), which are precursors, respectively, of macrophages that accumulate in response to infections and macrophages that are constitutively present in normal tissues. Macrophages in tissues become activated to perform antimicrobial and tissue repair functions in response to infections and tissue injury. Macrophages differentiate into specialized forms in particular tissues. CNS, central nervous system; DC, dendritic cell.
FIGURE 2–3 Morphology of mononuclear phagocytes.
A, Light micrograph of a monocyte in a peripheral blood smear. B, Electron micrograph of a peripheral blood monocyte.
(Courtesy of Dr. Noel Weidner, Department of Pathology, University of California, San Diego.)
C, Electron micrograph of an activated tissue macrophage showing numerous phagocytic vacuoles and cytoplasmic organelles.
(From Fawcett DW. Bloom and Fawcett: A Textbook of Histology, 12th ed. Chapman & Hall, New York, 1994. With kind permission of Springer Science and Business Media.)
Once they enter tissues, these monocytes mature and become macrophages. Macrophages in different tissues have been given special names to designate specific locations. For instance, in the central nervous system, they are called microglial cells; when lining the vascular sinusoids of the liver, they are called Kupffer cells; in pulmonary airways, they are called alveolar macrophages; and multinucleate phagocytes in bone are called osteoclasts.
Macrophages perform several important functions in innate and adaptive immunity.
Macrophages are activated to perform their functions by recognizing many different kinds of microbial molecules as well as host molecules produced in response to infections. These various activating molecules bind to specific signaling receptors located on the surface of or inside the macrophage. An example of these receptors is the Toll-like receptors, which are of central importance in innate immunity and will be discussed in detail in Chapter 4. Macrophages are also activated when receptors on their plasma membrane bind opsonins on the surface of microbes. Opsonins are substances that coat particles for phagocytosis. Examples of these opsonin receptors are complement receptors and antibody Fc receptors, discussed in Chapter 12. In adaptive immunity, macrophages are activated by secreted cytokines and membrane proteins made by T lymphocytes, discussed in Chapter 10.
Macrophages can acquire distinct functional capabilities, depending on the types of activating stimuli. The clearest example of this is the response of macrophages to different cytokines made by subsets of T cells. Some of these cytokines activate macrophages to become efficient at killing microbes, called classical activation. Other cytokines activate macrophages to promote tissue remodeling and repair, called alternative activation. The details of these different forms of activation, and the cytokines involved, are discussed in Chapter 10. Macrophages may also assume different morphologic forms after activation by external stimuli, such as microbes. Some develop abundant cytoplasm and are called epithelioid cells because of their resemblance to epithelial cells of the skin. Activated macrophages can fuse to form multinucleate giant cells.
Macrophage-like cells are phylogenetically the oldest mediators of innate immunity. Drosophila responds to infection by surrounding microbes with “hemocytes,” which are similar to macrophages, and these cells phagocytose the microbes and wall off the infection by inducing coagulation of the surrounding hemolymph. Similar phagocyte-like cells have been identified even in plants.
Macrophages typically respond to microbes nearly as rapidly as neutrophils do, but macrophages survive much longer at sites of inflammation. Unlike neutrophils, macrophages are not terminally differentiated and can undergo cell division at an inflammatory site. Therefore, macrophages are the dominant effector cells of the later stages of the innate immune response, several days after infection.
Mast Cells, Basophils, Eosinophils
Mast cells, basophils, and eosinophils are three additional cells that play roles in innate and adaptive immune responses. All three cell types share the common feature of having cytoplasmic granules filled with various inflammatory and antimicrobial mediators. Another common feature of these cells is their involvement in immune responses that protect against helminths and immune responses that cause allergic diseases. We will describe the major features of these cells in this section and discuss their functions in more detail in Chapter 19.
Mast Cells
Mast cells are bone marrow–derived cells that are present in the skin and mucosal epithelium and contain abundant cytoplasmic granules filled with cytokines histamine, and other mediators. Stem cell factor (also called c-Kit ligand) is a cytokine that is essential for mast cell development. Normally, mature mast cells are not found in the circulation but are constitutively present in healthy tissues, usually adjacent to small blood vessels and nerves. Human mast cells vary in shape and have round nuclei, and the cytoplasm contains membrane-bound granules (see Fig. 2-1B). The granules contain acidic proteoglycans that bind basic dyes. Mast cells express plasma membrane receptors for IgE and IgG antibodies and are usually coated with these antibodies. When these antibodies on the mast cell surface also bind antigen, signaling events are induced that lead to release of the cytoplasmic granule contents into the extracellular space. The released contents of the granules, including cytokines and histamine, promote changes in the blood vessels that cause inflammation. Mast cells also express other activating receptors that recognize complement proteins, neuropeptides, and microbial products. Mast cells provide defense against helminths but are also responsible for symptoms of allergic diseases (see Chapter 19).
Basophils
Basophils are blood granulocytes with many structural and functional similarities to mast cells. Like other granulocytes, basophils are derived from bone marrow progenitors (a lineage different from that of mast cells), mature in the bone marrow, and circulate in the blood. Basophils constitute less than 1% of blood leukocytes (see Table 2-1). Although they are normally not present in tissues, basophils may be recruited to some inflammatory sites. Basophils contain granules that bind basic dyes (see Fig. 2-1C), and they are capable of synthesizing many of the same mediators as mast cells. Like mast cells, basophils express IgG and IgE receptors, bind IgE, and can be triggered by antigen binding to the IgE. Because basophil numbers are low in tissues, their importance in host defense and allergic reactions is uncertain.
Eosinophils
Eosinophils are blood granulocytes that express cytoplasmic granules containing enzymes that are harmful to the cell walls of parasites but can also damage host tissues. The granules of eosinophils contain basic proteins that bind acidic dyes such as eosin (see Fig. 2-1D). Like neutrophils and basophils, eosinophils are bone marrow derived. GM-CSF, IL-3, and IL-5 promote eosinophil maturation from myeloid precursors. Some eosinophils are normally present in peripheral tissues, especially in mucosal linings of the respiratory, gastrointestinal, and genitourinary tracts, and their numbers can increase by recruitment from the blood in the setting of inflammation.
Antigen-Presenting Cells
Antigen-presenting cells (APCs) are cell populations that are specialized to capture microbial and other antigens, display them to lymphocytes, and provide signals that stimulate the proliferation and differentiation of the lymphocytes. By convention, APC usually refers to a cell that displays antigens to T lymphocytes. The major type of APC that is involved in initiating T cell responses is the dendritic cell. Macrophages and B cells present antigens to T lymphocytes in different types of immune responses, and a specialized cell type called the follicular dendritic cell displays antigens to B lymphocytes during particular phases of humoral immune responses. APCs link responses of the innate immune system to responses of the adaptive immune system, and therefore they may be considered components of both systems. In addition to the introduction presented here, APC function will be described in more detail in Chapter 6.
Dendritic Cells
Dendritic cells are the most important APCs for activating naive T cells, and they play major roles in innate responses to infections and in linking innate and adaptive immune responses. They have long membranous projections and phagocytic capabilities and are widely distributed in lymphoid tissues, mucosal epithelium, and organ parenchyma (Fig. 2-4). Dendritic cells are part of the myeloid lineage of hematopoietic cells and arise from a precursor that can also differentiate into monocytes but not granulocytes (see Fig. 2-2). Maturation of dendritic cells is dependent on a cytokine called Flt3 ligand, which binds to the Flt3 tyrosine kinase receptor on the precursor cells. Similar to macrophages, dendritic cells express receptors that recognize molecules typically made by microbes and not mammalian cells, and they respond to the microbes by secreting cytokines. The majority of dendritic cells are called conventional dendritic cells. In response to activation by microbes, conventional dendritic cells in skin, mucosa, and organ parenchyma become mobile, migrate to lymph nodes, and display microbial antigens to T lymphocytes. Thus, these cells function in both innate and adaptive immune responses and are a link between these two components of host defense. One subpopulation of dendritic cells, called plasmacytoid dendritic cells, are early cellular responders to viral infection. They recognize nucleic acids of intracellular viruses and produce soluble proteins called type I interferons, which have potent antiviral activities. We will discuss the role of dendritic cells as mediators of innate immunity and as APCs in Chapters 4 and 6, respectively.
The fluorescence photomicrograph shows a bone marrow–derived dendritic cell in which class II MHC molecules appear green, highlighting the fine cytoplasmic processes characteristic of dendritic cells, and the nucleus appears blue. Class II MHC molecules are highly expressed in dendritic cells and are important for their function (see Chapter 6).
(Courtesy of Scott Loughhead and Uli Van Andrian, Harvard Medical School, Boston, Massachusetts.)
Antigen-Presenting Cells for Effector T Lymphocytes
In addition to dendritic cells, macrophages and B lymphocytes perform important antigen-presenting functions in CD4+ helper T cell–mediated immune responses. Macrophages present antigen to helper T lymphocytes at the sites of infection, which leads to helper T cell activation and production of molecules that further activate the macrophages. This process is important for the eradication of microbes that are ingested by the phagocytes but resist killing; in these cases, helper T cells greatly enhance the microbicidal activities of the macrophages. B cells present antigens to helper T cells in lymph nodes and spleen, which is a key step in the cooperation of helper T cells with B cells in humoral immune responses to protein antigens. These functions of macrophages and B cells will be discussed in Chapters 10 and 11. Cytotoxic T lymphocytes (CTLs) are effector CD8+ T cells that can recognize antigens on any type of nucleated cell and become activated to kill the cell. Therefore, all nucleated cells are potentially APCs for CTLs.
Follicular Dendritic Cells
Follicular dendritic cells (FDCs) are cells with membranous projections that are found intermingled in specialized collections of activated B cells, called germinal centers, in the lymphoid follicles of the lymph nodes, spleen, and mucosal lymphoid tissues. FDCs are not derived from precursors in the bone marrow and are unrelated to the dendritic cells that present antigens to T lymphocytes. FDCs trap antigens complexed to antibodies or complement products and display these antigens on their surfaces for recognition by B lymphocytes. This is important for the selection of activated B lymphocytes whose antigen receptors bind the displayed antigens with high affinity (see Chapter 11).
Lymphocytes
Lymphocytes, the unique cells of adaptive immunity, are the only cells in the body that express clonally distributed antigen receptors, each with a fine specificity for a different antigenic determinant. Each clone of lymphocytes consists of the progeny of one cell and expresses antigen receptors with a single specificity. This is why the total population of antigen receptors in the adaptive immune system is said to be clonally distributed. As we shall discuss here and in later chapters, there are millions of lymphocyte clones in the body, enabling the organism to recognize and respond to millions of foreign antigens.
The role of lymphocytes as the cells that mediate adaptive immunity was established during decades of research by several lines of evidence. One of the earliest clues about the importance of lymphocytes in adaptive immunity came from the discovery that humans with congenital and acquired immune deficiency states had reduced numbers of lymphocytes in the peripheral circulation and in lymphoid tissues. Furthermore, physicians noted that depletion of lymphocytes with drugs or irradiation impaired immune protection against infection. Experiments done mainly with mice showed that protective immunity to microbes can be adoptively transferred from immunized to naive animals only by lymphocytes or their secreted products. In vitro experiments established that stimulation of lymphocytes with antigens leads to responses that show many of the characteristics of immune responses induced under more physiologic conditions in vivo. Following the identification of lymphocytes as the mediators of humoral and cellular immunity, many discoveries were made at a rapid pace about different types of lymphocytes, their origins in the bone marrow and thymus, and the consequences of the absence of each type of lymphocyte. These discoveries relied on many tools, including genetically modified mice and reagents that selectively deplete one or another type of lymphocyte. Among the most important of these discoveries was that clonally distributed, highly diverse and specific receptors for antigens are produced by lymphocytes but not by any other types of cells. During the past two decades, there has been an enormous expansion of information about lymphocyte genes, proteins, and functions. We probably now know more about lymphocytes than about any other cells in all of biology.
One of the most interesting questions about lymphocytes has been how the enormously diverse repertoire of antigen receptors, and therefore specificities, is generated from a small number of genes for these receptors in the germline. It is now known that the genes encoding the antigen receptors of lymphocytes are formed by recombination of DNA segments during the maturation of these cells. There is a random aspect to these somatic recombination events that results in the generation of millions of different receptor genes and a highly diverse repertoire of antigen specificities among different clones of lymphocytes (see Chapter 8).
The total number of lymphocytes in a healthy adult is about 5 × 1011. Of these, ~2% are in the blood, ~10% in the bone marrow, ~15% in the mucosal lymphoid tissues of the gastrointestinal and respiratory tracts, and ~65% in lymphoid organs (mainly the lymph nodes and spleen). We first describe the properties of these cells and then their organization in various lymphoid tissues.
Subsets of Lymphocytes
Lymphocytes consist of distinct subsets that are different in their functions and protein products (Table 2-2). The major classes of lymphocytes were introduced in Chapter 1 (see Fig. 1-5). Morphologically, all lymphocytes are similar, and their appearance does not reflect their heterogeneity or their diverse functions. B lymphocytes, the cells that produce antibodies, were so called because in birds they were found to mature in an organ called the bursa of Fabricius. In mammals, no anatomic equivalent of the bursa exists, and the early stages of B cell maturation occur in the bone marrow. Thus, “B” lymphocytes refer to bursa-derived lymphocytes or bone marrow–derived lymphocytes. T lymphocytes, the mediators of cellular immunity, were named because their precursors, which arise in the bone marrow, migrate to and mature in the thymus; “T” lymphocytes refer to thymus-derived lymphocytes. B and T lymphocytes each consist of subsets with distinct phenotypic and functional characteristics. The major subsets of B cells are follicular B cells, marginal zone B cells, and B-1 B cells, each of which is found in distinct anatomic locations within lymphoid tissues. The two major T cell subsets are helper CD4+ T lymphocytes and CD8+ CTLs, which express an antigen receptor called the αβ receptor. CD4+ regulatory T cells are a third unique subset of T cells expressing the αβ receptor. Another population of T cells, called γδ T cells, expresses a similar but structurally distinct type of antigen receptor. The different functions of these classes of B and T cells will be discussed in later chapters.
The major populations of B cells and T cells express highly diverse, clonally distributed sets of antigen receptors. Some numerically minor subsets of lymphocytes, including γδ T cells, marginal zone B cells, and B-1 B cells, are restricted in their use of DNA segments that contribute to their antigen receptor genes, and these lymphocyte subsets have very limited diversity.
In addition to B and T cells, there exist other populations of cells that are called lymphocytes on the basis of morphology and certain functional and molecular criteria but that are not readily categorized as T or B cells. Natural killer (NK) cells, which are described in Chapter 4, have similar effector functions as CTLs, but their receptors are distinct from B or T cell antigen receptors and are not encoded by somatically recombined genes. NKT cells are a numerically small population of T lymphocytes that are so named because they express a surface molecule typically found on NK cells. They express αβ antigen receptors that are encoded by somatically recombined genes, but like γδ T cells and B-1 B cells, they lack diversity. NKT cells, γδ T cells, and B-1 B cells may all be considered part of both adaptive and innate immune systems.
Membrane proteins are used as phenotypic markers to distinguish distinct populations of lymphocytes (see Table 2-2). For instance, most helper T cells express a surface protein called CD4, and most CTLs express a different surface protein called CD8. These and many other surface proteins are often called markers because they identify and discriminate between (“mark”) different cell populations. These markers not only delineate the different classes of lymphocytes but also have many functions in the cell types in which they are expressed. The most common way to determine if a surface phenotypic marker is expressed on a cell is to test if antibodies specific for the marker bind to the cell. In this context, the antibodies are used by investigators or clinicians as analytical tools. There are available thousands of different pure antibody preparations, called monoclonal antibodies, each specific for a different molecule and labeled with probes that can be readily detected on cell surfaces by use of appropriate instruments. (Monoclonal antibodies are described in Chapter 5, and methods to detect labeled antibodies bound to cells are discussed in Appendix IV.) The cluster of differentiation (CD) system is a widely adopted uniform method for naming cell surface molecules that are characteristic of a particular cell lineage or differentiation stage, have a defined structure, and are recognized by a group (“cluster”) of monoclonal antibodies. Thus, all structurally well defined cell surface molecules are given a CD number designation (e.g., CD1, CD2). A current list of CD markers for leukocytes that are mentioned in the book is provided in Appendix III.
Development of Lymphocytes
After birth, lymphocytes, like all blood cells, arise from stem cells in the bone marrow. The origin of lymphocytes from bone marrow progenitors was first demonstrated by experiments with radiation-induced bone marrow chimeras. Lymphocytes and their precursors are radiosensitive and are killed by high doses of γ-irradiation. If a mouse of one inbred strain is irradiated and then injected with bone marrow cells or small numbers of hematopoietic stem cells of another strain that can be distinguished from the host, all the lymphocytes that develop subsequently are derived from the bone marrow cells or hematopoietic stem cells of the donor. Such approaches have proved useful for examining the maturation of lymphocytes and other blood cells.
All lymphocytes go through complex maturation stages during which they express antigen receptors and acquire the functional and phenotypic characteristics of mature cells. The anatomic sites where the major steps in lymphocyte development occur are called the generative lymphoid organs. These include the bone marrow, where precursors of all lymphocytes arise and B cells mature, and the thymus, where T cells mature (Fig. 2-5). We will discuss the processes of B and T lymphocyte maturation in much more detail in Chapter 8. These mature B and T cells are called naive lymphocytes. After activation by antigen, lymphocytes go through sequential changes in phenotype and functional capacity.
FIGURE 2–5 Maturation of lymphocytes.
Lymphocytes develop from bone marrow stem cells and mature in the generative lymphoid organs (bone marrow and thymus for B and T cells, respectively) and then circulate through the blood to secondary lymphoid organs (lymph nodes, spleen, regional lymphoid tissues such as mucosa-associated lymphoid tissues). Fully mature T cells leave the thymus, but immature B cells leave the bone marrow and complete their maturation in secondary lymphoid organs. Naive lymphocytes may respond to foreign antigens in these secondary lymphoid tissues or return by lymphatic drainage to the blood and recirculate through other secondary lymphoid organs.
Populations of Lymphocytes Distinguished by History of Antigen Exposure
In adaptive immune responses, naive lymphocytes that emerge from the bone marrow or thymus migrate into peripheral lymphoid organs, where they are activated by antigens to proliferate and differentiate into effector and memory cells, some of which then migrate into tissues (Fig. 2-6). The activation of lymphocytes follows a series of sequential steps beginning with the synthesis of new proteins, such as cytokine receptors and cytokines, which are required for many of the subsequent changes. The naive cells then undergo proliferation, resulting in increased size of the antigen-specific clones, a process that is called clonal expansion. In some infections, the numbers of microbe-specific T cells may increase more than 50,000-fold, and the numbers of specific B cells may increase up to 5000-fold. This rapid clonal expansion of microbe-specific lymphocytes is needed to keep pace with the ability of microbes to rapidly replicate and expand in numbers. Concurrently with clonal expansion, antigen-stimulated lymphocytes differentiate into effector cells whose function is to eliminate the antigen. Some of the progeny of antigen-stimulated B and T lymphocytes differentiate into long-lived memory cells, whose function is to mediate rapid and enhanced (i.e., secondary) responses to subsequent exposures to antigens. Distinct populations of lymphocytes (naive, effector, and memory) are always present in various sites throughout the body, and these populations can be distinguished by several functional and phenotypic criteria (Table 2-3).
FIGURE 2–6 The anatomy of lymphocyte activation.
Naive T cells emerging from the thymus and immature B cells emerging from the bone marrow migrate into secondary lymphoid organs, including lymph nodes and spleen. In these locations, B cells complete their maturation; naive B and T cells activated by antigens differentiate into effector and memory lymphocytes. Some effector and memory lymphocytes migrate into peripheral tissue sites of infection. Antibodies secreted by effector B cells in lymph node, spleen, and bone marrow (not shown) enter the blood and are delivered to sites of infection.
The details of lymphocyte activation and differentiation as well as the functions of each of these populations will be addressed later in this book. Here we summarize the phenotypic characteristics of each population.
Naive Lymphocytes
Naive lymphocytes are mature T or B cells that reside in the peripheral lymphoid organs and circulation and have never encountered foreign antigen. (The term naive refers to the idea that these cells are immunologically inexperienced because they have not encountered antigen.) Naive lymphocytes typically die after 1 to 3 months if they do not recognize antigens. Naive and memory lymphocytes, discussed later, are both called resting lymphocytes because they are not actively dividing, nor are they performing effector functions. Naive (and memory) B and T lymphocytes cannot be readily distinguished morphologically and are both often called small lymphocytes when observed in blood smears or by flow cytometry (a technique described in Appendix IV). A small lymphocyte is 8 to 10 µm in diameter and has a large nucleus with dense heterochromatin and a thin rim of cytoplasm that contains a few mitochondria, ribosomes, and lysosomes but no visible specialized organelles (Fig. 2-7). Before antigenic stimulation, naive lymphocytes are in a state of rest, or in the G0 stage of the cell cycle. In response to stimulation, they enter the G1 stage of the cell cycle before going on to divide. Activated lymphocytes are larger (10 to 12 µm in diameter), have more cytoplasm and organelles and increased amounts of cytoplasmic RNA, and are called large lymphocytes or lymphoblasts (see Fig. 2-7).
FIGURE 2–7 Morphology of lymphocytes.
A, Light micrograph of a lymphocyte in a peripheral blood smear.
(Courtesy of Jean Shafer, Department of Pathology, University of California, San Diego. Copyright 1995-2008, Carden Jennings Publishing Co., Ltd.)
B, Electron micrograph of a small lymphocyte.
(Courtesy of Dr. Noel Weidner, Department of Pathology, University of California, San Diego.)
C, Light micrograph of a large lymphocyte (lymphoblast).
(Courtesy of Jean Shafer, Department of Pathology, University of California, San Diego. Copyright 1995-2008, Carden Jennings Publishing Co., Ltd.)
D, Electron micrograph of a large lymphocyte (lymphoblast).
(From Fawcett DW. Bloom and Fawcett: A Textbook of Histology, 12th ed. Chapman & Hall, New York, 1994. With kind permission of Springer Science and Business Media.)
The survival of naive lymphocytes depends on two types of signals, some of which are generated by antigen receptors and others by cytokines. It is postulated that the antigen receptor of naive B cells generates survival signals even in the absence of antigen, and naive T lymphocytes recognize various self antigens “weakly,” enough to generate survival signals but without triggering the stronger signals that are needed to initiate clonal expansion and differentiation into effector cells. The need for antigen receptor expression to maintain the pool of naive lymphocytes in peripheral lymphoid organs has been demonstrated by studies with mice in which the genes that encode the antigen receptors of B cells or T cells were deleted after the lymphocytes matured. (The method used, called the Cre/lox recombinase technique, is described in Appendix IV.) In these studies, naive lymphocytes that lost their antigen receptors died within 2 or 3 weeks.
Cytokines are also essential for the survival of naive lymphocytes, and naive T and B cells constitutively express receptors for these cytokines. The most important of these cytokines are interleukin-7 (IL-7), which promotes survival and, perhaps, low-level cycling of naive T cells, and B cell–activating factor (BAFF) belonging to the TNF family, which is required for naive B cell survival.
In the steady state, the pool of naive lymphocytes is maintained at a fairly constant number because of a balance between spontaneous death of these cells and the generation of new cells in the generative lymphoid organs. Any loss of lymphocytes leads to a compensatory proliferation of the remaining ones and increased output from the generative organs. A demonstration of the ability of the lymphocyte population to “fill” the available space is the phenomenon of homeostatic proliferation. If naive cells are transferred into a host that is deficient in lymphocytes (said to be lymphopenic) because of inherited defects or the effects of irradiation, the transferred lymphocytes begin to proliferate and increase in number until they reach roughly the numbers of lymphocytes in normal animals. Homeostatic proliferation appears to be driven by the same signals—weak recognition of some self antigens and cytokines, mainly IL-7—that are required for the maintenance of naive lymphocytes.
Effector Lymphocytes
After naive lymphocytes are activated, they become larger and proliferate and are called lymphoblasts. Some of these cells differentiate into effector lymphocytes that have the ability to produce molecules capable of eliminating foreign antigens; effector lymphocytes include helper T cells, CTLs, and antibody-secreting plasma cells. Helper T cells, which are usually CD4+, express surface molecules such as CD40 ligand (CD154) and secrete cytokines that interact with macrophages and B lymphocytes, leading to their activation. CTLs have cytoplasmic granules filled with proteins that, when released, kill the cells that the CTLs recognize, which are usually virus-infected and tumor cells. Both CD4+ and CD8+ effector T cells usually express surface proteins indicative of recent activation, including CD25 (a component of the receptor for the T cell growth factor IL-2), and altered patterns of adhesion molecules (selectins and integrins, discussed in Chapter 3). The majority of differentiated effector T lymphocytes are short-lived and not self-renewing.
Many antibody-secreting B cells are morphologically identifiable as plasma cells. They have characteristic nuclei, abundant cytoplasm containing dense, rough endoplasmic reticulum that is the site where antibodies (and other secreted and membrane proteins) are synthesized, and distinct perinuclear Golgi complexes where antibody molecules are converted to their final forms and packaged for secretion (Fig. 2-8). It is estimated that half or more of the messenger RNA in plasma cells codes for antibody proteins. Plasma cells develop in lymphoid organs and at sites of immune responses and some of them migrate to the bone marrow, where they may live and secrete antibodies for long periods after the immune response is induced and even after the antigen is eliminated. Circulating antibody-secreting cells, called plasmablasts, are rare, and may be precursors of long-lived plasma cells in tissues.
Memory Lymphocytes
Memory cells may survive in a functionally quiescent or slowly cycling state for months or years without a need for stimulation by antigen and presumably after the antigen is eliminated. They can be identified by their expression of surface proteins that distinguish them from naive and recently activated effector lymphocytes, although it is still not clear which of these surface proteins are definitive markers of memory populations (see Table 2-3). Memory B lymphocytes express certain classes (isotypes) of membrane Ig, such as IgG, IgE, or IgA, as a result of isotype switching, whereas naive B cells express only IgM and IgD (see Chapters 5 and 11). In humans, CD27 expression is a good marker for memory B cells. Memory T cells, like naive but not effector T cells, express high levels of the IL-7 receptor (CD127). Memory T cells also express surface molecules that promote their migration into sites of infection anywhere in the body (discussed later in the chapter). In humans, most naive T cells express a 200-kD isoform of a surface molecule called CD45 that contains a segment encoded by an exon designated A. This CD45 isoform can be recognized by antibodies specific for the A-encoded segment and is therefore called CD45RA (for “restricted A”). In contrast, most activated and memory T cells express a 180-kD isoform of CD45 in which the A exon RNA has been spliced out; this isoform is called CD45RO. However, this way of distinguishing naive from memory T cells is not perfect, and interconversion between CD45RA+ and CD45RO+ populations has been documented.
Memory cells appear to be heterogeneous, and there are subsets that differ, especially with respect to their location and migratory properties. More details about memory T and B cells will be discussed in Chapters 9 and 11, respectively.
The distinguishing features of naive, effector, and memory lymphocytes reflect different programs of gene expression that are regulated by transcription factors and by stable epigenetic changes, including DNA methylation and chromatin remodeling. Our understating of these molecular determinants of mature lymphocyte phenotype is still incomplete and evolving. For example, a transcription factor called Kruppel-like factor 2 (KLF-2) is required for maintenance of the naive T cell phenotype. The phenotypes of functionally different types of CD4+ effector T cells, called TH1, TH2, and TH17 cells, depend on transcription factors T-bet, GATA-3, and RORγT, respectively, as well as epigenetic changes in cytokine gene loci (see Chapter 9). Other transcription factors are required for maintaining the phenotypes of memory T and B cells.
