Cellular Components of the Immune System

About 5 to 15% of lymphocytes in the blood are B cells; they are also present in the bone marrow, spleen, lymph nodes, and mucosa-associated lymphoid tissues.

B cells can present antigen to T cells and release cytokines, but their primary function is to develop into plasma cells, which manufacture and secrete antibodies.

Patients with B-cell immunodeficiencies (eg, X-linked agammaglobulinemia) are especially susceptible to recurrent bacterial infections.

After random rearrangement of the immunoglobulin (Ig) variable, diversity, and joining genes that encode the variable region of an antibody, B cells collectively have the potential to recognize an almost limitless number of unique antigens. Gene rearrangement occurs in programmed steps in the bone marrow during B-cell development. The process starts with a committed stem cell, continues through pro‒B and pre‒B cell stages, and results in an immature B cell. At this point, any cells that interact with self antigen (autoimmune cells) are removed from the immature B cell population via inactivation (anergy) or apoptosis, thereby ensuring immune tolerance. Cells that are not removed (ie, those that recognize nonself antigen) continue to develop into mature naive B cells, leave the bone marrow, and enter peripheral lymphoid organs, where they may encounter antigens.

Their response to antigen has 2 stages:

• Primary immune response: When mature naive B cells first encounter antigen, they become lymphoblasts, undergo clonal proliferation, and differentiate into memory cells, which can respond to the same antigen in the future, or into mature antibody-secreting plasma cells. After first exposure, there is a latent period of days before antibody is produced. Initially, only IgM is produced. After that, with the help of T cells, B cells can further rearrange their Ig genes and undergo class switching to produce antigen-specific IgG, IgA, or IgE. Thus, at first exposure, the response is slow and initially provides limited protective immunity.

• Secondary (anamnestic or booster) immune response: When memory B cells and Th cells are re-exposed to the antigen, the memory B cells rapidly proliferate, differentiate into mature plasma cells, and promptly produce large amounts of antibody (chiefly IgG because of a T cell–induced class switch). The antibody also has stronger binding affinity to the antigen due to mutation in the genes encoding the antibody variable regions. The antibody is released into the blood and other tissues, where it can react with antigen. Thus, after re-exposure, the immune response is faster and more effective.

T cells develop from bone marrow stem cells that travel to the thymus, where they go through rigorous selection. There are 3 main types of T cell:

• Helper

• Regulatory (suppressor)

• Cytotoxic

In thymic selection, T cells that react to self antigen presented by self MHC molecules (or react strongly to self MHC molecules regardless of the antigen presented) are eliminated by apoptosis, limiting the likelihood of autoimmunity. Only T cells that can recognize nonself antigen complexed to self MHC molecules survive; they leave the thymus for peripheral blood and lymphoid tissues.

Most mature T cells express either CD4 or CD8 and have an antigen-binding, Ig-like, surface receptor called the T-cell receptor (TCR). There are 2 types of TCR:

• Alpha-beta TCR: Composed of TCR alpha and beta chains; present on most T cells

• Gamma-delta TCR: Composed of TCR gamma and delta chains; present on a minor population of T cells

Genes that encode the TCR, like Ig genes, are rearranged, resulting in defined specificity and affinity for antigen. Most T cells (those with an alpha-beta TCR) recognize antigen-derived peptide displayed along with the MHC molecule of an antigen-presenting cell. T cells with gamma-delta TCR recognize protein antigen directly or recognize lipid antigen displayed by an MHC-like molecule called CD1. The number of T-cell specificities is almost limitless.

For alpha-beta T cells to be activated, the TCR must engage with antigen-MHC (see figure Two-Signal Model for T-cell Activation). Costimulatory accessory molecules must also interact (eg, CD28 on the T cell interacts with CD80 and CD86 on the antigen-presenting cell); otherwise, the T cell exposed to antigen becomes anergic or dies by apoptosis. Some accessory molecules (eg, CTLA-4 [cytotoxic T-lymphocyte antigen 4] on the T cell, which also interacts with CD80 and CD86 on the antigen-presenting cell, PD-1 [programmed cell death protein 1] on the T cell, which interacts with PD-L1 [programmed cell death protein ligand 1] on the antigen-presenting cell) inhibit previously activated T cells and thus dampen the immune response.

Molecules such as CTLA-4 and PD-1, and their ligands, are termed checkpoint molecules because they signal that the T cell needs to be restrained from continuing its activity. Cancer cells that express checkpoint molecules may thus be protected from the immune system by restraining the activity of tumor-specific T cells.

Monoclonal antibodies that target checkpoint molecules on either T cells or on tumor cells (termed checkpoint inhibitors, see table Some Immunotherapeutic Agents in Clinical Use) are used to prevent downregulation of antitumor immune responses and effectively treat some heretofore resistant cancers and boost the antitumor response. However, because checkpoint molecules are also involved in preventing other types of immune response (such as self-directed autoimmune reactions), checkpoint inhibitors can permit severe immune-related inflammatory and autoimmune reactions to occur (both systemic and organ specific) or exacerbate autoimmune disorders.

Polymorphisms in the CTLA-4 gene are associated with certain autoimmune disorders, including Graves disease and type 1 diabetes mellitus.

Two-Signal Model for T-Cell Activation

The alpha (α) and beta (β) chains of the T-cell receptor (TCR) bind to antigen (Ag)–major histocompatibility complex (MHC) on an antigen-presenting cell (APC), and CD4 or CD8 interacts with the MHC. Both actions stimulate the T cell (first signal) through the accessory CD3 chains. However, without a second (coactivation) signal, the T cell is anergic or tolerant. The TCR is structurally homologous to the B-cell receptor; the α and β (or gamma [γ] and delta [δ]) chains have constant (C) and variable (V) regions. (1) = first signal; (2) = second signal.

Helper T (Th) cells are usually CD4 but may be CD8. They differentiate from Th0 cells into one of the following:

• Th1 cells: In general, Th1 cells promote cell-mediated immunity via cytotoxic T cells and macrophages and are thus particularly involved in defense against intracellular pathogens (eg, viruses). They can also promote the production of some antibody classes.

• Th2 cells: Th2 cells are particularly adept at promoting antibody production by B cells (humoral immunity) and thus are particularly involved in directing responses aimed at extracellular pathogens (eg, bacteria, parasites).

• Th17 cells: Th17 cells promote tissue inflammation.

• Tfh cells (T follicular helper cells): Tfh cells promote B-cell responses in the germinal centers of secondary lymphoid tissues

Each cell type secretes several cytokines (see table Functions of T Cells). Different patterns of cytokine production identify other Th-cell functional phenotypes. Depending on the stimulating pathogen, Th1 and Th2 cells can, to a certain extent, downregulate each other's activity, leading to dominance of a Th1 or a Th2 response.

Any T cells that can recognize peptides derived from self antigens are normally eliminated during T cell development in the thymus. If such T cells are not deleted, they can potentially develop into autoimmune Th1, Th2, or Th17 cells that may drive development of autoimmune disease.

The distinction between the different Th cells is clinically relevant. For example, a Th1 response dominates in tuberculoid leprosy, and a Th2 response dominates in lepromatous leprosy. A Th1 response is characteristic of certain autoimmune disorders (eg, type 1 diabetes mellitus, multiple sclerosis). A Th2 response promotes IgE production and development of allergic disorders, as well as helps B cells produce autoantibodies in some autoimmune disorders (eg, Graves disease, myasthenia gravis). Th17 cells, via their role in inflammation, may also contribute to autoimmune disorders such as psoriasis and rheumatoid arthritis. Patients with immunodeficiencies characterized by defective Th17 cells (eg, hyper-IgE [Job] syndrome) are especially susceptible to infection with Candida albicans and Staphylococcus aureus.

Regulatory (suppressor) T (Treg) cells mediate suppression of immune responses and usually express the Foxp3 transcription factor. They comprise functional subsets of CD4 or CD8 T cells:

• Natural Treg cells develop within the thymus

• Induced Treg cells develop from conventional T cells upon encounter with antigen in the periphery

Regulatory T cells secrete cytokines with immunosuppressive properties (eg, transforming growth factor [TGF]-beta and interleukin [IL]-10) deplete immunostimulatory interleukin-2 (IL-2) by expressing high levels of the CD25 component of the IL-2 receptor, or suppress the immune response by mechanisms that require cell-to-cell contact and involve cell surface molecules (eg, CTLA-4). The main role of Treg cells is to prevent excessive immune responses by contributing to the resolution of inflammation and by curbing allergic and autoimmune activity, Patients with functional mutations in Foxp3 develop the autoimmune disorder IPEX syndrome (immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome).

Cytotoxic T (Tc) cells are usually CD8 but may be CD4. They are vital for eliminating intracellular pathogens, especially viruses. Tc cells play a role in organ transplant rejection.

Tc-cell development involves 3 phases:

• A precursor cell that, when appropriately stimulated, can differentiate into a Tc cell

• An effector cell that has differentiated and can kill its appropriate target

• A memory cell that is quiescent (no longer stimulated) but is ready to become an effector when restimulated by the original antigen-MHC combination

Fully activated Tc cells, like natural killer (NK) cells, can kill an infected target cell by inducing apoptosis.

Tc cells can secrete cytokines and, like Th cells, have been divided into types Tc1, Tc2, and Tc17 based on their patterns of cytokine production.

Tc cells may be

• Syngeneic: Generated in response to self (autologous) cells whose MHC presents peptides derived from viral infection or other foreign proteins

• Allogeneic: Generated in response to cells that express foreign MHC products (eg, in organ transplantation when the donor’s MHC molecules differ from the recipient’s)

Some Tc cells can directly recognize foreign MHC (direct pathway); others may recognize fragments of foreign MHC presented by self MHC molecules of the transplant recipient (indirect pathway).

Natural killer T (NKT) cells are a distinct subset of T cells. Activated NKT cells secrete IL-4 and interferon-gamma and may help regulate immune responses. NKT cells differ from NK cells in phenotype and certain functions.

Neutrophils constitute 40 to 70% of total circulating white blood cells; they are a first line of defense against infection. Mature neutrophils have a half-life of about 2 to 3 days.

During acute inflammatory responses (eg, to infection), neutrophils, drawn by chemotactic factors, use adhesion molecules on blood vessel endothelium to leave the circulation and enter tissues. Their purpose is to phagocytose and digest pathogens. Microorganisms are killed when phagocytosis generates lytic enzymes and reactive oxygen compounds (eg, superoxide, hypochlorous acid) and triggers release of granule contents (eg, defensins, proteases, bactericidal permeability-increasing protein, lactoferrin, lysozymes). DNA and histones are also released, and they, with granule contents such as elastase, generate fibrous structures called neutrophil extracellular traps (NETs) in the surrounding tissues; these structures facilitate killing by trapping bacteria and focusing enzyme activity.

Patients with immunodeficiencies that affect the ability of phagocytes to kill pathogens (eg, chronic granulomatous disease) are especially susceptible to chronic bacterial and fungal infections.

Eosinophils constitute up to 5% of circulating white blood cells.

They target organisms too large to be engulfed. Eosinophils kill by secreting toxic substances (eg, reactive oxygen compounds similar to those produced in neutrophils), major basic protein (which is toxic to parasites), eosinophil cationic protein, and several enzymes.

Circulating eosinophils, which increase in response to invasive parasitic infections, some cancers, and as part of an allergic response, are also a major source of inflammatory mediators (eg, prostaglandins, leukotrienes, platelet-activating factor, many cytokines).

Basophils constitute < 5% of circulating white blood cells. Basophils share several characteristics with mast cells, although the 2 cell types have distinct lineages. Both have high-affinity receptors for IgE called Fc-epsilon RI (FcεRI). When these cells encounter certain antigens, the bivalent IgE molecules bound to the receptors become cross-linked, triggering cell degranulation with release of preformed inflammatory mediators (eg, histamine, platelet-activating factor) and generation of newly synthesized mediators (eg, leukotrienes, prostaglandins, thromboxanes).