 Gamma/delta (/) T lymphocytes represent a small population of T cells that possess a distinct TCR on their surface made of one -chain and one -chain. Most other TCRs are composed of two glycoprotein chains called -and -TCR chains. These cells develop in the thymus and migrate into various epithelial tissues (e.g., the skin, oral mucosa, intestines, and vagina). Once they colonize an epithelial tissue, they do not recirculate between blood and lymphatic organs.
Gamma/delta (/) T cells are strategically positioned at the interfaces of the external and internal environments and function as the first line of defense against invading organisms. They encounter antigen on the surface of the epithelial cells even before it enters the body.
B lymphocytes differentiate in the bursa-equivalent organs and participate in humoral immunity.
B lymphocytes (B cells) are so named because they were first recognized as a separate population in the bursa of Fabricius in birds (page 447) or bursa-equivalent organs such as bone marrow and GALT in mammals. They have variable life spans and are involved in the production and secretion of the various circulating antibodies, also called immunoglobulins (Ig), the immune proteins associated with humoral immunity (Fig. 14.2 and Table 14.2). B cells account for 20% to 30% of the circulating lymphocytes. In addition to secreting circulating immunoglobulins, B cells express membrane-bound forms of immunoglobulin called B-cell receptors (BCRs) that serve as the antigen-specific binding site. During diff erentiation, the BCR isotope switches from immunoglobulin M (IgM) in immature B cells to immu noglobulin D (IgD) in mature B cells. B cells also express the major histocompatibility complex II (MHC II) molecules on the cell surface. Their CD markers are CD9, CD19, and CD20.
tion in the primary lymphatic organs.
In humans and other mammals, the bone marrow and GALT region (together called the bursa-equivalent organ) and the thy mus have been identified as primary (central) lymphatic organs. Lymphocytes differentiate into immunocompetent cells in these organs. Initially, lymphocytes are genetically programmed to recognize a single antigen out of virtually an infinite number of possible antigens, a process called
These immunocompetent cells then enter the blood or lymph antigen-independent proliferation and differentiation.
region of IgG, IgM carboxy terminus
FIGURE 14.2 • Schematic diagram of an antibody molecule.
Antibodies are Y-shaped molecules produced by plasma cells. They consist of two heavy (H) and two light (L) polypeptide chains connected by disulfide bonds (SOS). Both H and L chains are composed of domains of amino acids that are constant (at the carboxy terminus) or variable (at the amino terminus) in their sequence. The five different immunoglobulin (Ig) isotypes (see Table 14.2) are determined by the type of heavy chain present. An antibody molecule binds an antigen (Ag) at the two sites of the amino terminus, where the heavy and light chains are associated with each other. Digestion of an antibody molecule by the proteolytic enzyme papain cleaves the antibody into two Fab fragments and one crystallizable Fc fragment. The Fab fragments impart the specific antigen binding, whereas the Fc fragment, which is composed of two carboxy-terminus heavy-chain segments (CH2 and CH3), fulfills the effector functions (e.g., in complement activation). Many cells express Fc receptors on their surfaces, which anchor antibodies at the Fc fragment.
Natural killer lymphocytes (NK cells) are neither T nor B cells and are specialized to kill certain types of target cells.