There are two major types of lymphocytes in the vertebrate immune system, the B lymphocytes (B cells) and T lymphocytes (T cells). These express distinct types of antigen receptors and have quite different roles in the immune system, as was discovered in the 1960s. Most lymphocytes circulating in the body appear as rather unimpressive small cells with few cytoplasmic organelles and a condensed, inactive-appearing nuclear chromatin (Fig. 1.12). Lymphocytes manifest little functional activity until they encounter a specific antigen that interacts with an antigen receptor on their cell surface. Lymphocytes that have not yet been activated by antigen are known as naive lymphocytes; those that have met their antigen, become activated, and have differentiated further into fully functional lymphocytes are known as effector lymphocytes.
B cells and T cells are distinguished by the structure of the antigen receptor that they express. The B-cell antigen receptor, or B-cell receptor (BCR), is formed by the same genes that encode antibodies, a class of proteins also known as immunoglobulins (Ig) (Fig. 1.13). Thus, the antigen receptor of B lymphocytes is also known as membrane immunoglobulin (mIg) or surface immunoglobulin (sIg). The T-cell antigen receptor, or T-cell receptor (TCR), is related to the immunoglobulins but is quite distinct in its structure and recognition properties.
After antigen binds to a B-cell antigen receptor, or B-cell receptor (BCR), the B cell will proliferate and differentiate into plasma cells. These are the effector form of B lymphocytes, and they secrete antibodies that have the same antigen specificity as the plasma cell’s B-cell receptor. Thus the antigen that activates a given B cell becomes the target of the antibodies produced by that B cell’s progeny.
Principles of adaptive immunity.
Fig. 1.13 Schematic structure of antigen receptors. upper panel: an antibody molecule, which is secreted by activated B cells as an antigen-binding effector molecule. A membrane-bound version of this molecule acts as the B-cell antigen receptor (not shown). An antibody is composed of two identical heavy chains (green) and two identical light chains (yellow). each chain has a constant part (shaded blue) and a variable part (shaded red). each arm of the antibody molecule is formed by a light chain and a heavy chain, with the variable parts of the two chains coming together to create a variable region that contains the antigen-binding site. the stem is formed from the constant parts of the heavy chains and takes a limited number of forms. this constant region is involved in the elimination of the bound antigen. lower panel: a t-cell antigen receptor. this is also composed of two chains, an α chain (yellow) and a β chain (green), each of which has a variable and a constant part. As with the antibody molecule, the variable parts of the two chains create a variable region, which forms the antigen-binding site. the t-cell receptor is not produced in a secreted form.
When a T cell first encounters an antigen that its receptor can bind, it proliferates and differentiates into one of several different functional types of effector T lymphocytes. When effector T cells subsequently detect antigen, they can manifest three broad classes of activity. Cytotoxic T cells kill other cells that are infected with viruses or other intracellular pathogens bearing the antigen. Helper T cells provide signals, often in the form of specific cytokines that activate the functions of other cells, such as B-cell production of antibody and macrophage killing of engulfed pathogens. Regulatory T cells suppress the activity of other lymphocytes and help to limit the possible damage of immune responses. We discuss the detailed functions of cytotoxic, helper, and regulatory T cells in Chapters 9, 11, 12, and 15.