B Cell Activation: Three Steps and Their Variations Review

* Base1 Institute for Immunology Basel, Switzerland +Biomedicum University of Uppsala Uppsala, Sweden When the immune system recognizes foreign antigens three different cell types collaborate in the induction of antibody formation. Only one of them, the B lymphocyte, produces antibody. The other two cells, which do not produce antibodies but cooperate in their production, are helper T lymphocytes and macrophages or accessory (A) cells. B cells express immunoglobulin (Ig), most of them IgM, on their surface membrane. Each B cell produces only one particular structure of the variable (V) regions of the heavy (H) and light (L) chains of Ig that have been generated by unique somatic rearrangements of the gene segments coding for VH and VL (Hozumi and Tonegawa, 1976; Sakano et al., 1980). B cells are different from each other, since each has experienced a unique set of V-gene rearrangements. T cells also are different from each other, as each one of them produces a unique antigen-specific receptor. The structures of these antigen-specific recep- tors are being discovered (Acuto et al., 1983; Kappler et al., 1983; McIntyre and Allison, 1983; Hedrick et al., 1984a, 1984b; Yanagi et al., 1984). When antigen enters the system it is taken up by A cells, reappears on the surface of these A cells, and presents itself to helper T cells together with surface membrane proteins called histocompatibility antigens. Most frequently these histocompatibility antigens are the la-antigens (class II antigens) encoded in the l-region of the major histocompatibility complex (Schwartz et al., 1978).