THE CHANGING PREFERENCE OF T-CELL AND B-CELL FOR PARTNERS AS T-DEPENDENT ANTIBODY-RESPONSES DEVELOP

Recirculating virgin CD4(+) T cells spend their life migrating between the T zones of secondary lymphoid tissues where they screen the surfa ce of interdigitating dendritic cells. T-cell priming starts when proc essed peptides or superantigen associated with class II MHC molecules are recognised. Those primed T cells that remain within the lymphoid t issue move to the outer T zone, where they interact with B cells that have taken up and processed antigen. Cognate interaction between these cells initiates immunoglobulin (Ig) class switch-recombination and pr oliferation of both B and T cells; much of this growth occurs outside the T zones. B cells migrate to follicles, where they form germinal ce ntres, and to extrafollicular sites of B-cell growth, where they diffe rentiate into mainly short-lived plasma cells. T cells do not move to the extrafollicular foci, but to the follicles; there they proliferate and are subsequently involved in the selection of B cells that have m utated their Ig variable-region genes. During primary antibody respons es T-cell proliferation in follicles produces many times the peak numb er of T cells found in that site; a substantial proportion of the CD4( +) memory T-cell pool may originate from growth in follicles.