CELL INTERACTIONS IN CELL MEDIATED IMMUNITY SEE APPENDIX (11) MIXED LYMPHOCYTE REACTION The diverse manifestations of CELL-MEDIATED IMMUNITY

The diverse manifestations of CELL-MEDIATED IMMUNITY (CMI) include several mechanisms by which target cells may be killed. The induction of one of these mechanisms, namely killing by CYTOTOXIC T-CELLS, involves the participation of two distinct, antigenically precommitted lymphocytes (THand TC -cellCELLS) as well as non-specific antigen-presenting cells (APCs, typified by DENDRITIC CELLS and MACROPHAGES). TH cells can be subdivided into two major sub-types, TH1 cells which promote inflammatory reactions and CMI, and TH2 cells which largely provide "help" to B-cells in generating humoral immunity. T-cell receptors recognize antigenic peptides only when they are associated with MHC molecules, which accounts for the phenomenon of MHC-RESTRICTED RECOGNITION. The existence of two DISTINCT CELLULAR PATHWAYS by which CLASS I and CLASS II molecules become associated with antigenic peptides is described. Resistance to the intracellular bacterium LISTERIA serves to illustrate a distinct mode of lymphoid cell interactions and cell-mediated effector functions which does not depend on TC cells.