THE IMMUNE-RESPONSE - THE EFFERENT ARM

Once naive T cells encounter antigen, they become primed effector cell s. The scope of effector functions mediated by these cells defines the efferent arm of the immune response. The change from naive to primed effector cell is known as adaptive immunity and takes 2 forms: cell me diated, in which T cells mediate effector function, and humoral, in wh ich antibodies are the effector molecules. There are 3 types of effect or T cells: inflammatory CD4 T cells, which activate macrophages; help er CD4 T cells, which help B lymphocytes produce antibody; and cytotox ic CDS T cells, which kill their target cells. The interaction of prim ed effector cells with their targets results in phenotypic changes in the cells and the secretion of cytokines. These cytokines may be secre ted by the primed effector T cell, the target cell, or both. Cytokines function in either autocrine (secreted and used by the same cell) or paracrine (secreted by 1 cell and used by a different cell) circuits a nd have marked regulatory effects on cells in both the immune and skel etal systems. Many of these cytokines, which were once thought to be p roducts exclusively of immune cells, are now known to be produced by c ells of the skeletal system. Both the specific and nonspecific compone nts of the immune response have profound effects on remodeling of the musculoskeletal system during normal and pathologic states.