The immune response: the efferent arm.

Once naive T cells encounter antigen, they become primed effector cells. 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 mediated, in which T cells mediate effector function, and humoral, in which antibodies are the effector molecules. There are 3 types of effector T cells: inflammatory CD4 T cells, which activate macrophages; helper CD4 T cells, which help B lymphocytes produce antibody; and cytotoxic CD8 T cells, which kill their target cells. The interaction of primed effector cells with their targets results in phenotypic changes in the cells and the secretion of cytokines. These cytokines may be secreted 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 and have marked regulatory effects on cells in both the immune and skeletal systems. Many of these cytokines, which were once thought to be products exclusively of immune cells, are now known to be produced by cells of the skeletal system. Both the specific and nonspecific components of the immune response have profound effects on remodeling of the musculoskeletal system during normal and pathologic states.