The priming of helper T cells.

Before helper T cells and B cells can interact to produce specific antibody, the antigen-specific helper T cell precursors must be primed. Priming is a fairly abrupt event which occurs in vivo about 5 days after immunization. Priming involves several steps: Antigen uptake, processing, and cell surface expression bound to class II MHC molecules by an antigen presenting cell, recirculation and antigen-specific trapping of helper T cell precursors in lymphoid tissue, and T cell proliferation and differentiation. Helper T cells express CD4, but not all CD4 T cells are helper cells. The signals required for clonal expansion of helper T cells differ from those required by other CD4 T cells. The critical antigen-presenting cell for helper T cell priming appears to be a macrophage, and the critical second signal for helper T cell growth is the macrophage product interleukin-1.