Activation of T cells requires:

A. Stimulation of the antigen receptor.

C. Co-stimulation.

E. CD3.

DISCUSSION: The activation of T cells generally involves the delivery of two types of signals. One signal is initiated when the T-cell antigen receptor binds in a cognate manner to an MHC antigen bearing an antigenic peptide expressed on the surface of an antigen-presenting cell. This interaction is enhanced by the co-ordinate binding of CD4 or CD8 to the MHC antigen complex. This interaction initiates signaling through CD3, as well as through CD4 or CD8, both of which are associated with tyrosine kinases. Full activation of the T cell also requires the delivery of “co-stimulatory” signals. These signals may arise through the interaction of CD28 expressed by the T cell with B7-1 or B7-2 expressed on antigen-presenting cells. If only the T-cell antigen receptor is stimulated (and co-stimulation is not provided) the Tcell becomes anergic, that is, resistant to further stimulation. Anergy may be an important mechanism contributing to tolerance.