Model of CD4 T-cell autoregulation of an immune response. (1) At the onset of the immune response, naive (and also memory in the case of a secondary response, which is not represented here) CD4 T cells are recruited and stimulated by activated APCs displaying antigen. (2) During activation, these CD4 T cells capture the specific MHC-peptide complexes bound by their TCRs. (3) At later time points, the number of activated CD4 T cells that have captured MHC-peptide complexes increases. As these activated Ag-bearing T cells begin to outnumber the APCs, there is increasing probability that newly arriving T cells (and any T cells that have just been activated and need a second hit to continue to divide) will encounter them before encountering a proper professional APC. (4) This T-T interaction leads to inhibition of the Ag-experienced T cells, whereas the normal interaction of naive cells with the professional APCs leads to proliferation.