Figure 1
From: Aging, immunosenescence and membrane rafts: the lipid connection
Signal 1 and 2 during T cell activation. Upon antigen presentation by professional antigen presenting cells (APC), multiple stimulatory molecules control full activation of T cells. Signal 1 is delivered by CD4/CD8 and T cell receptor (TCR)/major histocompatibility (MHC) interaction while signal 2 is delivered by CD28/B7 interaction (or other co-stimulator-receptor pairs) The two signals together are required to induce IL-2 production which will have autocrine and paracrine effects.