Immunologic synapse

APCs are illustrated as presenting a peptide bound to human leukocyte antigen (HLA) forming an "immunologic synapse" with the T cell immunoreceptor. CD8-positive T cells bind to MHC class I APCs, and CD4-positive T cells bind to MHC class II-positive APCs. Notice the presence of secondary messenger systems such as the CD28 and B7 molecules (CD80/B7-1 and CD86/B7-2), which also come into apposition to allow a T cell response. Local adhesion molecules are thought to help stabilize the synapse.