Fig. 8. Antigen presentation: major histocompatibility complex (MHC) class I. Antigen-presenting target cells, especially tumors or viral infected cells bearing MHC class I determinants (α chain and β2-microglobulin), present antigen to the CD8+ cytotoxic T cells through the T-cell receptor (TCR) complex. Binding of the Ag-MHC complex to the TCR leads to signal transduction via the CD3 complex. The CD8 molecule also may act as an accessory signal to enhance T-cell activation through tyrosine protein kinase (TPK p56) phosphorylation of the Þgz chain of the CD3 complex. (After Medical Knowledge Self-Assessment Program [MKSAP]: Allergy and Immunology, Book 1, p 161. Philadelphia: American College of Physicians [ACP], 1993, Fig. 28.)