Fig. 1.

Diagram showing antigen presentation by MHC II molecules. (1) An antigen enters intracellular vesicles. (2) Acidification of vesicles activates proteases that hydrolyze the antigen into peptide fragments. (3) Vesicles containing the peptide fragments merge with vesicles containing MHC II molecules (green). (4) The invariant chain (Ii) (violet) binds to the newly synthesized MHC II molecules and partially occupies the peptide-binding groove. (5) The invariant chain undergoes proteolytic degradation; as a result, the CLIP peptide (blue) remains bound in the groove. (6) DM (orange) binds to the MHC II molecules and catalyzes the peptide exchange. (7) The MHC II molecules, loaded with an antigenic peptide (red), are transported to the cell surface where they can be recognized by a CD4 + T cell receptor TCR (cyan blue). The CD4 co-receptor molecule (brown) present on T cells also binds to the MHC II molecules. For T-cell activation to occur, the CD80 or CD86 co-stimulating molecules (pink) expressed on the antigen-presenting cell must bind to the CD28 co-stimulating molecule (beige) expressed on the T cells