Figure 1. Antigen presentation to T lymphocytes. (a) CD8+ cytotoxic T cells recognize antigenic peptides presented by class I major histocompatibility complex (MHC) molecules. (b) CD4+ helper T cells recognize antigenic peptides presented by class II MHC molecules. (c) Certain T-cell populations, including natural killer (NK) T cells and T cells, recognize lipid and glycolipid antigens presented by MHC-related molecules, CD1.

Figure 2. Processing of endogenous and exogenous proteins into peptides that are going to be loaded on to MHC molecules. Nearly 30% of newly synthesized proteins are for some reason misfolded. Even intact endogenous proteins are denatured by various sources of stress like heat shock. Those unfolded proteins are rapidly ubiquitinylated by ubiquitin-specific activating enzyme (E1), conjugating enzyme (E2) and ubiquitin ligase (E3). The polyubiquitinylated proteins are degraded by the proteasome into peptides some of which are loaded on to MHC class I molecules in the ER. On the other hand, internalized exogenous proteins or even endogenous proteins engulfed by autophagosome are transported into the endosome/MIIC compartment and degraded by acid proteases. The produced peptides are loaded on to MHC class II molecules that have been escorted by the invariant chain (Ii) to MIIC compartment.

Figure 3. Antigen presentation to B lymphocytes by follicular dendritic cells (FDCs) in a germinal centre. B cells undergo rapid proliferation accompanied by somatic hypermutation in their rearranged immunoglobulin genes, resulting in the generation of both antigen-specific high-affinity B cells and B cells that no longer recognize the antigen. High-affinity B cells that recognize antigens presented by FDCs survive and differentiate, with the help of T cells, into either memory or plasma (antibody-secreting) cells. In contrast, B cells that no longer recognize the antigen die through programmed cell death (apoptosis).