APCs

Antigen-presenting cells (APCs) are specialized cells that internalize, process, and display antigens on MHC molecules to activate T lymphocytes. The three main professional APCs are: (1) Dendritic cells—the most efficient APCs, constitutively express high levels of MHC II and co-stimulatory molecules (B7-1, B7-2), excellent at priming naive T cells; found in peripheral tissues and lymph nodes; (2) Macrophages—express MHC II after activation by IFN-gamma, kill pathogens via NADPH oxidase and nitric oxide, produce IL-12 and TNF-alpha to promote Th1 differentiation; (3) B cells—express MHC II and co-stimulatory molecules, particularly when activated, and present antigen specifically to cognate CD4+ helper T cells to receive help for antibody production. APC function includes: (1) pathogen uptake (phagocytosis or receptor-mediated endocytosis); (2) lysosomal degradation to peptide fragments; (3) loading peptides onto MHC molecules in endosomal compartments; (4) transport to cell surface; (5) T cell activation via TCR-MHC interaction + co-stimulation (CD28-B7). Non-professional cells can present MHC II under inflammatory conditions (e.g., endothelial cells, fibroblasts express MHC II when stimulated by IFN-gamma). Absence of co-stimulation leads to T cell anergy or regulatory T cell differentiation. Understanding APC biology is critical for immunology board questions.