APCs

Antigen-presenting cells (APCs) are immune cells that capture, process, and present protein fragments (antigens) to T lymphocytes, bridging innate and adaptive immunity. By displaying peptides on major histocompatibility complex (MHC) molecules and delivering co-stimulatory signals, APCs determine whether T cells become activated, tolerant, or anergic. Professional APCs include dendritic cells, macrophages, and B cells; they are specialized for efficient antigen presentation and can initiate primary T cell responses. Other cells can present antigen at lower levels but generally lack the robust co-stimulatory capacity of professional APCs.

Dendritic cells are the most potent APCs, continually sampling tissues and migrating to draining lymph nodes when activated. They express high levels of MHC class II and costimulatory molecules (e.g., CD80/CD86, CD40) and secrete cytokines that shape T cell differentiation. Macrophages phagocytose and process antigens, present peptides on MHC II, and provide inflammatory cues to sustain T cell responses. B cells present antigen to CD4+ T helper cells via MHC II, linking antibody production to T cell help, and can present in germinal centers during humoral responses.

Antigen processing follows two main pathways. Exogenous antigens derived from outside the cell are endocytosed and loaded onto MHC class II molecules for presentation to CD4+ T cells. Endogenous antigens, derived from the cell's own proteins, are processed for MHC class I presentation to CD8+ cytotoxic T cells. Dendritic cells can cross-present extracellular antigens on MHC I, enabling CD8+ T cell activation against pathogens that do not infect APCs.

APCs are central to vaccination, tumor immunity, and transplant rejection. Defects or modulation of APC function influence infection susceptibility, vaccine efficacy, and the development of autoimmune diseases or immune tolerance.