Question
Antigen presenting cells.
Answer
Antigen-presenting cells (APCs) are specialized cells that capture, process, and display antigenic peptides bound to MHC molecules on their surface, for recognition by T lymphocytes — the essential link between innate detection of a pathogen and initiation of the adaptive immune response.
Professional APCs (express both MHC class II and co-stimulatory molecules, essential for fully activating naive T cells):
- Dendritic cells — the most potent and important APC for initiating primary T-cell responses; capture antigen in peripheral tissues, migrate to lymph nodes, and present it to naive T cells.
- Macrophages — present antigen after phagocytosing pathogens, particularly important in re-stimulating previously activated (memory/effector) T cells at sites of infection.
- B lymphocytes — internalize antigen bound by their surface immunoglobulin (BCR) and present it to helper T cells, a crucial step for T-cell-dependent antibody responses (allowing B cells to receive T-cell help for class switching and affinity maturation).
Mechanism: APCs process antigen (via phagocytosis/endocytosis and intracellular proteolytic degradation) and load the resulting peptide fragments onto MHC class II molecules, which are then recognized by CD4+ T-helper cells; (nucleated cells generally, including non-professional APCs, also present endogenously synthesized peptides on MHC class I to CD8+ cytotoxic T cells, though this is distinct from the professional-APC MHC class II pathway).
Co-stimulation: full T-cell activation requires not just MHC-peptide/TCR engagement (signal 1) but also a co-stimulatory signal (signal 2, e.g., B7-CD28 interaction) provided by professional APCs — antigen presentation without adequate co-stimulation can instead induce T-cell anergy, an important mechanism of peripheral tolerance.