Paper I
Question
List antigen presenting cells and discuss their role in immune response.
Answer
Antigen-Presenting Cells (APCs) 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, needed for fully activating naive T cells):
- Dendritic cells — the most potent APC, crucial for initiating primary T-cell responses; capture antigen in peripheral tissues, migrate to lymph nodes, present it to naive T cells.
- Macrophages — present antigen after phagocytosing pathogens, particularly important in re-stimulating previously activated (memory/effector) T cells at infection sites.
- B lymphocytes — internalize antigen bound by their surface immunoglobulin (BCR) and present it to helper T cells, crucial for T-cell-dependent antibody responses.
Role in immune response:
- Antigen processing and presentation: APCs process antigen (via phagocytosis/endocytosis and intracellular proteolytic degradation) and load resulting peptides onto MHC class II molecules, recognized by CD4+ T-helper cells.
- Cross-presentation: dendritic cells can also present exogenously acquired antigen on MHC class I to CD8+ cytotoxic T cells (important for generating CTL responses against viruses/tumours that dendritic cells have not themselves been infected by).
- 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, a mechanism of peripheral tolerance.
- Cytokine secretion: APCs (particularly dendritic cells) secrete cytokines (e.g., IL-12) that direct the differentiation of activated T cells into specific functional subsets (Th1, Th2, Th17).
- Bridging innate and adaptive immunity: APCs, via pattern recognition receptors, sense the nature of the pathogen (PAMPs) and tailor the subsequent adaptive response accordingly, linking the two arms of the immune system.