- Antigen-presenting cells (APCs) are specialized immune cells that serve as critical intermediaries between the innate and adaptive immune responses. Their primary function is to capture, process, and present antigens – fragments of pathogens or foreign substances – to T cells (also known as lymphocytes), thereby initiating specific immune responses. Without APCs, T cells would be unable to effectively recognize and respond to pathogens.
- The process begins when APCs engulf pathogens or foreign substances through phagocytosis or endocytosis. These materials are broken down into smaller peptide fragments, which are then loaded onto major histocompatibility complex (MHC) molecules for display on the cell surface.
- MHC class I molecules present endogenous antigens to CD8+ cytotoxic T cells, while MHC class II molecules present exogenous antigens to CD4+ helper T cells. Additionally, APCs express co-stimulatory molecules like CD80 and CD86, which are essential for full T cell activation and prevention of immune tolerance.
- There are three main types of professional APCs: dendritic cells, macrophages, and B cells.
- Dendritic cells are considered the most potent APCs due to their exceptional ability to capture antigens and activate naive T cells. They are strategically located in tissues that contact the external environment, such as the skin and mucosal surfaces. Upon encountering antigens, dendritic cells mature and migrate to lymphoid tissues to present antigens to T cells.
- Macrophages, derived from monocytes, are found in virtually all tissues and excel at phagocytosis. Beyond engulfing and destroying pathogens, they process antigens for presentation to T cells and secrete cytokines that help regulate immune responses.
- B cells, while primarily known for antibody production, can also internalize specific antigens through their B cell receptors and present them to helper T cells, facilitating antibody production and immune memory.
- APCs also play a crucial role in immune regulation and homeostasis through cytokine secretion, which influences the type and magnitude of immune responses. They shape T cell differentiation into various subsets, including Th1, Th2, and regulatory T cells.
- Their ability to modulate immune responses makes them essential for both protective immunity against pathogens and the maintenance of immune tolerance.
- Dysfunction of APCs can lead to various immune disorders, including autoimmune diseases, chronic infections, and cancer.
