Antigénpresentáló

Antigénpresentáló sejtek, often abbreviated as APCs, are a crucial component of the adaptive immune system. Their primary function is to recognize, engulf, and process foreign antigens, such as those found on bacteria, viruses, or other pathogens, and then present fragments of these antigens on their surface to T cells. This presentation is essential for initiating an adaptive immune response.

APCs encompass several distinct cell types, including dendritic cells, macrophages, and B cells. Dendritic cells are considered the most potent APCs and are strategically located in tissues that are likely to encounter pathogens, such as the skin and mucosal linings. Macrophages are also phagocytic cells that can engulf and digest pathogens, then present antigens. B cells, while primarily known for antibody production, can also act as APCs, particularly when encountering antigens that bind to their B cell receptors.

The process of antigen presentation involves several steps. APCs engulf or endocytose the antigen, breaking it down into smaller peptide fragments within specialized cellular compartments. These fragments are then loaded onto molecules called Major Histocompatibility Complex (MHC) proteins. There are two main classes of MHC molecules: MHC class I and MHC class II. Antigens derived from intracellular pathogens or abnormal self-proteins are typically presented on MHC class I molecules, while antigens from extracellular pathogens are presented on MHC class II molecules. The MHC-antigen complex is then transported to the APC's surface and displayed to T cells. Naive T cells, which have not yet encountered their specific antigen, circulate throughout the body searching for APCs presenting their cognate antigen. When a T cell recognizes the presented antigen-MHC complex, it becomes activated, leading to the proliferation and differentiation of T cells, which then orchestrate the targeted immune response against the pathogen.