Antigénpresentáció

Antigénpresentáció, also known as antigen presentation, is a crucial process in the adaptive immune system. It is the mechanism by which immune cells, primarily antigen-presenting cells (APCs), display fragments of antigens on their surface to T lymphocytes. This presentation is essential for initiating an immune response against pathogens or abnormal cells.

The primary APCs are dendritic cells, macrophages, and B cells. These cells engulf foreign particles, such as bacteria or viruses, or self-antigens released from damaged cells. Inside the APC, these antigens are broken down into smaller peptide fragments through a process called antigen processing. These peptide fragments are then loaded onto specialized molecules called Major Histocompatibility Complex (MHC) molecules.

There are two main classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules are found on almost all nucleated cells and present endogenous antigens, typically those derived from intracellular pathogens like viruses or from tumor cells. When an infected or cancerous cell presents viral or tumor peptides on MHC class I, it signals cytotoxic T lymphocytes (CTLs), also known as CD8+ T cells, to kill the abnormal cell.

MHC class II molecules are primarily expressed on professional APCs. They present exogenous antigens, which are taken up from the extracellular environment, such as from bacteria or toxins. When APCs present these exogenous peptides on MHC class II, they activate helper T lymphocytes (Th cells), also known as CD4+ T cells. Helper T cells then orchestrate a broader immune response, which can include helping B cells produce antibodies or activating macrophages to destroy pathogens.

The interaction between the T cell receptor (TCR) on the T lymphocyte and the peptide-MHC complex on the APC is highly specific. This ensures that T cells are activated only against the appropriate antigen, thereby preventing autoimmune reactions and mounting effective defenses against foreign invaders.