Final answer:
Antigen-presenting cells, such as macrophages, dendritic cells, and B cells, internalize antigens and present them on their surface via MHC molecules to T cells. This interaction is essential for the immune system to recognize and respond to foreign antigens. There are two types of MHC molecules involved in this process: MHC class I for intracellular pathogens and MHC class II for extracellular antigens presented by professional APCs.
Step-by-step explanation:
Antigen-Presenting Cells (APCs) are specialized cells such as macrophages, dendritic cells, and B cells that are capable of capturing antigens and presenting them on their surface to T cells. This process involves the internalization of antigens, which are then processed into smaller pieces and displayed on the cell surface associated with Major Histocompatibility Complex (MHC) molecules.
There are two types of MHC molecules involved in antigen presentation: MHC class I and MHC class II. MHC class I molecules are found on all nucleated cells and are responsible for presenting antigens derived from intracellular pathogens like viruses, signaling cytotoxic T cells to destroy infected cells.
In contrast, MHC class II molecules are expressed only on the surface of professional APCs and are essential for initiating the immune response by presenting extracellular antigenic fragments to helper T cells.
MHC molecules have a specific feature called a peptide-binding cleft where the processed antigen fragments sit. This is what is recognized by the T cell receptors (TCR) on T cells. The combination of the MHC molecule and the antigenic fragment forms a complex that is essential for T cells to recognize and mount an immune response against specific antigens.