Final answer:
Antigen-presenting cells display processed antigens to T cells through Major Histocompatibility Complex (MHC) molecules, which come in two forms: MHC class I and MHC class II. MHC class I presents intracellular antigens and is found on all nucleated cells, while MHC class II presents extracellular antigens and is exclusive to antigen-presenting cells.
Step-by-step explanation:
Antigen-presenting cells, such as dendritic cells, macrophages, and B cells, process antigens and present them on their surface through Major Histocompatibility Complex (MHC) molecules for T cell recognition. Antigens internalized by these cells are broken down into smaller fragments through enzymatic cleavage, known as antigen processing. These fragments are then presented on the cell surface bound to MHC molecules. There are two primary 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 intracellular antigens, typically from viruses or other intracellular pathogens. Conversely, MHC class II molecules are exclusively found on antigen-presenting cells and are key for presenting extracellular antigens to helper T cells. The combination of an antigen fragment and an MHC molecule is what is recognized by the T cell receptor, initiating the immune response.