Britannica Sources used in this answer

How do major histocompatibility complex molecules help T cells recognize foreign antigens?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration of a blood vessel with a cutaway view showing red blood cells, white blood cells, platelets, and plasma, each labeled to highlight the main components of blood. — white blood cell Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and water-rich plasma.

Major histocompatibility complex (MHC) molecules are essential for the immune system because they allow T cells to detect foreign antigens that are otherwise inaccessible to circulating antibodies. Unlike B cells, T cells cannot bind to free-floating antigens; they must recognize foreign peptide fragments displayed on the surface of body cells.21

The process functions as follows:
* Antigen Processing: When a cell (such as a macrophage) ingests a microorganism or is infected by a virus, it partially digests the invader.21
* Presentation: The cell moves a fragment of the foreign protein to its surface, where it is bound to an MHC molecule.21
* Recognition: The T-cell receptor binds to this specific complex of the foreign peptide and the MHC molecule.23
* Coreceptor Support: To strengthen this interaction, T cells use coreceptors: CD4 binds to class II MHC molecules (typically on macrophages or B cells), while CD8 binds to class I MHC molecules (found on most nucleated cells).1

This mechanism ensures that T cells only respond to antigens presented in the context of "self" MHC molecules, triggering an appropriate immune response.21