Definition
Major Histocompatibility Complex (MHC) molecules are cell-surface proteins that present antigenic peptides to T cells.
They play a crucial role in T-cell recognition and activation, which is essential for immune responses.
Types of MHC
A) MHC Class I
Found on all nucleated cells.
Composed of:
A heavy (α) chain with three domains (α1, α2, α3)
β2-microglobulin (a non-MHC-encoded component)
Function:
Presents endogenous antigens (from inside the cell, e.g., viral peptides) to CD8+ Cytotoxic T cells.
The peptide-binding groove is formed by α1 and α2 domains.
B) MHC Class II
Expressed primarily on professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells.
Composed of:
Two polypeptide chains: α and β (both encoded in the MHC).
Function:
Presents exogenous antigens (from outside the cell, e.g., extracellular microbes that have been phagocytosed) to CD4+ Helper T cells.
The peptide-binding groove is formed by α1 and β1 domains.
Function of MHC
Self vs. Non-self Recognition: T cells detect infected or altered cells.
T-cell Activation: Antigens must be presented on MHC molecules to trigger an immune response.
Transplantation: Mismatched MHC molecules can cause transplant rejection.
