What is the Difference Between MHC I and II?

Major Histocompatibility Complex (MHC) proteins are essential for adaptive immunity and are responsible for presenting antigens to the immune system. There are two major classes of MHC proteins: MHC Class I and MHC Class II. The main differences between them are:

1. Structure and Composition: MHC Class I proteins are composed of a heavy chain (α chain) and a smaller non-covalently associated protein called β2-microglobulin, while MHC Class II proteins are composed of an α chain and a β chain.
2. Peptide-binding Groove: MHC Class I molecules typically bind peptides of 8-11 amino acids, most commonly of 9 amino acids, within a closed groove formed by their membrane-distal domains. In contrast, MHC Class II molecules typically bind peptides of 12-25 amino acids that extend beyond the ends of their open groove.
3. Cellular Distribution: MHC Class I proteins are present on almost all cells with a nucleus, while MHC Class II proteins have a more limited distribution and are typically found on dendritic cells, macrophages, and B cells.
4. Antigen Sources: MHC Class I proteins present antigens derived from endogenous sources, such as cytosolic proteasome, while MHC Class II proteins present antigens derived from exogenous sources, processed by lysosomal and endosomal proteases.
5. T Cell Recognition: MHC Class I proteins are recognized by the CD8+ receptor of cytotoxic T cells, while MHC Class II proteins are recognized by the CD4+ receptor of helper T cells.

In summary, MHC Class I and MHC Class II proteins differ in their structure, composition, cellular distribution, antigen sources, and T cell recognition. These differences enable them to present antigens to the immune system in distinct ways, contributing to the overall functioning of the immune response.

Comparative Table: MHC I vs II

MHC I and MHC II are two classes of molecules within the Major Histocompatibility Complex (MHC), which plays a crucial role in the immune system. Here is a table summarizing the differences between MHC I and MHC II:

The structural differences between MHC I and MHC II molecules result in different antigen processing and presentation pathways. MHC I presents endogenous antigens to cytotoxic T cells, while MHC II presents exogenous antigens to helper T cells.