MHC Structure and Antigen Presentation

Exam Tip: The Invariant Chain To prevent MHC II molecules from accidentally binding endogenous peptides while they are still in the RER, a protein called the invariant chain occupies the binding groove. The invariant chain is only released in the acidified endosome, allowing the MHC II to bind the intended exogenous antigen.

Structural Comparison of MHC I and MHC II

While both molecules serve to present peptides, their molecular architectures are distinct, reflecting their different roles in the immune response.

MHC Class I Structure

MHC I is characterized by a single long $\alpha$ chain that spans the cell membrane, which pairs with a smaller, non-transmembrane protein called $\beta_{2}$-microglobulin.

The $\alpha$ chain forms the peptide-binding groove. Because MHC I is expressed on all nucleated cells (except red blood cells), it allows the immune system to monitor the internal health of every cell in the body.

MHC Class II Structure

MHC II consists of two roughly equal-length chains: an $\alpha$ chain and a $\beta$ chain. Both chains span the cell membrane. Unlike MHC I, MHC II is primarily expressed only on specialized Antigen-Presenting Cells (APCs), such as dendritic cells, macrophages, and B cells.

Functional Outcomes: The "Rule of 8"

The most critical functional difference lies in which type of T cell recognizes the MHC-antigen complex. This relationship is governed by a mathematical principle known as the Rule of 8.

CD8+ Cytotoxic T Cells (MHC I)

MHC I molecules present endogenous antigens to CD8+ cytotoxic T cells. When a CD8+ T cell recognizes a foreign peptide on MHC I, it is activated to directly kill the presenting cell using perforin and granzymes, a process essential for eliminating virus-infected or tumor cells.

The Rule of 8 for this interaction is: $\text{MHC I} \times \text{CD8} = 8$

CD4+ Helper T Cells (MHC II)

MHC II molecules present exogenous antigens to CD4+ helper T cells. Once activated, CD4+ T cells act as "conductors" of the immune response, secreting cytokines to recruit phagocytes and helping B cells differentiate into antibody-producing plasma cells.

The Rule of 8 for this interaction is: $\text{MHC II} \times \text{CD4} = 8$

Summary Table: MHC Comparison

Clinical Relevance: HLA and Disease

In humans, the MHC is also known as the Human Leukocyte Antigen (HLA) system. Specific HLA subtypes are strongly associated with various autoimmune and inflammatory diseases. For example, HLA-B27 is linked to the "PAIR" mnemonic: Psoriatic arthritis, Ankylosing spondylitis, IBD-associated arthritis, and Reactive arthritis. Similarly, HLA-DQ2 and DQ8 are major risk factors for Celiac disease. Understanding these associations is vital for both diagnosis and understanding the genetic basis of immune-mediated pathology.