Which of the following statements is false regarding MHC molecules?

Explanation

## Correct Answer: B. MHC Class II presents endogenous antigens MHC Class II molecules present **exogenous antigens**, not endogenous antigens—this is the fundamental discriminator. MHC Class II is expressed on antigen-presenting cells (APCs: dendritic cells, macrophages, B cells) and processes antigens via the **exocytic/endocytic pathway**. Extracellular pathogens (bacteria, toxins, parasites) are phagocytosed, degraded in endosomes/lysosomes, and peptide fragments are loaded onto MHC II in specialized compartments (MIICs). These peptide–MHC II complexes then traffic to the cell surface and present to CD4+ T helper cells via TCR recognition. In contrast, MHC Class I presents **endogenous antigens** (viral proteins, tumor antigens, intracellular pathogens) processed via the proteasomal pathway in the cytosol. This distinction is critical in Indian immunology curricula (Jawetz, Robbins) and underpins vaccine design and immune response to infections common in India (TB, dengue, malaria). The question tests whether students confuse the antigen source for each MHC class—a classic NBE trap. ## Why the other options are wrong **A. MHC II is present on antigen presenting cells** — This statement is **true**. MHC Class II is constitutively expressed on professional APCs: dendritic cells, macrophages, and B lymphocytes. In Indian clinical practice, this is why dendritic cells are the most potent APCs for initiating CD4+ T cell responses in infections like TB and leprosy. This is a correct statement, not the false one. **C. MHC class II presents peptide antigen to CD4 T cells** — This statement is **true**. MHC Class II–peptide complexes are recognized by CD4+ T helper cells via their TCR and CD4 co-receptor. This interaction is essential for humoral immunity and delayed-type hypersensitivity responses seen in Indian patients with TB and leprosy. The statement correctly describes the functional outcome of MHC II presentation. **D. The action of MHC II occurs via endocytic pathway** — This statement is **true**. MHC Class II antigen processing occurs via the **endocytic/exocytic pathway**: extracellular antigens are endocytosed, processed in endosomes and lysosomes, and peptides are loaded onto MHC II in MIICs (MHC Class II compartments). This is distinct from the proteasomal pathway used by MHC Class I, and is a high-yield distinction in immunology. ## High-Yield Facts - **MHC Class II presents exogenous antigens** (not endogenous); these are extracellular pathogens processed via endocytic pathway. - **MHC Class I presents endogenous antigens** (viral proteins, tumor antigens); processed via proteasomal pathway in cytosol. - **MHC II is expressed on APCs** (dendritic cells, macrophages, B cells) and presents to CD4+ T helper cells. - **MHC II peptide loading occurs in MIICs** (MHC Class II compartments), specialized endosomal vesicles. - **Exogenous antigen pathway** (MHC II) is critical for responses to extracellular bacteria (TB, pneumococcus) and parasites common in India. ## Mnemonics **MHC I vs II: **E**xo vs **E**ndo** MHC **II** = **E**xogenous (outside) + **E**ndocytic pathway. MHC **I** = **E**ndogenous (inside) + proteasomal pathway. The double-E for Class II helps recall both the antigen source and processing route. **APC Rule: **II** for APCs** MHC **II** is on **A**ntigen **P**resenting **C**ells. The Roman numeral II matches the concept that APCs are the 'second' cells in the immune response (after innate recognition). Use this when identifying which cells express MHC II. ## NBE Trap NBE pairs "MHC II" with "endogenous antigens" to exploit students who memorize that MHC II is on APCs and processes antigens, but forget the critical distinction that MHC II specifically handles **exogenous** (extracellular) antigens, not endogenous ones. The trap conflates "MHC II processes antigens" with "MHC II processes all types of antigens." ## Clinical Pearl In Indian TB patients, dendritic cells (MHC II+) phagocytose *Mycobacterium tuberculosis* and present exogenous mycobacterial antigens to CD4+ T cells, driving Th1 responses and granuloma formation. This is why BCG vaccination (which primes CD4+ responses via exogenous antigen presentation) is part of the Indian immunization schedule—the distinction between MHC I and II antigen sources directly impacts vaccine design and immune protection strategies. _Reference: Jawetz Melnick & Adelberg's Medical Microbiology Ch. 8 (Immunology); Robbins Pathologic Basis of Disease Ch. 6 (Immune System)_