MHC II are presented by –
**Core Concept**
Major Histocompatibility Complex (MHC) II molecules play a crucial role in the immune system by presenting peptides from extracellular pathogens to CD4+ T cells, facilitating antigen recognition and immune response. This process is essential for initiating a specific immune response against foreign substances.
**Why the Correct Answer is Right**
MHC II molecules are expressed on the surface of antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. These APCs engulf and digest pathogens, processing their proteins into peptides that are then loaded onto MHC II molecules. The MHC II-peptide complex is then presented on the surface of the APC, allowing CD4+ T cells to recognize and bind to the complex via their T-cell receptor (TCR). This recognition event triggers the activation of CD4+ T cells, which then orchestrate the immune response against the invading pathogen.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because MHC II molecules are not typically expressed on the surface of epithelial cells or red blood cells.
* **Option B:** This option is incorrect because MHC I molecules are responsible for presenting endogenously synthesized peptides to CD8+ T cells, not extracellular pathogens.
* **Option C:** This option is incorrect because B cells primarily function as APCs in the absence of T cell help, but they do not exclusively present antigens via MHC II.
**Clinical Pearl / High-Yield Fact**
MHC II molecules are essential for initiating the adaptive immune response against extracellular pathogens. A deficiency in MHC II molecule expression or function can lead to impaired antigen presentation and increased susceptibility to infections.
**Correct Answer:** D. Antigen-presenting cells such as dendritic cells, macrophages, and B cells.