MHC Class II molecules are expressed on the surface of:

AIIMS CRE, SNO-2024

Applied Microbiology & Infection control

Easy

Appeared in: AIIMS CRE, SNO-2024

Explanation

Major Histocompatibility Complex (MHC) Class II molecules are specifically expressed on the surface of professional Antigen-Presenting Cells (APCs).
Macrophages and B cells are primary examples of APCs.
The function of MHC Class II on these cells is to present fragments of extracellular pathogens (exogenous antigens) to CD4+ helper T-lymphocytes.
This interaction is essential for initiating and coordinating the adaptive immune response, including the activation of B cells to produce antibodies.

Option A: This describes the expression pattern of MHC Class I molecules, not Class II. MHC Class I molecules are present on all nucleated cells to display internal proteins.
Option C: T cells are the cells that recognize antigens presented by MHC molecules via their T-cell receptors. They are the responders, not the primary presenters of exogenous antigens via MHC Class II.
Option D: Mature red blood cells are anucleated, meaning they lack a nucleus and other organelles. Consequently, they do not express any MHC molecules (neither Class I nor Class II).

Visual 1: Diagram: An Antigen-Presenting Cell (e.g., macrophage) with an MHC Class II molecule on its surface presenting a peptide antigen to the T-cell receptor of a CD4+ Helper T-cell. This visualizes the core interaction.
Visual 2: Table: A side-by-side comparison of MHC Class I and MHC Class II, detailing their cellular location, antigen source, the type of T-cell they interact with, and their primary function in the immune system.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Cellular expression of MHC Class II molecules as background academic context rather than a clinical decision trigger.
Understanding the difference between MHC Class I and II is fundamental to nursing knowledge of immunology, including vaccine mechanisms, autoimmune disorders, and transplant medicine.
In transplant rejection, the recipient's T-cells recognize the donor's MHC molecules (called HLA in humans) as foreign, leading to an immune attack on the transplanted organ.
In autoimmune diseases like rheumatoid arthritis or Type 1 diabetes, the immune system mistakenly attacks self-antigens presented by MHC molecules on the body's own cells.

How to Approach the Question

First, identify the key term in the question: 'MHC Class II molecules'.
Recall the basic division in the MHC system: Class I and Class II have different locations and functions.
Associate MHC Class I with 'self-surveillance' on nearly all body cells (all nucleated cells). Its job is to show what's happening inside the cell.
Associate MHC Class II with 'professional antigen presentation' by a specialized group of immune cells (APCs). Its job is to show what's been captured from outside the cell.
Scan the options to find the one that lists professional APCs. Macrophages and B cells are classic examples.
Eliminate the other options based on this core knowledge: 'All nucleated cells' is for MHC Class I, 'T cells' are primarily responders, and 'Red blood cells' lack MHC molecules entirely.

Concept Tested & Keywords

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