Q.27 Class II MHC molecules are NOT expressed by
(A) B–cells
(B) dendritic cells
(C) macrophages
(D) T–cells
Class II MHC molecules are not expressed by T-cells under normal resting conditions, making (D) the correct answer in this immunology question relevant to CSIR NET Life Sciences exam preparation.
Option Analysis
A. B-cells: These professional antigen-presenting cells (APCs) constitutively express MHC class II molecules on their surface to present exogenous antigens to CD4+ T cells.
B. Dendritic cells: As the most potent APCs, dendritic cells express high levels of MHC class II, enabling efficient antigen processing and presentation during immune activation.
C. Macrophages: These phagocytic APCs display MHC class II molecules after antigen uptake and processing in endosomal compartments for CD4+ T cell stimulation.
D. T-cells: Resting T cells do not express MHC class II; expression occurs only upon activation in humans (not typically in mice), but standard textbook knowledge for exams considers T cells as non-expressors.
Class II MHC molecules NOT expressed by T-cells forms a key concept in immunology for CSIR NET Life Sciences aspirants. Understanding which cells express MHC class II helps differentiate professional antigen-presenting cells (APCs) from effector cells like T lymphocytes. This article breaks down the MCQ with precise explanations.
Core Concept
MHC class II molecules present exogenous peptides to CD4+ helper T cells and appear constitutively on APCs such as B-cells, dendritic cells, and macrophages. T-cells lack this expression in their resting state, preventing self-presentation loops.
Cell-Wise Breakdown
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B-cells: Express MHC II via CIITA transactivator for humoral immunity support.
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Dendritic cells: Highest MHC II density among APCs for naive T cell priming.
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Macrophages: Induce MHC II post-phagocytosis for inflammation coordination.
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T-cells: No baseline expression; activated human T cells may show low levels, but not standard for exams.
Exam Relevance
For CSIR NET, focus on classical APC definition excluding T-cells. Exceptions like activated T cell expression are advanced and species-specific (human vs mouse).
