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Adaptive Immunity Quiz
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Which of the following best describes the role of MHC class II molecules in the adaptive immune response?
- A They present extracellular antigen fragments to CD4+ T lymphocytes on antigen-presenting cells You picked ✓
- B They present intracellular antigen fragments to CD8+ T lymphocytes on all nucleated cells You picked ✕
- C They bind whole, unprocessed antigens on the cell surface You picked ✕
- D They are expressed on all body cells to detect viral infection You picked ✕
- E They directly activate B lymphocytes without T cell involvement You picked ✕
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Incorrect
The answer is A: They present extracellular antigen fragments to CD4+ T lymphocytes on antigen-presenting cells
Explanation
Why A is correct
See Antigen Presentation. Option A is correct. MHC class II molecules are mainly found on professional antigen-presenting cells: dendritic cells, macrophages and B cells. These cells take in material from outside, for example bacteria engulfed by phagocytosis (swallowing), and digest it into small fragments called peptides. The peptides are then loaded on to MHC class II molecules and shown on the cell surface to CD4+ T cells, also called helper T cells. CD4 acts as a co-receptor, steadying the contact between the T-cell receptor and the peptide-MHC complex. Together with signals called co-stimulation and cytokines, this activates the helper T cell, which then coordinates the rest of the immune response, including helping B cells make antibodies. The rule of eight is a quick way to hold this: class II times CD4 equals eight. Class II presentation is conventionally about material from outside the cell, whereas class I is about material made inside the cell.
- Location: MHC class II is mainly on dendritic cells, macrophages and B cells.
- Function: It shows peptides from material taken up from outside the cell to CD4+ helper T cells.
- Outcome: The activated helper T cell coordinates the response, including helping B cells make antibodies.
Why the other options are incorrect
- B. They present intracellular antigen fragments to CD8+ T lymphocytes on all nucleated cells: This describes MHC class I, which presents peptides made inside the cell to CD8 T cells and is found on almost all nucleated cells. It is not class II. Your answer
- C. They bind whole, unprocessed antigens on the cell surface: MHC molecules do not bind whole, unprocessed antigens. The antigen is first broken into small peptides, and only these fragments are held in the groove of the MHC molecule. Your answer
- D. They are expressed on all body cells to detect viral infection: This describes MHC class I, which is on nearly every nucleated cell and lets cytotoxic T cells detect viral infection. MHC class II is mainly on specialised antigen-presenting cells. Your answer
- E. They directly activate B lymphocytes without T cell involvement: Most B cells need help from CD4 T cells that have been activated by antigen presentation, so MHC class II does not activate B cells on its own. It is part of the helper pathway instead. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which combination correctly pairs an MHC class with the type of T lymphocyte that recognises it?
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2
Which T-helper response is most closely associated with promoting neutrophil recruitment after antigen presentation on MHC class II?
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