Q.59 Given below are two statements :
Statement I : Major Histocompatibility Complex (MHC) is present on the cell surface of T-cells for antigen
recognition.
Statement II : During organ transplantation, the major Histocompatibility Complex (MHC) is involved in
allograft rejection.
In the light of the above statements, choose the correct answer from the options given below :
1. Both Statement I and Statement II are true
2. Both Statement I and Statement II are false
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
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Correct Answer
Option 4: Statement I is false but Statement II is true
Why This Answer?
Statement I Analysis: False
Claim: “Major Histocompatibility Complex (MHC) is present on the cell surface of T-cells for antigen recognition.”
Why False?
MHC molecules are NOT primarily expressed on T-cells for antigen recognition. Instead:
-
MHC Class I: Present on all nucleated cells (not just T-cells)
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MHC Class II: Present on antigen-presenting cells (APCs) like dendritic cells, macrophages, B-cells
-
T-cells express T-cell receptors (TCRs), NOT MHC molecules for antigen recognition
Correct Process:
Antigen-Presenting Cell (MHC+) → Presents antigen → T-cell (TCR+) recognizes
T-cells are the recognizers, not the MHC presenters.
Statement II Analysis: True
Claim: “During organ transplantation, MHC is involved in allograft rejection.”
Why True?
MHC mismatch is the primary cause of acute transplant rejection:
Donor organ MHC ≠ Recipient T-cells → Recognize as foreign → Cytotoxic response → Rejection
Mechanism:
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Recipient T-cells recognize foreign MHC on donor organ
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CD8+ T-cells → Direct killing of graft cells
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CD4+ T-cells → Activate immune response
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Results in graft rejection within days-weeks
Complete Options Analysis
| Option | Statement I | Statement II | Correct? | Reason |
|---|---|---|---|---|
| 1 | True | True | ❌ | Statement I false |
| 2 | False | False | ❌ | Statement II true |
| 3 | True | False | ❌ | Both wrong |
| 4 | False | True | ✅ | Correct Answer |
Key Immunology Concepts for Exams
MHC Distribution Quick Reference
MHC Class I: All nucleated cells → CD8+ T-cells
MHC Class II: APCs only → CD4+ T-cells
T-cells: TCRs (no MHC expression)
Transplant Rejection Types
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Hyperacute: Pre-formed antibodies
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Acute: MHC-mediated T-cell response (most common)
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Chronic: Long-term fibrosis
Memory Trick for Exams
“T-cells have TCRs, not MHCs—APCs do the MHC job!”
Pro tip: In MCQs, if they say “MHC on T-cells,” it’s always false.
