Before We Start
A number trick that's more than just a mnemonic
MHC class I and class II aren't interchangeable — they're expressed on completely different cell populations and read by completely different T cell subsets. Getting the pairing backwards is one of the most common errors in this entire topic.
💡 Memory Trick
MHC I: ALL nucleated cells → CD8+. MHC II: APCs only → CD4+. Memory: "1×8=8, 2×4=8" — both equations equal 8, letting you self-check the correct pairing under exam pressure.
The Key Points
Which cells express each class, and which T cell reads it
I
MHC I — on every nucleated cell, read by CD8+
MHC class I (HLA-A, B, C) is expressed on virtually every nucleated cell in the body — this is how the immune system checks what's happening inside a cell. It displays intracellular peptides: fragments of viral proteins if the cell is infected, or abnormal tumor proteins if the cell has become cancerous. CD8+ Cytotoxic T cells specifically read MHC I.
🦠 Red blood cells lack a nucleus, so they don't express MHC I at all — they're the one major exception to the "all nucleated cells" rule, worth remembering specifically because it's a common trick question.
II
MHC II — on professional APCs only, read by CD4+
MHC class II (HLA-DR, DP, DQ) is restricted to professional antigen-presenting cells: dendritic cells, macrophages, and B cells. These cells phagocytose material from outside the cell, then display fragments of that extracellular material on MHC II. CD4+ Helper T cells specifically read MHC II.
🦠 A macrophage that has engulfed a bacterium processes bacterial proteins and displays fragments of them on MHC II, which a CD4+ Helper T cell then recognizes to kick off a coordinated immune response.
1×8
The math check — why "1×8=8, 2×4=8" actually helps
MHC I paired with CD8 equals 8 (1×8=8), and MHC II paired with CD4 also equals 8 (2×4=8) — both equations landing on the same number is exactly the point: it gives you a fast, self-verifiable way to confirm the correct pairing rather than relying on pure memorization alone.
🦠 Under exam pressure, running the quick math check (1×8=8, 2×4=8) confirms MHC I goes with CD8 and MHC II goes with CD4, rather than relying on memory alone in a stressful moment.
🏥 Applied Scenario
A patient receives a kidney transplant, and their body begins rejecting the new organ within weeks.
Step 1
Identify what's actually being recognized as foreign: Both MHC class I and class II molecules on the donor organ's cells differ from the recipient's own MHC molecules — the recipient's T cells recognize the mismatched MHC itself as foreign, triggering rejection.
Step 2
Contrast with a viral infection scenario: In a viral infection, MHC I on an infected cell displays a piece of the virus itself, not a mismatched MHC molecule — the T cell response there targets the infected cell specifically, based on what it's displaying, not based on the MHC molecule's own identity.
Step 3
Recognize the exam-relevant distinction: MHC mismatch driving transplant rejection versus MHC correctly displaying a viral antigen during infection is a key way exams separate transplant immunology questions from infection immunology questions — the mechanism looks superficially similar (T cell response involving MHC) but the actual trigger is fundamentally different.
Step 4
Conclusion: In transplant rejection, the MHC molecule ITSELF is the problem; in infection, the MHC molecule is doing its normal job correctly, displaying something abnormal it picked up.
📌 Exam Application
Exams commonly test which cells express which MHC class (all nucleated cells vs. APCs only), which T cell subset reads each class (CD8 reads MHC I, CD4 reads MHC II), and the clinical relevance — transplant rejection involves MHC mismatch across both classes, while HLA-B27 associates with ankylosing spondylitis and HLA-DR3/4 with Type 1 diabetes.
⚠️ The Trap — Reversing the CD4/CD8-MHC Pairing
The most common trap is reversing the pairing — assuming CD4 reads MHC I or CD8 reads MHC II. Use the math check to self-verify quickly: 1×8=8 (MHC I, CD8), 2×4=8 (MHC II, CD4). A second common trap is forgetting that red blood cells, lacking a nucleus, don't express MHC I at all — a frequently tested exception to the "all nucleated cells" rule.
✓ Quick Self-Test
Answer before checking:
1. Which cells express MHC class I, and which T cell subset reads it?
2. Which cells express MHC class II, and which T cell subset reads it?
3. Why don't red blood cells express MHC class I?
4. What causes transplant rejection at the molecular level?
5. What does the memory trick "1×8=8, 2×4=8" help you recall?
Answers:
1. Virtually all nucleated cells express MHC I; CD8+ Cytotoxic T cells read it.
2. Only professional antigen-presenting cells (dendritic cells, macrophages, B cells) express MHC II; CD4+ Helper T cells read it.
3. Because they lack a nucleus, and MHC I is expressed on nucleated cells.
4. A mismatch between donor and recipient MHC molecules (both Class I and Class II), which the recipient's T cells recognize as foreign.
5. MHC I pairs with CD8 (1×8=8) and MHC II pairs with CD4 (2×4=8).