Step by Step
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 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.
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 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 — why "1×8=8, 2×4=8" is more than a memory device
MHC I × CD8 = 8, and MHC II × CD4 = 8 — both equations equal 8, which is exactly the point: it ties the two classes to their correct T cell partner using a number that's easy to check yourself against under exam pressure.
Applied Walkthrough
1
A patient receives a kidney transplant, and their body begins rejecting the new organ within weeks.
2
Ask: 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.
3
Contrast: in a viral infection (not a transplant), 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, not a mismatched organ.
4
This 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.
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.
⚠ Common Trap
The most common trap is reversing the pairing — assuming CD4 reads MHC I or CD8 reads MHC II. Use the math check: 1×8=8 (MHC I, CD8), 2×4=8 (MHC II, CD4) to self-verify quickly. Another trap: forgetting that red blood cells, lacking a nucleus, don't express MHC I.
✓ Quick Self-Check
1. Which cells express MHC class I, and which T cell subset reads it?
Virtually all nucleated cells express MHC I; CD8+ Cytotoxic T cells read it.
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2. Which cells express MHC class II, and which T cell subset reads it?
Only professional antigen-presenting cells (dendritic cells, macrophages, B cells) express MHC II; CD4+ Helper T cells read it.
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3. Why don't red blood cells express MHC class I?
Because they lack a nucleus, and MHC I is expressed on nucleated cells.
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4. What causes transplant rejection at the molecular level?
A mismatch between donor and recipient MHC molecules (both Class I and Class II), which the recipient's T cells recognize as foreign.
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5. What does the memory trick "1×8=8, 2×4=8" help you recall?
MHC I pairs with CD8 (1×8=8) and MHC II pairs with CD4 (2×4=8).
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