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MHC I × CD8 = 8 & MHC II × CD4 = 8
Picture This (Mental Hook): Immune cells follow the sacred Rule of Eight! Take MHC Class 1 and multiply by cytotoxic CD8: 1 x 8 = 8! Take MHC Class 2 and multiply by helper CD4: 2 x 4 = 8! The product is ALWAYS 8!
📊 Visual Diagram & Mental Flow:
CYTOTOXIC T-CELL (Killing Tumor / Virus infected cells):
[MHC Class I] × [CD8 T-Cell] = 8
* Present on ALL NUCLEATED CELLS (not RBCs!)
* Presents ENDOGENOUS ANTIGENS (viral proteins made inside cell)
* Heavy chain + beta-2-microglobulin (Chr 15)
HELPER T-CELL (Activating B-cells & Macrophages):
[MHC Class II] × [CD4 T-Cell] = 8
* Present ONLY on Professional APCs (Dendritic cells, Macrophages, B-cells)
* Presents EXOGENOUS ANTIGENS (engulfed bacteria digested in phagolysosomes)
* alpha + beta chains (both encoded in MHC locus)🔍 The Decoded Sequence:
1 x 8 = 8
CD8 Cytotoxic
CD8 co-receptor binds invariant alpha-3 domain of MHC Class I
2 x 4 = 8
CD4 Helper
CD4 co-receptor binds beta-2 domain of MHC Class II
APCs
Antigen Presenting Cells
Dendritic cells (most potent), Macrophages, B-lymphocytes
Examiner Trap / Never Forget: Red blood cells (erythrocytes) do NOT express MHC Class I because they are non-nucleated in adult mammals!
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ACID: Anaphylactic (I), Cytotoxic (II), Immune Complex (III), Delayed (IV)
Picture This (Mental Hook): Gell and Coombs poured ACID on allergic immune reactions: Type I is Anaphylactic (IgE triggers mast explosions). Type II is Cytotoxic (IgG/IgM destroys blood cells). Type III is Immune complex (antigen-antibody clumps settle in kidneys). Type IV is Delayed (T-cells show up 48 hours later with contact dermatitis)!
📊 Visual Diagram & Mental Flow:
┌──────────┬──────────────────┬─────────────────┬────────────────────────────────────────┐ │ Type │ Landmark Name │ Immune Mediator │ Clinical Prototype Example │ ├──────────┼──────────────────┼─────────────────┼────────────────────────────────────────┤ │ Type [A] │ Anaphylactic │ IgE, Mast cells │ Asthma, Hay fever, Penicillin allergy, │ │ │ (Immediate) │ Histamine │ Food allergies, Bee venom shock │ │ Type [C] │ Cytotoxic │ IgG, IgM, │ ABO incompatibility, Rh erythroblast. │ │ │ (Antibody-med) │ Complement (MAC)│ Goodpasture's, Myasthenia gravis │ │ Type [I] │ Immune Complex │ Ag-Ab complexes │ SLE (Lupus), Post-strep glomeruloneph,│ │ │ │ Neutrophils │ Serum sickness, Farmer's lung │ │ Type [D] │ Delayed-Type │ T-Cells (CD4/8) │ Tuberculin (Mantoux) test, Contact │ │ │ (Cell-mediated) │ Macrophages │ dermatitis (Poison ivy, Nickel allergy)│ └──────────┴──────────────────┴─────────────────┴────────────────────────────────────────┘
🔍 The Decoded Sequence:
A
Anaphylactic
Type I: IgE cross-linking on mast cells releases histamine within minutes
C
Cytotoxic
Type II: IgG/IgM directed against cell-surface antigens causing ADCC or lysis
I
Immune Complex
Type III: Circulating Ag-Ab complexes deposit in capillary walls & glomeruli
D
Delayed
Type IV: The ONLY cell-mediated hypersensitivity; requires 48-72 hours
Examiner Trap / Never Forget: Type IV is the ONLY hypersensitivity that CANNOT be transferred by serum antibodies! It can only be transferred passively using sensitized T-lymphocytes.
