Type II hypersensitivity is one of those “easy to recognize once you see it” immunology patterns: IgG/IgM bind to antigens on cells or extracellular matrix → the immune system destroys or malfunctions that target. If you can picture the mechanism, you can usually nail the question stem in seconds.
The 10-second one-liner (what Step wants)
Type II (cytotoxic) hypersensitivity = IgG or IgM against fixed antigens (cell surface or ECM) → complement/opsinization/ADCC or receptor dysfunction.
Memory Palace: “The Antibody Police at Cell Surface City”
Walk into Cell Surface City, where antibodies act like cops tagging “criminals” stuck on buildings (cell membranes) or city walls (basement membrane/ECM).
Room 1: The “Badge & Siren” Station — Opsonization + Phagocytosis
You see IgG cops putting handcuffs on a red blood cell, then dragging it to the Spleen “Jail”.
Mnemonic image: IgG handcuffs + spleen jail cell
USMLE translation:
- IgG opsonizes cells → macrophages in spleen/liver remove them (extravascular hemolysis).
- Classically tested in:
- Autoimmune hemolytic anemia
- Hemolytic disease of the newborn (Rh incompatibility; IgG crosses placenta)
- ITP (platelet destruction)
- Transfusion reactions
High-yield clue words: spherocytes, splenomegaly, extravascular hemolysis, positive Coombs (DAT)
Room 2: The “Complement Cannon” — MAC-mediated cell lysis
In the next room, an IgM “pentamer tank” fires a complement cannon at a cell, punching holes with the MAC (C5b-9).
Mnemonic image: IgM pentamer tank launching complement cannonballs
USMLE translation:
- IgM is the best complement activator (also IgG can do it).
- Complement can:
- Lyse cells directly (MAC)
- Promote inflammation via C3a/C5a
- Increase opsonization via C3b
High-yield clue words: intravascular hemolysis (classically), low complement in immune complex processes (context matters)
Room 3: The “Natural Killer Bouncer” — ADCC
At a club door, an NK cell bouncer grabs an antibody-tagged target using Fc receptors (CD16) and kicks it out.
Mnemonic image: NK bouncer scanning Fc “VIP pass” (CD16)
USMLE translation:
- ADCC = antibody-coated target killed by NK cells (also eosinophils can do ADCC in parasites, but that’s more Type I/Th2 contexts).
Room 4: The “Broken Receptor Switchboard” — Antibodies alter receptor function
Finally, you enter a switchboard room where antibodies either jam the “OFF” button or hold the “ON” button down.
Mnemonic image: Sticky antibodies on a light switch: stuck OFF or ON
USMLE translation (mega-testable):
- Myasthenia gravis: antibodies block ACh receptors → decreased neuromuscular transmission
- Graves disease: antibodies stimulate TSH receptor → increased thyroid hormone
High-yield clue words:
- MG: ptosis, diplopia, fatigability; improves with rest; thymic abnormalities
- Graves: exophthalmos, pretibial myxedema, diffuse goiter, thyroid-stimulating immunoglobulins
Rapid-fire “Type II = Cytotoxic” anchor mnemonic
“C-I-T-Y” mnemonic (Cell surface City)
- Complement activation (IgM > IgG)
- Inflammation + opsonization (C3b, Fc-mediated phagocytosis)
- Target is Tethered (cell surface or ECM—not soluble antigen)
- You can get receptor dysfunction (on/off)
High-yield table: Type II vs common confusion points
| Feature | Type II (Cytotoxic) | Type III (Immune complex) | Type IV (Delayed, T-cell) |
|---|---|---|---|
| Main immune player | IgG/IgM | IgG immune complexes | T cells (Th1/Th17, CD8+) |
| Antigen location | Fixed (cell surface/ECM) | Soluble → deposits in tissues | Cellular antigens |
| Key mechanisms | Complement, opsonization, ADCC, receptor blockade/stimulation | Complement + neutrophils at deposition sites | Cytokine-mediated inflammation or direct cytotoxicity |
| Classic examples | AIHA, ITP, Goodpasture, MG, Graves | SLE, PSGN, serum sickness, Arthus reaction | Contact dermatitis, TB test, type 1 DM |
Classic diseases to attach to the palace (know these cold)
Goodpasture syndrome (Type II)
- Anti–type IV collagen (basement membrane) in glomeruli + alveoli
- Findings: hemoptysis + hematuria
- IF pattern: linear staining
Pemphigus vulgaris (often tested as Type II)
- Anti-desmoglein (desmosomes) → intraepidermal blister
- Findings: flaccid bullae, +Nikolsky sign
Bullous pemphigoid (also antibody-mediated, high-yield)
- Anti-hemidesmosomes → subepidermal blister
- Findings: tense bullae, typically negative Nikolsky
Exam tip: If the stem screams “antibodies against a structural protein in skin or basement membrane,” Type II should jump to mind.
The “2-step” exam approach (fast identification)
- Is the antigen fixed on a cell or basement membrane?
- Yes → think Type II
- Is it destruction or dysfunction?
- Destruction: complement/opsonization/ADCC
- Dysfunction: receptor blockade or stimulation
Mini self-check (one question worth of clarity)
If IgG binds RBC antigens and the patient gets anemia + jaundice + splenomegaly, what’s happening?
- Type II → IgG opsonization → splenic macrophages remove RBCs (extravascular hemolysis) → positive direct Coombs test.