Expansion
ACID: Anaphylactic, Cytotoxic, Immune complex, Delayed
Mnemonic
ACID, for types I to IV:
- A - type I, Anaphylactic (or Allergic). IgE on mast cells. Immediate. Anaphylaxis, asthma, hay fever
- C - type II, Cytotoxic. IgG or IgM against cell surface antigen. Autoimmune haemolytic anaemia, Goodpasture’s, myasthenia gravis, rheumatic fever, ABO reaction
- I - type III, Immune complex. Antigen-antibody complexes deposit. SLE, serum sickness, post-streptococcal glomerulonephritis, extrinsic allergic alveolitis
- D - type IV, Delayed. T cell mediated, 48 to 72 hours. Contact dermatitis, tuberculin (Mantoux) test, graft rejection, coeliac disease
“1, 2 and 3 are antibody; 4 is cells”, and “only type 4 is delayed”, which is why the Mantoux is read at 48 to 72 hours.
The timing separates them clinically: type I in minutes, type II in hours to days, type III at about 7 to 10 days as complexes form, type IV at 48 to 72 hours.
Expansion
- Type I: Anaphylactic - IgE mediated mast cell degranulation, within minutes. Anaphylaxis, asthma, hay fever, food allergy
- Type II: Cytotoxic - IgG or IgM against cell surface antigens, with complement and phagocytosis. Autoimmune haemolytic anaemia, ABO transfusion reaction, Goodpasture disease, myasthenia gravis, Graves disease
- Type III: Immune complex - antigen-antibody complexes deposit in tissues and activate complement. Serum sickness, systemic lupus erythematosus, post-streptococcal glomerulonephritis, extrinsic allergic alveolitis
- Type IV: Delayed - T cell mediated, taking 48 to 72 hours. Tuberculin test, contact dermatitis, graft rejection, coeliac disease
The first three are antibody mediated and therefore transferable in serum; only type IV is cell mediated and requires transfer of cells.
Timing is the most useful discriminator at the bedside: minutes for type I, hours for type III, and days for type IV.