By Haley Masterson
The 4 categories of hypersensitivity reactions is one of those subjects many students suspect we’ll never need to remember. But, in fact, this topic will likely haunt us for the rest of our medical career no matter what field we go into (even surgical residents have to review this topic for their ABSITE exam), so you may as well memorize the 4 categories now.
To remember the 4 categories, trying using the First Aid for the USMLE Step 1 mnemonic “ACID” (To remember that “ACID” is referring specifically to hypersensitivity reactions, consider that your skin would be fairly hypersensitive to someone pouring acid on it.).
The “A” stands for Allergic/Anaphylactic (Type I), the “C” stands for Cell-mediated (Type II), the “I” is for Immune Complex Deposition (Type III), and the “D” stands for ‘Delayed’ (Type IV).
Type I – Allergic. People commonly use the term “allergic” to mean “any sort of undesirable side-effect,” so you have to keep in mind that the true definition of the word means the reaction is IgE-mediated. Examples: Anaphylaxis, asthma, atopy.
Type II – Cell-mediated (Cytotoxic). Examples: Goodpasture’s, Grave’s disease, Myasthenia Gravis.
Type III – Immune complex deposition (Antigen-antibody). Examples: SLE, serum sickness.
Type IV – Delayed: Think of “Dermatitis from contact” examples such as poison ivy exposure and cheap jewelry. Several other Type IV reactions start with “T,” which is convenient, since they happen to be “T”-Cell mediated. (Examples: the TB skin test, and transplant rejection.)
Note that many of these examples all start with the same letter. Therefore, while understanding the mechanism for why each disease fits into its specific category is undeniably important, a second technique for remembering some of these categories is:
AnGST –(as in, you’d probably feel angst about someone pouring acid on your skin.) A is for “anaphylaxis, asthma, atopy”, G for “Goodpasture’s disease, Grave’s disease, and myasthenia Gravis”, S for “Systemic lupus erythematous and serum sickness”, and T for “T-cell mediated diseases like the TB test and transplant rejection”.
1. Kumar, Vinay, and Stanley L. Robbins. Robbins Basic Pathology. 8th ed. Philadelphia, PA: Saunders/Elsevier, 2007.
2. Le, Tao, and Vikas Bhushan. First Aid for the USMLE Step 1 2010. New York: McGraw-Hill Medical, 2010.