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 – 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”.