knowmedge:

There are four types of hypersensitivity reactions.

• Type I is IgE-mediated and occurs very quickly after exposure. It is associated with allergens such as bee stings, peanuts, and certain medications, to name a few. This can lead to a life-threatening condition called anaphylaxis.
• Type II hypersensitivity reactions are cytotoxic/antibody-mediated. Some examples in this category are hemolytic reactions, goodpasture syndrome and hyperacute graft rejection.
• Type III is known as immune complex/IgG/IgM mediated and includes certain diagnoses like hypersensitivity pneumonitis, systemic lupus erythematosus, polyarteritis nodosa and serum sickness.
• Type IV is known as delayed or cell-mediated hypersensitivity reaction. Examples include chronic graft rejections, purified protein derivative (PPD), latex, nickel and poison ivy.

A quick mnemonic to use to remember these is ACID:

• Type I - Allergic
• Type II - Cytotoxic
• Type III - Immune complex deposition
• Type IV - Delayed