Serum sickness is which type of hypersensitivity reaction: September 2011
**Core Concept**
Serum sickness is a type of immune-mediated reaction that occurs in response to the administration of foreign serum proteins, such as antivenom or antiserum. It is characterized by the formation of immune complexes and subsequent activation of the complement system, leading to tissue damage and inflammation.
**Why the Correct Answer is Right**
Serum sickness is a classic example of a type III hypersensitivity reaction, which involves the formation of immune complexes (antigen-antibody complexes) that deposit in tissues and activate the complement system. This leads to the release of pro-inflammatory cytokines and the activation of various cell types, including neutrophils and macrophages, resulting in tissue damage and inflammation. The immune complexes in serum sickness are composed of antibodies directed against the foreign serum proteins, which are deposited in tissues such as the skin, joints, and kidneys.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because serum sickness is not a type I hypersensitivity reaction, which is mediated by IgE antibodies and involves the release of histamine from mast cells.
**Option B:** This option is incorrect because serum sickness is not a type II hypersensitivity reaction, which involves the binding of antibodies to cell surface antigens and the activation of complement or cellular cytotoxicity.
**Option C:** This option is incorrect because serum sickness is not a type IV hypersensitivity reaction, which is a cell-mediated reaction that involves the activation of T cells and the release of cytokines and other inflammatory mediators.
**Clinical Pearl / High-Yield Fact**
The clinical presentation of serum sickness can be mimicked by other conditions, such as systemic lupus erythematosus (SLE), so it is essential to consider the patient's history of recent exposure to foreign serum proteins when making a diagnosis.
**Correct Answer: C. Type III hypersensitivity reaction.