**Core Concept**
Serum sickness is an immune-mediated reaction that occurs in response to the presence of foreign antigens, typically resulting from the administration of antiserum or other foreign proteins. It is characterized by the formation of antigen-antibody complexes that deposit in various tissues, leading to inflammation and tissue damage.

**Why the Correct Answer is Right**
Serum sickness is a classic example of a Type III hypersensitivity reaction, which is mediated by the formation of antigen-antibody complexes. These complexes activate the complement system, leading to the release of inflammatory mediators and the recruitment of immune cells to the site of deposition. The resulting inflammation and tissue damage are hallmarks of serum sickness. The involvement of the complement system and the formation of antigen-antibody complexes are key features of Type III hypersensitivity reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** Type I hypersensitivity reactions are mediated by IgE antibodies and are characterized by immediate allergic responses, such as anaphylaxis. Serum sickness does not involve IgE antibodies or immediate allergic responses.
**Option B:** Type II hypersensitivity reactions are mediated by IgG or IgM antibodies and are characterized by the direct cytotoxic effects on cells, such as in autoimmune hemolytic anemia. Serum sickness does not involve direct cytotoxic effects on cells.
**Option D:** Type IV hypersensitivity reactions are mediated by T cells and are characterized by delayed hypersensitivity responses, such as contact dermatitis. Serum sickness is not a delayed hypersensitivity response.

**Clinical Pearl / High-Yield Fact**
To remember the different types of hypersensitivity reactions, use the mnemonic "IgE for Type I, IgG for Type II, IgM for Type III, and T cells for Type IV." This will help you associate the correct antibodies or immune cells with each type of reaction.

**✓ Correct Answer: C. Type III. Serum sickness is a Type III hypersensitivity reaction.**