**Core Concept**
Henoch-Schönlein purpura (HSP) is a form of vasculitis that primarily affects small vessels, particularly in the skin, kidneys, and gastrointestinal tract. It is characterized by the deposition of immune complexes, which play a crucial role in its pathogenesis.

**Why the Correct Answer is Right**
The immune complexes in HSP are primarily composed of IgA antibodies, which are deposited in the walls of small blood vessels. This deposition of IgA immune complexes leads to activation of the complement system, resulting in inflammation and damage to the affected tissues. The IgA antibodies are thought to be produced in response to an infectious agent, such as a bacterial or viral infection, which triggers an immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** IgM - While IgM is an important antibody in the immune response, it is not the primary antibody deposited in HSP.
**Option B:** IgE - IgE is primarily involved in allergic reactions and is not associated with HSP.
**Option C:** Complement C3 - While complement C3 is involved in the inflammatory process, it is not the primary component deposited in HSP.

**Clinical Pearl / High-Yield Fact**
HSP is often associated with a preceding upper respiratory tract infection, and the deposition of IgA immune complexes is thought to play a key role in its pathogenesis.

**Correct Answer:** D. IgA