**Core Concept**
Henoch Schonlein Purpura (HSP) is a form of vasculitis that affects small vessels, characterized by the deposition of immune complexes in the vessel walls. This leads to inflammation and damage, resulting in symptoms such as purpura, arthritis, gastrointestinal symptoms, and renal involvement.

**Why the Correct Answer is Right**
The correct answer is IgA. IgA immune complexes are deposited in the endothelial cells of small vessels, triggering an inflammatory response. This is mediated by the activation of complement and the release of various cytokines, leading to the characteristic symptoms of HSP. The deposition of IgA immune complexes is a hallmark of HSP and is often seen in biopsy specimens.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is not directly associated with HSP, although it may be present in small amounts. The primary antibody involved in HSP is IgA.
**Option B:** IgM is not typically associated with HSP, and its deposition is not a characteristic feature of the disease.
**Option C:** IgE is primarily involved in allergic reactions and is not directly associated with HSP.

**Clinical Pearl / High-Yield Fact**
IgA nephropathy, also known as Berger's disease, is a common cause of kidney disease worldwide and is often associated with HSP. Patients with HSP are at increased risk of developing IgA nephropathy, and renal involvement is a significant complication of the disease.

**Correct Answer: D. IgA.**