**Question:** Immune complex deposition is the basic underlying pathology in which type of hypersensitivity reaction?

A. Type I Hypersensitivity (Immediate)
B. Type II Hypersensitivity (Cardiovascular)
C. Type III Hypersensitivity (Nephritis)
D. Type IV Hypersensitivity (Delayed)

**Core Concept:** Hypersensitivity reactions are a group of pathological conditions resulting from an abnormal immune response against foreign substances. They are classified into five types based on the immune complex formation and activation of immune cells.

**Why the Correct Answer is Right:**
Immune complex deposition is a key feature in Type III Hypersensitivity (Nephritis) reaction. In this type of hypersensitivity reaction, immune complexes are formed when antibodies bind to antigens, leading to their deposition in tissues, particularly in the glomerulus of the kidney. This deposition triggers a cascade of events, including inflammation and tissue damage, resulting in clinical symptoms and signs characteristic of Type III Hypersensitivity reaction (e.g., proteinuria, hematuria, hypertension).

**Why Each Wrong Option is Incorrect:**

A. Type I Hypersensitivity (Immediate) is characterized by the release of histamine and other mediators directly from mast cells and basophils without immune complex formation.

B. Type II Hypersensitivity (Cardiovascular) involves the activation of complement system leading to membrane attack complex formation and cell lysis, typically affecting red blood cells.

C. Type IV Hypersensitivity (Delayed) is characterized by a cell-mediated immune response, which is not directly related to immune complex deposition.

D. Type IV Hypersensitivity (Delayed) is characterized by a cell-mediated immune response, which is not directly related to immune complex deposition.

**Clinical Pearl:** Hypersensitivity reactions are crucial to understand as they can manifest as various clinical syndromes, which are essential to recognize and manage appropriately in clinical practice. Adequate knowledge of the different types of hypersensitivity reactions can help in formulating an appropriate diagnosis and treatment plan for patients presenting with organ-specific symptoms, such as nephritis in Type III Hypersensitivity reaction.