**Question:** Which of the following is the test for immediate hypersensitivity reaction?

A. Prick test
B. RAST (Radioallergosorbent test)
C. Intradermal test
D. Skin prick test with histamine

**Core Concept:** Immediate hypersensitivity reactions are a group of responses involving immunoglobulin E (IgE) antibodies and mast cells, leading to rapid release of inflammatory mediators. These reactions occur within minutes after exposure to an allergen.

**Why the Correct Answer is Right:** The correct answer, D, is the skin prick test with histamine because:

* Histamine is a potent vasodilator and mediator of immediate hypersensitivity reactions.
* When applied to the skin, histamine causes vasodilation, edema, and increased vascular permeability, which are characteristic features of immediate hypersensitivity reactions.
* The skin prick test with histamine serves as a positive control, demonstrating the presence of functional mast cells and the capacity of the skin to mount an immediate hypersensitivity response.

**Why Each Wrong Option is Incorrect:**

A. Prick test (Option A): The prick test is a test for delayed hypersensitivity reactions (Type IV hypersensitivity), which occur hours after allergen exposure.
B. RAST (Radioallergosorbent test) (Option B): RAST is a laboratory test for quantifying allergen-specific IgE antibodies in serum, not a test for immediate hypersensitivity reactions.
C. Intradermal test (Option C): The intradermal test is a test for both immediate and delayed hypersensitivity reactions, not specifically for immediate reactions.
D. Skin prick test with histamine (Option D): As explained above, the skin prick test with histamine serves as a positive control for immediate hypersensitivity reactions, not a test for these reactions themselves.

**Clinical Pearl:**

When assessing patients with suspected allergic disorders, it is crucial to differentiate between immediate and delayed hypersensitivity reactions. The skin prick test with histamine is a valuable tool in demonstrating the presence of functional mast cells and the capacity of the skin to mount an immediate hypersensitivity response.

This knowledge is essential for accurate diagnosis and appropriate management of allergic conditions, particularly anaphylaxis, which is a severe, potentially life-threatening reaction due to an immediate hypersensitivity reaction. Understanding the principles of skin tests helps in determining the appropriate treatment and preventive measures for these patients.