**Question:** RAST test is used in diagnosis of

A. allergy to specific IgE
B. allergy to specific IgG
C. allergy to specific IgA
D. allergy to specific IgM

**Core Concept:**
The RAST (Radioallergosorbent test) is a laboratory test that measures the level of specific immunoglobulin E (IgE) in a patient's serum. IgE is a type of antibody produced in response to allergens (foreign substances that trigger an immune response). The test is used to diagnose allergic reactions to specific allergens, which are substances that trigger an allergic response.

**Why the Correct Answer is Right:**
The correct answer is A. RAST test is used to diagnose allergy to specific IgE. IgE is a key antibody in the allergic response, and its level in the blood can indicate the presence of specific allergens to which the patient is sensitized. By measuring the level of specific IgE, the RAST test helps determine if a patient is allergic to a particular allergen, such as pollen, dust, or food.

**Why Each Wrong Option is Incorrect:**

B. RAST test is primarily used to diagnose allergy to specific IgE, not IgG (which is involved in antibody-dependent cellular cytotoxicity).

C. The RAST test is used to diagnose IgE-mediated allergies, not IgA-mediated ones. IgA is involved in humoral immunity and is not directly related to allergic reactions.

D. RAST test is designed to diagnose IgM-mediated allergies, which are unrelated to allergic reactions. IgM is involved in the initial immune response and is not directly related to allergic reactions.

**Clinical Pearl:**
The RAST test is an essential tool in diagnosing IgE-mediated allergies, which are crucial in understanding the severity and management of allergic disorders. A positive RAST test result indicates an increased specific IgE level, leading to the identification of allergens and guiding appropriate allergen avoidance or treatment strategies.