**Question:** Eosinophils are activated by -

A. Interleukin-5
B. Interleukin-3
C. Interleukin-4
D. Interleukin-2

**Core Concept:** Eosinophils are a type of white blood cell involved in the immune response, particularly in the defense against parasitic infections. They are activated and recruited to the site of infection by specific cytokines (chemical messengers).

**Why the Correct Answer is Right:** Eosinophils are activated by Interleukin-5 (IL-5) because IL-5 is a cytokine produced by T cells and plays a crucial role in eosinophil activation, differentiation, and survival. IL-5 promotes eosinophil proliferation and differentiation from hematopoietic stem cells, and also enhances eosinophil survival at the site of infection.

**Why Each Wrong Option is Incorrect:**

A. Interleukin-3 (IL-3) is involved in the proliferation and differentiation of various hematopoietic cells, including basophils and mast cells, but not eosinophils. Therefore, IL-3 is incorrect.

B. Interleukin-2 (IL-2) is primarily involved in the activation of T cells and natural killer cells, not eosinophil activation. Thus, IL-2 is incorrect.

C. Interleukin-4 (IL-4) is involved in the activation of B cells, not eosinophil activation. Therefore, IL-4 is incorrect.

**Clinical Pearl:** Understanding cytokine interactions is crucial in understanding immune responses and various immunological disorders. In asthma, for example, elevated levels of IL-5 and other cytokines contribute to excessive eosinophil activation and recruitment, leading to asthma exacerbation.

**Correct Answer:** Interleukin-5 (IL-5) is the correct answer because it plays a vital role in eosinophil activation, proliferation, and survival, making it the key cytokine involved in eosinophil-related immune responses.