Which of the following interleukin is secreted by T helper 2 cells?
## **Core Concept**
The question tests understanding of the cytokine profile associated with T helper 2 (Th2) cells, a subset of CD4+ T lymphocytes. Th2 cells play a crucial role in the immune response, particularly in the context of allergic reactions and combating parasitic infections. They are characterized by the secretion of specific cytokines that promote humoral immunity.
## **Why the Correct Answer is Right**
Th2 cells are known to secrete **IL-4**, **IL-5**, **IL-6**, and **IL-13** among other cytokines. These cytokines are pivotal in inducing B cell class switching to IgE, eosinophil activation, and mucosal immunity. **IL-4** is particularly significant as it is a key cytokine that drives the differentiation of naive helper T cells (Th0 cells) to Th2 cells. It also promotes the production of IgE and IgG1 antibodies.
## **Why Each Wrong Option is Incorrect**
- **Option A:** While **IL-2** is indeed a cytokine involved in the immune response, it is primarily secreted by T helper 1 (Th1) cells and to some extent by all T cells, not specifically Th2 cells.
- **Option B:** **IL-12** is not typically associated with Th2 responses; instead, it is produced by antigen-presenting cells and promotes Th1 cell differentiation.
- **Option D:** **IFN-γ** is a hallmark cytokine of Th1 cells, not Th2 cells, and plays a critical role in cell-mediated immunity.
## **Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that an imbalance between Th1 and Th2 responses is implicated in various diseases. For instance, an overactive Th2 response is associated with allergic diseases like asthma and atopic dermatitis. Mnemonically, Th2 diseases can be remembered as "Allergy, Asthma, Atopy."
## **Correct Answer:** . **IL-4**