Final answer:
IL-4 plays a major role in the induction of allergic responses.
Step-by-step explanation:
The cytokine that plays a major role in the induction of allergic responses is IL-4. IL-4 is released by T helper cells (TH2 cells) and stimulates B cells to produce IgE antibodies. These IgE antibodies bind to mast cells, which are then primed for a subsequent exposure to the allergen. When the allergen is encountered again, mast cells release cytokines and histamines, causing inflammation and allergic symptoms.