IL-13, produced primarily by Th2 cells and to a lesser extent by mast and NK cells, is an immunoregulatory cytokine. Its various functions include induction of matrix metalloproteinases, enhancement of IgE secretion from activated B cells and B cell proliferation. Additionally, IL-13 plays a role in the inhibition of the expression of inflammatory cytokines and modulates resistance to various parasites. IL-13 is also secreted by eosinophils and in the lung is essential in regulating eosinophilic inflammation, mucus secretion, and airway hyperresponsiveness. Its function as a central mediator of allergen-induced asthma is well documented. Single nucleotide polymorphisms result in variants associated with specific asthmatic or allergic phenotypes, and certain variants will show enhanced functional activity as compared to wild-type IL-13.