Genetic studies of atopic families have identified regions on chromosome 11q and 5q that affect IgE production. Chromosome 5 contains multiple genes, including those for IL-4, IL-5, and granulocyte-macrophage colony-stimulating factor. Eosinophil survival and mast cell proliferation are just a few pro-allergic effects of these cytokines. Chromosome 11 encodes the beta subunit of the high-affinity IgE receptor. Increased expression of this receptor on mast cells leads to more vehement response to small numbers of antigens. This increased expression explains how exposure to minute amounts of allergen, such as venom from a stinging insect, can produce systemic anaphylaxis.