The most common anti-insulin antibody is immunoglobulin (Ig) G, but IgA, IgM, IgD, and IgE antibodies also have anti-insulin properties. These antibodies usually do not cause clinical problems, but they may complicate insulin assay testing. IgM is thought to participate in insulin resistance and IgE in insulin allergy. Improvements in the purity of animal insulin and increased use of human insulin have resulted in a significant decrease in the incidence of insulin antibody formation.