IgG4 represents only 2-5% of the human IgG subclasses in serum. IgG4 lacks effector functions, such as antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity. IgG4 plays a protective role in allergy by acting as a blocking antibody, and inhibiting mast cell degranulation, but a deleterious role in malignant melanoma, by impeding IgG1-mediated anti-tumor immunity.