Human reaginic antibodies in sera of atopic individuals are associated with immunoglublin E which represents a distinct immunoglobulin class. The γE antibody agglutinated red cells coated with antigen, indicating that the antibodies are probably divalent. However, the antibodies do not have complement-fixing activity. The antibodies are responsible for P–K reactions in humans, passive cutaneous anaphylaxis reactions in monkeys and sensitize human leucocytes and monkey lung tissues, but not the guinea-pig skin. Immunoglobulin E combines with the tissues which are involved in the reaginic hypersensitivity reactions through the Fc portion of the molecules.