Inhibition of reagin-mediated PCA reactions in monkeys and histamine release from human leukocytes by human IgG4 subclass.

Human myeloma proteins of IgG4 subclass in contrast to myeloma proteins IgG1, IgG2 and IgG3, were capable of blocking PCA reactions in monkeys mediated by human reaginic antibodies of IgE class. In addition to IgE, IgG4 myeloma protein was also capable of sensitizing leukocytes from normal individuals and gave histamine release (HR) upon challenge with anti-human IgG4. Leukocytes from 11 allergic individuals and from 9 normal subjects sensitized with the serum of allergic patients, were capable of releasing histamine with anti-human IgG4, anti-human IgE, and the specific allergen. No response was obtained with anti-human IgG1 and IgG3 sera. Leukocytes from the normal individuals released histamine from 3 to 20% with anti-human IgG4 and from 6 to 30% with anti-human IgE. Moreover, normal leukocytes sensitized with IgG4 myeloma protein or a serum of an allergic patient heated at 56 degrees C for 2 h, released a significant amount of histamine on challenge with anti-human IgG4 whereas no response was obtained with anti-human IgE. The biological role of human IgG4 in immediate hypersensitivity reactions is discussed in relation to human IgE.