Inhibition of Reagin-Mediated PCA Reactions in Monkeys and Histamine Release from Human Leukocytes by Human IgG<sub>4</sub> Subclass

Abstract

Human myeloma proteins of IgG<sub>4</sub> subclass in contrast to myeloma proteins IgG<sub>1</sub>, IgG<sub>2</sub> and IgG<sub>3</sub>, were capable of blocking PCA reactions in monkeys mediated by human reaginic antibodies of IgE class. In addition to IgE, IgG<sub>4</sub> myeloma protein was also capable of sensitizing leukocytes from normal individuals and gave histamine release (HR) upon challenge with anti-human IgG<sub>4</sub>. 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 IgG<sub>4</sub>, anti-human IgE, and the specific allergen. No response was obtained with anti-human IgG<sub>1</sub> and IgG<sub>3</sub> sera. Leukocytes from the normal individuals released histamine from 3 to 20% with anti-human IgG<sub>4</sub> and from 6 to 30% with anti-human IgE. Moreover, normal leukocytes sensitized with IgG<sub>4</sub> myeloma protein or a serum of an allergic patient heated at 56°C for 2 h, released a significant amount of histamine on challenge with anti-human IgG<sub>4</sub> whereas no response was obtained with anti-human IgE. The biological role of human IgG<sub>4</sub> in immediate hypersensitivity reactions is discussed in relation to human IgE.