Chronic urticaria is caused by a complement fixing, IgG antibody directed to the ⍺-subunit of the IgE receptor, which is present in 35% to 45% of patients. This autoimmune subgroup can be identified by an autologous skin test or histamine release from human basophils or cutaneous mast cells. However, binding assays do not correlate with these functional assays. We considered the possibility that pathogenic antibody may be present within particular IgG subclasses, which might facilitate development of a binding method that can reliably screen patients.

To determine the subclass distribution of IgG antireceptor antibodies on the basis of histamine release, and to assess the possibility that a subclass specific ELISA binding method could be used to screen patients.

We isolated patient IgG by protein G affinity chromatography and then isolated patient IgG subclasses 1, 2, 3, and 4 by a combination of antibody affinity chromatography and protein A affinity chromatography. The ability of each subclass to activate basophils was assayed by histamine release.

Patient IgG subclasses IgG₁, IgG₃, and to a lesser degree IgG₄ have antibody capable of activating basophils to release histamine, whereas IgG₂ is inactive. Immunoblot or RAST assay that is subclass-specific does not correlate with histamine release as a result of nonfunctional but binding antibody within IgG subclasses 1, 3, or 4, and complement activation by IgG₁ and IgG₃.

Purification of IgG subclasses from patients with chronic urticaria demonstrates functional antibody in IgG₁ and IgG₃ and occasionally IgG₄. Nonfunctional antibody within IgG₂ plus nonfunctional antibody mixed with functional antibody within IgG₁, IgG₃, and IgG₄ and effects of complement are responsible for a lack of correlation of histamine release with binding assays even if subclass-specific.