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To the Editor.—
Fry et al found decreased serum levels of IgM in four of 12 patients with dermatitis herpetiformis (Lancet 2:729-733,1967). As far as I know, this work has not been discredited. Yet, according to the answer to the Test Your Knowledge question in the Archives (110:252, 1974), one may not expect to find "decreased or undetermined serum levels of IgM" in dermatitis herpetiformis. When wrong answers are given, these quizzes really do "test your knowledge"!