In 1963, two British immunologists, Robert Coombs and Philip Gell, published in a book a new scheme for classifying different types of hypersensitivity reactions. Although knowledge on the underlying immunological processes has greatly expanded over the past 50 years, this simple classification is still used nowadays.

Coombs and Gell’s classification divides allergies into four pathophysiological types, namely the immediate (type I), cytotoxic (type II), immune complex-mediated (type III), and delayed hypersensitivity (type IV) reactions. The different pathophysiological mechanisms lead to varying latency periods for the four classes: type I allergic symptoms already appear after a few seconds to minutes; symptoms of type II reactions appear after minutes to hours; signs of type III allergic reactions set in after several hours; and finally, for type IV reactions, a long latency of hours to days is expected. Types I to III are also termed ‘humoral’ reactions, because these are mediated by soluble factors (ie antibodies), whereas in type IV, primarily cells (ie T cells) are involved. While the three humoral types have remained basically the same over the past 50 years, for the cell-mediated type IV reaction, further subclassifications have been proposed.

Please have a look at the PDF attached to this article now. There you’ll find a table summarizing the Coombs and Gell classification, including common allergens, latencies, mediators, effector mechanisms, and the clinical signs of each type. Immediate type I allergies are most common and, for this reason, in the upcoming chapter we’ll mainly focus on this class. You might want to have a look at the allergens, the effector mechanisms, and the clinical signs of this type and compare these to the characteristics of the remaining subtypes. Can you classify allergic contact eczema based on this table?

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