It has recently been established that mast cells display functional heterogeneity. The question then arises whether the IgE‐type of antibody, which avidly binds to and thereby sensitizes mast cells and basophils for allergen‐induced release of mediators, also expresses functional heterogeneity. In the present article we bring together several experimental observations, mainly from the rat system, which are difficult to explain unless one postulates that mat cell/basophil‐sensitizing antibodies of the IgE‐type re heterogeneous in their cell‐binding p