Eosinophils, mast cells, and basophils all were first recognized and described by Paul Ehrlich in the late 19th century. Since then, it has become clear that these three cell types have much more in common than their recognition by the same scientist. All three cells are involved in the pathogenesis of allergic disease. This is a consequence of the receptors expressed on their surface and their arsenal of powerful immunologically active mediators that are released upon activation. These mediators can either be preformed (e.g., histamine, proteases, cytotoxic proteins) and released within seconds to minutes after activation, or de novo synthesized (e.g., arachidonic acid metabolites, chemokines, cytokines) and released minutes to hours to days after activation. While mature mast cells do not occur in blood, eosinophils are found both circulating in blood (normally less than 5% of leukocytes) and in hematopietic and lymphatic organs, such as the bone marrow, spleen, lymph nodes and thymus. Basophils are only found in blood in healthy individuals (normally less than 1% of leukocytes), but are known to be rapidly recruited to inflamed tissues, where they can reach high densities (e.g., Jones-Mote reaction, anti–tick immunity) and play non-redundant roles. Mast cells are resident in vascularized tissues throughout the body and are particularly prominent within tissues that interface the external environment.
The pathological roles of eosinophils, mast cells, and basophils in allergy are either directly or indirectly linked with the presence of allergen-specific IgE in allergic individuals.