Eosinophils tend to be a similar size to Basophils and Neutrophils with a diameter of 12-17 μm twelve to seventeen micrometers but can be recognized by their bi-lobed nucleus (sometimes you can see the thin ‘string’ joining the lobes). Another feature is the presence of granules in their cytoplasm, which stain bright red in the presence of eosin or acid dye. This coloring is a handy way to tell eosinophils apart from basophils, which stain blue in the presence of basic dye.

Eosinophils are phagocytic, which means that they detect, engulf, and destroy pathogens invading the body. Eosinophils’ granules contain cytotoxic proteins, which they use in the phagocytosis of parasites. These cells also play a role in wound healing and releasing cytokines to modulate inflammatory responses such as those involved in allergies.

An illustration of an eosinophil. A round cell with a big irregularly shaped nucleus and numerous granules present within its cytoplasm.

Figure 1. Eosinophil

Reference: Murphy, K.M. and Weaver, C. (2016) Janeway’s Immunobiology: Ninth International Student Edition. W.W. Norton & Company.