New Light Shed On The Enigmatic Eosinophil Granulocyte; A Versatile Cell Of The Immune System.

Eosinophils normally constitute only a few per cent of circulating leukocytes, though they are more numerous in tissues vulnerable to attack by environmental microorganisms. Eosinophils can kill invasive parasites, but they also have immunoregulatory functions and may be involved in, for example, the connective tissue remodeling that occurs in conjunction with inflammation. Although their effects may be beneficial to the host, for instance in the event of helminthic infestation, they may also cause tissue damage, for example in allergy and asthma. Recent years have witnessed significant advances in our knowledge of these fascinating but still enigmatic cells. The eosinophil granulocyte was first described in 1879 by Paul Ehrlich, who discovered a blood cell that had high affinity for acid dyes and, in particular, eosin (1) (Figure 1). Eosin, which gives the cells their characteristic red-orange color, is named after Eos, the goddess of dawn in Greek mythology. Paul Ehrlich also put forward the hypothesis that eosinophils develop in the bone marrow (2) and exert their functions in the peripheral tissues. All vertebrates seem to have eosinophils in their blood, but these cells have also been described in more primitive organisms such as sharks, turtles and snakes (3). However. there are differences in morphology, and a comparison of eosinophils from different species also reveals considerable differences in the protein content of their characteristic cytoplasmic granules. For example, in contrast to many vertebrates, eosinophils from the cat, rhino, hyena and okapi lack peroxidase (3).