Eosinophils are white blood cells (leukocytes) involved in allergic reactions and in defense against parasitic infestations. In the blood, eosinophils account for only about 1-3% of the leukocyte population; their concentration in those tissues exposed to environmental agents, such as the digestive tract, lungs, genitourinary epithelia and cutaneous connective tissue, is higher. It is at this level, in fact, that the lymphocytes protect the body from the possible attack of parasites, which fight releasing substances capable of damaging them or killing them. For this reason, eosinophils are included, together with Tc lymphocytes, in the category of cytotoxic leukocytes. Moreover, due to the presence of many small cytoplasmic granules, they fall into the category of granulocytes (particular types of white blood cells) to which also basophils and neutrophils belong.
The name eosinophils derives from the fact that their cytoplasmic granules turn pink-red with a particular dye called eosin. By examining the contents of these granules, many chemicals have been discovered that are able to mediate the various defense and regulatory reactions in which they are involved. Eosinophils, for example, are particularly active during inflammatory and allergic reactions, where they contribute to the inflammatory process and tissue damage through the release of oxidizing substances and toxic enzymes. In addition to promoting inflammatory responses, eosinophils also have regulatory action. The phagocytosis propensity shown in vitro seems to be absent in vivo.
High eosinophils
Counting of eosinophils in the blood varies with age, time of day (low in the morning, higher in the evening), physical exercise, environmental stimuli and, in particular, allergenic exposure.
Although important for the defense of the organism against various pathogenic noxae, eosinophils can produce tissue damage and states due to the massive release of their cytotoxic substances. An increase in circulating eosinophils (eosinophilia) accompanies many IgE mediated allergic forms - including allergic asthma, hay fever or hypersensitivity to drugs such as aspirin - parasite infestations (such as malaria, amoebiasis, the ascidiasis or the dreaded toxoplasmosis), the dermopathies and particular forms of leukemia. Doctors talk about eosinophilia (high eosinophils) whenever their blood concentration exceeds 450/500 per mm3.
Low eosinophils
Eosinophils are produced from bone marrow, where they remain and mature for 8-10 days. At the end of this phase, they pass into the bloodstream and within 8-12 hours migrate into the tissues, where they remain for a few days without returning to the circulation. Compared to other granulocytes they have a longer life. In humans the ratio between tissue eosinophils and blood eosinophils is about 100: 1.
Bone marrow multiplication of eosinophils is stimulated by T helper lymphocytes, while it is inhibited by the administration of cortisone and ACTH.
