What Is An Eosinophil?
The eosinophil is a type of white blood cell first recognized by Paul Ehrlich approximately 125 years ago. Most of our current knowledge about this type cell came during the latter half of the twentieth century (during the early 1950's the eosinophil was wrongly believed to be a precursor of red blood cells).
Structure
The eosinophil has, on average, bilobed nuclei, and its cytoplasm is filled with approximately 200 large granules containing enzymes and proteins with different (known and unknown) functions (see picture below).
Location
Erosinophils are a normal cellular component of the blood and also of certain tissues, including spleen, lymph nodes, thymus and the submucosal areas of the gastrointestinal, respiratory and genitourinary tracts. Numbers between 0 to 450 eosinophils per cubic millimeter of blood are reported as within normal limits.
Development
Eosinophils are formed exclusively in the bone marrow where they spend about 8 days in the process of maturation, then move into the blood vessels with a life span of 8 to 12 hours before they finally arrive at destination tissues, and remain there for 1 to 2 weeks. Interleukin 5 (IL-5) appears to be the major growth factor for this type of cell (look at our anti-IL-5 study).
Function
Eosinophils are multifunctional proinflammatory white blood cells implicated in numerous inflammatory processes, especially allergic disorders; in addition, it has been recently recognized that eosinophils may have a physiological role in organ formation (e. g. postgestational mammary gland development). The functions of the eosinophil are varied, some very similar to other white cells. Known eosinophil functions include movement to inflamed areas, trapping substances, killing cells, antiparasitic and bactericidal activity, participant in immediate allergic reactions and modulator of inflammatory responses. Eosinophil granule proteins, such as major basic protein (MBP), eosinophilic cationic protein (ECP), eosinophil peroxidase (EPO) and eosinophil-derived neurotoxin (EDN), are capable of inducing tissue damage and dysfunction. MBP, EPO and ECP have been shown to be toxic to a variety of tissues, including heart, brain, bronchial and intestinal epithelium. The degree of tissue injury is related to the duration of eosinophilia and the level of eosinophil activation as well as the type of stimulus attracting the eosinophil. In one end of the extreme there are examples of illnesses such as erythema toxicum where the eosinophil plays the role of a beneficial modulatory element or an innocent bystander; the other end is represented by conditions like Loeffler's disease and the idiopathic hypereosinophilic syndrome where the eosinophil is linked with permanent pathologic changes.
