The eosinophil is a specialized cell of the immune system first recognized by Paul Ehrlich approximately 130 years ago. Most of our current knowledge about this type of white blood cell came during the latter half of the twentieth century as the eosinophil was wrongly believed to be a precursor of red blood cells during the early 1950's.
 

Structure

 
The eosinophil generally has a nucleus with two lobes (bilobed), and cytoplasm filled with approximately 200 large granules containing enzymes and proteins with different (known and unknown) functions [see picture below; nucleus (purple), granules (pink)].
 
 
 

Location

 
Eosinophils 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. Counts of 0 to 450 eosinophils per cubic millimeter of blood are generally considered within normal limits.
 

Development

 
Eosinophils are formed exclusively in the bone marrow where they spend about 8 days in the process of maturation before moving into the blood vessels. They travel through the vessels for 8 to 12 hours before they finally arrive at destination tissues, where they remain for 1 to 2 weeks. Interleukin 5 (IL-5) is a major growth factor for this type of cell.
 

Function

 
Eosinophils are proinflammatory white blood cells that have many functions. They are implicated in numerous inflammatory processes, especially allergic disorders. The functions of the eosinophil are varied, some of which are very similar to other white cells. Known functions include movement to inflamed areas, trapping substances, killing cells, antiparasitic and bactericidal activity, participating in allergic reactions, and modulating 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, the level of eosinophil activation, and the type of stimulus attracting the eosinophil.
 
Eosinophils can be both helpful or harmful. At one extreme, such as in the illness erythema toxicum, eosinophils have a transient and benign role. Also, during certain parasitic infections, eosinophils protect the host from the parasite by helping rid the body of the infection. At the other extreme, represented by conditions like Loeffler's disease and hypereosinophilic syndrome, eosinophils are linked with severe pathologic changes.