During an allergic reaction, the release of histamine from mast cells causes vasodilation which allows eosinophils to migrate from the blood and localize in affected tissues. Accumulation of eosinophils in tissues can be significantly damaging. Eosinophils, like other granulocytes, contain granules (or sacs) filled with digestive enzymes and cytotoxic proteins which under normal conditions are used to destroy parasites but in eosinophilia these agents can damage healthy tissues. In addition to these agents, the granules in eosinophils also contain inflammatory molecules and cytokines which can recruit more eosinophils and other inflammatory cells to the area and hence amplify and perpetuate the damage. This process is generally accepted to be the major inflammatory process in the pathophysiology of atopic or allergic asthma. Diagnosis
Diagnosis is by complete blood count (CBC). However, in some cases, a more accurate absolute eosinophil count may be needed. Medical history is taken, with emphasis on travel, allergies and drug use. Specific test for causative conditions are performed, often including chest x-ray, urinalysis, liver and kidney function tests, and serologic tests for parasitic and connective tissue diseases. The stool is often examined for traces of parasites (i.e. eggs, larvae, etc.) though a negative test does not rule out parasitic infection; for example, trichinosis requires a muscle biopsy. Elevated serum B12 or low white blood cell alkaline phosphatase, or leukocytic abnormalities in a peripheral smear indicates a disorder of myeloproliferation. In cases of idiopathic eosinophilia, the patient is followed for complications.
Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 5×108/L (500/μL). Hypereosinophilia is an elevation in an individuals circulating blood eosinophil count above 1.5 x 109/L  (i.e. 1,500/μL). The hypereosinophilic syndrome is a sustained elevation in this count above 1.5 x 109/L (i.e. 1,500/μL) that is also associated with evidence of eosinophil-based tissue injury. Eosinophils usually account for less than 7% of the circulating leukocytes. A marked increase in non-blood tissue eosinophil count noticed upon histopathologic examination is diagnostic for tissue eosinophilia. Several causes are known, with the most common being some form of allergic reaction or parasitic infection. Diagnosis of eosinophilia is via a complete blood count (CBC), but diagnostic procedures directed at the underlying cause vary depending on the suspected condition(s). An absolute eosinophil count is not generally needed if the CBC shows marked eosinophilia. The location of the causal factor can be used to classify eosinophilia into two general types: extrinsic, in which the factor lies outside the eosinophil cell lineage; and intrinsic eosinophilia, which denotes etiologies within the eosiniphil cell line. Specific treatments are dictated by the causative condition, though in idiopathic eosinophilia, the disease may be controlled with corticosteroids. Eosinophilia is not a disorder (rather, only a sign) unless it is idiopathic.Informally, blood eosinophil levels are often regarded as mildly elevated at counts of 500–1,500/μL, moderately elevated between 1,500 and 5,000/μL, and severely elevated when greater than 5,000/μL.