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Bone marrow produces blood granulocytes known as eosinophils, and IL-5, IL-3, and GM-CSF stimulate the production of cytoplasmic granules, which contain basic proteins and form bonds with acidic dyes like "eosin." For 4.5 to 8 hours at a time, they circulate. Particularly in the digestive and respiratory tracts, they might linger in tissues for 8 to 12 days. Eosinophils make up less than 5% of the circulating leukocytes. Eosinophilia is defined as an increase in the amount of circulating eosinophils above 500/mm3.

Based on counts, eosinophilia can be categorised as mild, moderate, or severe. Both detectable tissue eosinophilia and absolute eosinophil counts of more than 1500/mm3 on two different occasions, at least one month apart, are thought to be signs of hypereosinophilic syndrome.

Epidemiology

The incidence and prevalence of eosinophilia are poorly understood. Eosinophilia does not favour one sex over another. However, there may be geographical consequences depending on the cause. Parasitic infestations are more common in tropical countries. Allergy-related illnesses are relatively common in wealthy countries. Idiopathic hypereosinophilia is typically diagnosed between the ages of 20 and 50, while it has been documented to occur at both extreme ends of the age spectrum.

Pathophysiology

The bone marrow is where eosinophils differentiate and stop growing after departing. They spend much of their time in tissues, away from the vasculature. In eosinophil-related disorders, eosinophils are attracted to the afflicted tissues. Synthesis of IL-5 and immune reactions mediated by T helper-2 cells lead to eosinophilopoiesis and eosinophil activation. A crucial cytokine involved in the production and activation of eosinophils is IL-5. When triggered, eosinophils degranulate and release cationic proteins into the tissues via which they function. These released proteins, which may also be proteolytic enzymes, may also cause damage to the host wall. Eosinophils also secrete cytokines like IL-10 and IL-14 that help to maintain homeostasis and regulate the immune system.

An eosinophil has a segmented nucleus and a diameter of 12 to 17 m. It has many proteolytic enzyme-containing cytoplasmic granules. The granules are made up of four main proteins: eosinophil derived neurotoxin (EDN), major basic protein (MBP1), eosinophilic cationic protein (ECP), and eosinophil peroxidase (EOP). They use Romanowsky stains to colour the orange-red.

Comprehensive history taking and careful physical examination are vitally critical, and sometimes enough, for diagnosis due to the disease's varied symptoms and severity ranging from mild to end-organ destruction. Common organ systems affected include those of the gastrointestinal, respiratory, and skin. Numerous disorders, such as myeloproliferative and lymphoid neoplasms, Churg Strauss syndrome, and DRESS syndrome, can cause constitutional symptoms such low-grade fevers, night sweats, lethargy, and weight loss.