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Eosinophilia is classified as primary or secondary, in addition to the hypereosinophilic syndromes.
Primary eosinophilia is a rise in the peripheral eosinophil count without any underlying condition to account for it. It is subclassified into clonal and idiopathic types.
Clonal primary eosinophilia is the type of eosinophilia that is due to proliferation of a clone of eosinophils in the bone marrow, and is found in leukemias or other myeloproliferative disorders.
Idiopathic primary eosinophilia is a term used to refer to peripheral blood eosinophilia without any detectable cause.
Secondary eosinophilia is the result of other disorders not associated with bone marrow proliferation, such as atopy, asthma, and most commonly helminthic infestations.
Hypereosinophilic syndromes are disorders which are characterized by eosinophilia above 1500/µL persisting for at least 6 months, with no underlying disease condition, but associated with organ dysfunction due to eosinophil recruitment into tissues which suffer resulting damage. These include the syndromes of pulmonary eosinophilic infiltrate with eosinophilia, such as:
The following simple classification may help to understand the manifold conditions which may give rise to secondary eosinophilia.
Allergic conditions, especially of the skin and the respiratory tract, such as;
Connective tissue disorders such as;
Immune disorders such as;
Certain tumors, such as Hodgkin’s disease, also result in eosinophilia. Others include
Invasive parasite infestations result in a rise in the number of peripheral blood eosinophils, such as
With regard to parasitosis, a sustained rise in eosinophils is usually seen when the parasites migrate into tissues and come into contact with patrolling immune cells. Once the parasite is walled off within a cyst, or is dormant, eosinophilia tends to fade. Only if the cyst leaks and parasite products again become exposed to immune effector cells (T-lymphocytes) does eosinophilia again appear. Parasites which remain within the lumen of the gut during their whole lifecycle rarely cause persistent eosinophilia.
Again, protozoal infection is not usually a cause of a raised eosinophil count. However, ectoparasites such as the scabies mite are associated with eosinophilia.
Non-parasitic infestations such as:
Skin disorders due to hypersensitivity reactions:
Asymptomatic drug-induced eosinophilia
Eosinophilia associated with pulmonary infiltrates
Eosinophilia associated with hepatitis
Eosinophilia associated with interstitial nephritis
Other conditions that may be found in eosinophilia include: