Eosinophils are a type of white blood cell and are measured as part of a full blood count, making up approximately 1-6% of the total white blood cells. They ingest foreign cells such as bacteria, viruses and parasites. Eosinophils attack multicellular parasites such as worms and are also involved in allergic reactions.
Eosinophils have two functions in your immune system. They destroy substances related to parasitic infection and they also help regulate inflammation. Eosinophils help promote inflammation, which helps to isolate and control a disease site. In some people, the inflammatory response may be greater than necessary and therefore contribute to producing the symptoms of asthma and allergies.
Elevated levels of eosinophils is called eosinophilia and is most commonly caused by allergic disorders such as asthma, eczema and allergic rhinitis in the UK. Worldwide the most common cause of eosinophilia is parasitic infections. Increased levels of eosinophils can also occur in response to connective tissue diseases such as rheumatoid arthritis.
A low number of eosinophils in the blood is called eosinopenia. This can occur in blood infections (sepsis), and treatment with some steroid medications. However, a low number of eosinophils does not usually cause problems because other parts of the immune system compensate adequately. A low number of eosinophils is usually detected by chance when a complete blood count is done for other reasons.