Basophils are a type of white blood cell Overview of White Blood Cell Disorders White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances ( the immune system). To defend the body adequately, a sufficient... read more 
that have a role in immune surveillance Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more (such as detecting and destroying very early cancers) and wound repair. Basophils can release histamine and other mediators and play a role in the initiation of allergic reactions Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more 
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Basophils account for less than 3% of the circulating white blood cells (0 to 300 basophils per microliter of blood [0 to 0.3 × 109 per liter]).
A decrease in the number of basophils (basopenia) can occur as a response to thyrotoxicosis Hyperthyroidism Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more 
, acute hypersensitivity reactions Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more , and infections.
An increase in the number of basophils (basophilia) can occur in people with hypothyroidism Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more 
. In the myeloproliferative neoplasms Overview of Myeloproliferative Neoplasms In myeloproliferative neoplasms (myelo = bone marrow; proliferative = rapid multiplication; and neoplasm = new abnormal growth, such as a precancer or cancer), the blood-producing cells in the... read more (for example, polycythemia vera Polycythemia Vera Polycythemia vera is a myeloproliferative neoplasm of the blood-producing cells of the bone marrow that results in overproduction of all types of blood cells. Polycythemia vera is due to mutations... read more and myelofibrosis Myelofibrosis Myelofibrosis is a disorder in which fibrous tissue in the bone marrow replaces the blood-producing cells, resulting in abnormally shaped red blood cells, anemia, and an enlarged spleen. Myelofibrosis... read more ), a marked increase in the number of basophils can occur.
The symptoms usually depend on the disorder that is causing the change in the number of basophils, but increased numbers of basophils can cause itching and other allergic reactions.
The change in the number of basophils is usually detected by chance when a complete blood count is done for other reasons.
Treatment is directed at the disorder that is causing the change in the number of basophils.
