Basophils are a type of white blood cell produced in the bone marrow. They make up about 0.5% of the total number of white blood cells. Basophils can circulate in the blood and also are found outside blood vessels throughout many tissues in the body.
Basophils protect the body, killing bacteria and parasites, including external parasites such as ticks. Basophils can cause problems when they react incorrectly and cause allergies, asthma, and other inflammatory reactions in the body.
Basophils contain histamine and heparin, a blood thinning substance produced by the body. The histamine that is released by basophils (and mast cells) is the source of the symptoms of common seasonal allergies such as watery eyes, itchy skin, and runny nose. This is why antihistamines, which block the actions of histamine, are effective for reducing these symptoms.
Basophils are part of the innate immune system, which means that they can "non-specifically" destroy any invaders that they encounter in the body, such as bacteria and parasites. Non-specifically means that basophils do not have to recognize the invader specifically, but instead simply recognize the invader as something that should not be present and should be destroyed.