Basophils are the rarest type of white blood cell, making up less than 1% of circulating white cells. Despite their low numbers, they play important roles in allergic reactions and in protecting against parasitic infections. When activated, basophils release histamine, heparin, and other mediators that trigger the immediate hypersensitivity response.

Elevated basophils (basophilia) can be seen in allergic conditions, chronic inflammatory states, hypothyroidism, and certain myeloproliferative disorders — particularly chronic myeloid leukaemia (CML), where basophilia can be a distinctive finding. Very low basophil counts are common in acute allergic reactions and acute infections. Basophil count is reported as part of the white blood cell differential.