basophils

Basophils are a type of white blood cell and one of the granulocytes. They arise from the myeloid lineage and account for roughly 0.5 to 1 percent of circulating leukocytes. Named for their basophilic staining properties with basic dyes, basophils are the least abundant leukocytes in peripheral blood. Morphologically they are small cells with a lobed nucleus and cytoplasm that contains large granules.

The granules are rich in histamine, heparin, and proteases, and they release these mediators during activation. Activation most commonly occurs when the high-affinity IgE receptor FcεRI on basophils binds allergen-bound IgE, triggering degranulation and the synthesis of lipid mediators such as prostaglandin D2 and leukotrienes C4, D4, and E4. Basophils also secrete cytokines, including interleukins 4 and 13, which promote T helper 2 responses and IgE production.

Functionally, basophils contribute to defense against parasites and play a central role in allergic inflammation and anaphylaxis. They communicate with other immune cells and can amplify responses through mediator release and cytokine signaling. Although they are present in low numbers, their rapid response can influence vascular permeability, smooth muscle tone, and recruitment of other leukocytes.

Clinical relevance for basophils includes basophilia, an elevated basophil count seen in certain myeloproliferative disorders and during severe allergic reactions; basopenia is less commonly discussed. In the laboratory, basophil activation can be assessed by upregulation of surface markers such as CD63 or CD203c.