basophil

Basophils are granulocytic leukocytes derived from myeloid progenitors in the bone marrow. Morphologically, they have a bilobed nucleus often obscured by large, coarse, deeply basophilic (purple-blue) granules on Wright-Giemsa stain. These granules contain histamine, heparin, leukotrienes (especially LTC4), and platelet-activating factor.

Functionally, basophils play a key role in type I hypersensitivity reactions (immediate hypersensitivity). Upon antigen cross-linking of IgE bound to FcεRI receptors on their surface, basophils degranulate, releasing histamine and other mediators that cause vasodilation, increased vascular permeability, bronchoconstriction, and smooth muscle contraction—contributing to symptoms of allergy, anaphylaxis, and asthma.

Basophils are also important in the response to parasitic infections, working alongside eosinophils and mast cells, and secrete IL-4 and IL-13, which promote Th2 differentiation and IgE class switching in B cells.

Clinical significance: - Basophilia can be seen in chronic myeloid leukemia (CML), a hallmark finding along with the Philadelphia chromosome (BCR-ABL fusion). - Basophilia may also occur in myeloproliferative disorders, hypothyroidism, and chronic inflammatory conditions. - Basopenia can be seen with acute infections, hyperthyroidism, and stress reactions (glucocorticoid effect). - Basophils are distinct from mast cells: mast cells reside in tissue and have a different developmental lineage, while basophils circulate in blood and migrate to tissue during inflammation.

High-yield board points: Remember basophils are associated with CML (elevated basophils is a key differentiating feature from a leukemoid reaction), and their role in IgE-mediated hypersensitivity reactions parallels mast cell function.