Basophils are the rarest white blood cells (less than 1% of leukocytes) and hardly ever appear in blood smears because they only circulate in the bloodstream briefly before migrating into tissues.

Similar in size to both Neutrophils and Eosinophils, Basophils have a diameter of 14-16 μm fourteen to sixteen micrometers and like these other leukocytes are also granulocytes, they possess protein-containing granules in their cytoplasm. The high density of acid-containing granules makes Basophils stain dark purple in the presence of basic dye (hence their name, and a useful way to tell them apart from Eosinophils which stain bright red). The granules are often so densely packed into the cell that they obscure the bi-lobed, irregularly shaped nucleus from view completely.

Broadly, Basophils’ roles within the immune system are linked to inflammatory responses and allergic reactions:

  • Mediating allergic reactions in response to allergens through the release of chemicals such as histamine from granules.

  • Defense against parasites through the release of anti-parasitic molecules.

  • Production and release of cytokines that stimulate other immune cells.

  • Regulation of clotting through the release of molecules such as heparin.

An illustration of a basophil. A round cell containing a bi-lobed nucleus united by a string. Its cytoplasm contains a high number of granules, which are depicted as small spheres.

Figure 1. Basophil

Reference: Murphy, K.M. and Weaver, C. (2016) Janeway’s Immunobiology: Ninth International Student Edition. W.W. Norton & Company.