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The page below is a sample from the LabCE course White Cell and Platelet Disorders: Peripheral Blood Clues to Nonneoplastic Conditions. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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A basophil and a small lymphocyte are compared in the same field in the upper image, while a single basophil is shown in the lower image.
The cytoplasmic granules of the basophil are larger than the granules of toxic granulation. They contain chemical mediators of immediate hypersensitivity, and are found in the cytoplasm and overlying the nucleus (better seen in the lower image). Basophilic granules stain metachromatically with toluidine blue indicating the presence of acid mucopolysaccharide or proteoglycans, both thought to be heparin or heparin-like substances.
Basophils are related to mast cells (tissue basophils), each involved in hypersensitivity responses and following anaphylactic episodes. Under the stimulation of complement components C3a and C5a, many mediators are released from the basophil granules, including histamine, heparin, and eosinophil chemotactic factors of anaphylaxis, or ECF-A.
Basophils are the least common granulocytes in the peripheral blood, comprising 2% or less of the differential count. The presence of large granules of irregular size in basophils and the admixture of eosinophilic granules may indicate dysplastic changes associated with myelodysplastic disorders and leukemia. Basophils may be increased in:
  • Myeloproliferative disorders
  • Chronic metabolic conditions
    • Myxedema
    • Diabetes mellitus
  • Hypersensitivity responses and allergic reactions
  • Tuberculosis
  • Varicella infection
  • Collagen vascular disease