Basophilia

Excerpt

Basophilia refers to an increased number of basophils, a type of leukocyte, in the blood. Basophils are typically the least numerous myeloid cells in a peripheral blood smear (see Image. Monocyte, Neutrophil, and Basophil). These cells' numerous dark azurophilic granules easily distinguish them from other white blood cells. Basophilia is not a frequent finding in peripheral blood. This condition most commonly manifests as a reactive mechanism associated with eosinophilia and an absolute basophil count exceeding 200 cells/µL. Different ranges are set depending on the laboratory and the local population.

If performed, bone marrow aspirates may show an increase in basophils or precursors. Basophils express cluster of differentiation (CD) 45 and are positive for CD13, CD11b, and CD33 myeloid markers. These leukocytes also express CD22 (also positive in B-cells), bright CD38, and bright CD123. CD203c is a distinct marker specific to basophils, frequently utilized to distinguish basophils from other white blood cells, such as mast cells, eosinophils, and neutrophils.

Basophils originate from a cell type similar to a myeloblast. The earliest identifiable stage is a basophil myelocyte, which exhibits the characteristic basophil granules. These granules, measuring approximately 0.2 to 1 μm in diameter, are larger than the azurophilic granules found in promyelocytes and are often irregular. As the cell develops, the granules become more metachromatic (red-purple) due to an increasing content of acid mucopolysaccharides, particularly heparin. During maturation, the cytoplasmic ribonucleic acid (RNA) decreases, and the nucleus partially segments. Stages analogous to those observed in neutrophils are not easily distinguishable due to incomplete nuclear segmentation. The nucleus of mature basophils condenses but smudges chromatin, and the background cytoplasm lacks basophilia, which marks residual RNA.

Basophils have a lifespan comparable to eosinophils. The maturation process in the bone marrow takes approximately 7 days. Basophils circulate in the bloodstream and are not typically found in tissues. Several cytokines influence basophil production, including granulocyte-macrophage colony-stimulating factor, interleukin (IL)-3, and IL-5. However, IL-3 is the primary growth factor for basophil development and proliferation.

Basophil granules comprise several key components, including histamine, heparin, and peroxidase. Basophils can synthesize and store histamine and eosinophil chemotactic factors of anaphylaxis (ECF-A). These leukocytes also synthesize and release slow-reacting substances of anaphylaxis (SRS-A) and, likely, platelet-activating factor (PAF) upon stimulation but do not store these substances. Notably, as demonstrated by cytochemical analysis, basophils lack significant amounts of certain hydrolytic enzymes, such as alkaline and acid phosphatases.

Basophils (as well as mast cells) appear to be involved in immediate hypersensitivity reactions, such as allergic asthma. Immunoglobulin E (IgE) binds readily to basophil and mast cell membranes. Degranulation occurs when a specific antigen reacts with the membrane-bound IgE and releases mediators of immediate hypersensitivity (eg, histamine, SRS-A, PAF, heparin, ECF-A). Basophils are also involved in some delayed hypersensitivity reactions or cutaneous basophil hypersensitivity, such as contact allergies, where they appear to undergo a different degranulation response.