Basophil

A basophil is a leukocyte, or white blood cell, which plays a key role in immune response, especially in inflammation. Basophils are circulating blood cells, to which mast cells, in tissue, have a comparable function. Basophils are to mast cells as circulating macrophages are to tissue macrophages.

Basophils and mast cells are sensitized, to specific foreign triggers, by specific immunoglobulin E proteins. Triggering the cell causes the release of physiologically active chemicals, especially histamine. They also release leukotrienes, platelet aggregation factor, eosinophil chemotactic factor-A, and a basophil kallikrein of anaphylaxis, BK-A.

These released mediators are responsible both for defense, but also diseases of immunity and inflammation, such as asthma and anaphylactic shock.

Morphology
Leukocytes break into types with obvious granules in their cytoplasm, and those that do not. Basophils are granular, with coarse dark-staining granules of variable size and stainable by basic dyes. Using Wright's stain, the most common reagent used to make different leukocyte types have a different appearance on microscopic examination, basophils have prominent blue granules. They are beautiful, but in a dark and somber way, in comparison with the cheerful red polka-dots of eosinophils.

Their nucleus, when stained, is pale, and is lobed rather than a single ovoid.

Role in disease
These cells become sensitized to a specific substance recognized by an IgE a molecule, generated by a lymphocyte, which binds to their surface high-affinity IgE receptors (FcεRI). When an antigen binds to the sensitized cells, it causes both degranulation (i.e., release of mediators stored inside the granules of the cytoplasm) and de novo synthesis of additional mediators.

Histamine, released by basophils and mast cells, is perhaps the most important factor in triggering anaphylaxis, although other products of these cells influence the process.