Benign Basophil and Mast Cell Disorders

Basophils are a type of granulocyte in the peripheral blood. They are so named because their granules stain with basic dyes. Basophils play an important role in allergic late-phase reactions and atopic disease and may bridge innate and adaptive immunity by driving T helper 2 (Th2) cell differentiation and IgE class switching in response to both antigenic and nonantigenic inputs. The normal range for basophils in the peripheral blood is only 0.5% to 1% of the total white blood cell count. Basophils are similar in size to neutrophils (10–14 μm across) with a 2- to 3-lobed or reniform nucleus that is usually obscured by coarse, basophilic (blue-black), or purple-red granules. These granules are often unevenly distributed and at times appear as a washed-out pattern. An increase in absolute basophils (usually greater than 0.2 × 109/L) is referred to as basophilia, which can be associated with reactive and neoplastic processes. We focus on reactive basophilia and distinguish it from its neoplastic counterpart. Mast cells are singly scattered throughout loose connective tissue and are especially abundant in the skin, lungs, and gastrointestinal mucosa. They are important mediators of allergic reactions and have several functions which overlap basophils, although the two cell types are distinct. Mast cells also play a variety of regenerative and protective roles in the human body. They are morphologically characterized by numerous cytoplasmic granules (metachromatic in Wright stain, eosinophilic on H&E) which partially obscure the nucleus, like basophils. However, mast cells are larger (15–30 μm across) and have a nonsegmented nucleus. Normal mast cells are round-to-oval and scattered, whereas neoplastic mast cells are often spindled and form mast cell aggregates [9]. Abnormal proliferation of mast cells can be primary, derived from a mutant hematopoietic stem cell, or secondary to infection, allergy/anaphylaxis, inflammatory conditions, medications, or paraneoplastic syndromes. Mast cells can also be increased in idiopathic disorders, such as mast cell activation syndrome.