FIGURE 10.9 • Electron micrograph of a human eosinophil. The nucleus is bilobed, but the connecting segment is not within the plane of section. The granules are of moderate size, compared with those of the basophil, and show a crystalline body (Cr) within the less electron-dense matrix of the granule. M, mitochondria. 26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic image of an eosinophil from a blood smear. 1,800.
tract and at other sites of potential chronic inﬂammation (i.e., lung tissues in patients with asthma).
Basophils are about the same size as neutrophils and are so named because the numerous large granules in their cytoplasm stain with basic dyes (Plate 17, page 302).
Basophils are the least numerous of the WBCs, accounting for less than 0.5% of total leukocytes.
Often, several hundred WBCs must be examined in a blood smear before one basophil is found. The lobed basophil nucleus is usually obscured by the granules in stained blood smears, but its characteristics are evident in electron micro-graphs (Fig. 10.10). Heterochromatin is chiefly in a peripheral location, and euchromatin is chiefly centrally located; typical cytoplasmic organelles are sparse. The basophil plasma membrane possesses numerous high-affinity Fc receptors for IgE antibodies. In addition, a specific 39-kilodalton protein called CD40L is expressed on the basophil’s surface. CD40L interacts with a complementary receptor (CD40) on B lymphocytes, which results in increased synthesis of IgE.
FIGURE 10.10 • Electron micrograph of a human basophil. The nucleus appears as three separate bodies; the connecting strands are not in the plane of section. The basophil granules (B) are very large and irregularly shaped. Some granules reveal myelin figures (MF ). M, mitochondria. 26,000. (Courtesy of Dr. Dorothea Zucker-Franklin.) Inset. Light microscopic appearance of a basophil from a blood smear. 1,800.
The basophil cytoplasm contains two types of granules: specific granules, which are larger than the specific granules of the neutrophil, and nonspecific azurophilic granules.
 Specific granules exhibit a grainy texture and myelin figures when viewed with the TEM. These granules contain a variety of substances, namely, heparin, histamine, heparan sulfate, leukotrienes, IL-4, and IL-13. Heparin, a sulfated glycosaminoglycan, is an anticoagulant. Histamine and heparan sulfate are vasoactive agents that among other actions cause dilation of small blood vessels. Leukotrienes are modified lipids that trigger prolonged constriction of smooth muscles in the pulmonary airways (see page 186). Interleukin-4 (IL-4) and interleukin-13 (IL-13) promote synthesis of IgE antibodies. The intense basophilia of these specific granules correlates with the high concentration of sulfates within the glycosaminoglycan molecules of heparin and heparan sulfate.
 Azurophilic granules are the lysosomes of basophils and contain a variety of the usual lysosomal acid hydrolases that are similar to those in other leukocytes.
The function of basophils is closely related to that of mast cells.