Basophils vary in diameter from 12 μm to 15 μm but are usually slightly
smaller than neutrophils. Their nuclei are less heterochromatic than
other granulocytes and usually consist of 3 irregular lobes which are
often obscured by the large, dark-staining cytoplasmic granules. The
specific granules of basophils are their most characteristic feature.
These granules have irregular shapes and vary in size; the largest are
the size of the specific granules of eosinophils,
the smallest nearly as small as those of neutrophils. The granules
stain metachromatically and appear reddish-violet to nearly black in
stained blood smears.
The specific granules of basophils (like those of
the mast cells of connective tissue) contain heparin and histamine,
which may be released by exocytosis in response to certain types of
antigenic stimuli. The granules may contain inclusions, but they appear
more homogeneously electron-dense than do those of eosinophils.
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