Histamine causes dilation and increased permeability of capillaries close to the basophil. The most important pathophysiologic mechanism of mast cell and basophil histamine release is immunologic. May manifest basophil and mast cell granules by EM. The test is commonly designated as basophil activation test (BAT). In humans, HDC is primarily expressed in mast cells and basophil granulocytes. It would most likely be classified as M0 without electron microscopic confirmation of basophil lineage. CD200. Herpesvirus-6, herpesvirus-7, and herpesvirus-8 produce a CD200 homolog which also inhibits basophil function. Chronic hemolytic anemia and infectious diseases such as smallpox also demonstrate elevated basophil levels. Characteristically, basophil (e.g. CD11b, CD123) and monocyte markers (CD14, CD15) are absent.