Basophils, or basophil granulocytes, are rare granulocytes. If you collected 1000 white blood cells, only 13 of them would be basophils. Their nucleus is hidden by granules which turn dark blue in color when stained. Basophils carry histamine and heparin. They appear at the sites of ectoparasite infection, or allergies. We don't know exactly how they work.