Source: http://www.kumc.edu/instruction/medicine/pathology/ed/keywords/kw_basophil.html
The least common leukocyte. They are very similar to mast cells, both in terms of their appearance and functions; however, they are a completely distinct cell type. Basophils and Mast Cells are important contributors to allergic responses. They are a rich source of vasoactive mediators, such as histamine, leukotrienes, and platelet activating factor.