Basophils represent less than 1% of circulating leukocytes and are rarely found in peripheral tissues, but they are thought to play a central role in allergen- and helminth-induced immune responses. Like mast cells, basophils arise from CD34+ progenitor cells in the bone marrow, and upon activation induced by IgE-Fc epsilon RI receptor cross-linking, they degranulate and release histamine from cytoplasmic granules. Basophils are involved in defending the host against parasitic infections, but they are also thought to contribute to the pathogenesis of allergic inflammation in the skin, lungs, and intestine, anaphylaxis, and autoimmune and inflammatory disorders. In addition, it has recently been suggested that basophils may modulate B cell functions. While basophils and mast cells express a number of common cell surface markers, flow cytometry can be used to identify basophils based on their low side scatter properties along with high level expression of IL-3 R alpha/CD123 and lack of expression of CD117/c-kit. Other common basophil markers include CRTH-2, CCR3, ENPP-3/CD203c, Fc epsilon RI, Integrin alpha 2/CD49b, CD200 R3 in mouse, and the transcription factor, CEBP alpha. Basophils also display dim/variable expression of CD45, lack expression of MHC class II and lineage markers such as CD3 and CD19, and upregulate expression of ENPP-3/CD203c and CD63 following activation.