Leukotriene B4 (LTB4) is a potent lipid mediator of allergic and inflammatory reactions, as well as a modulator of immune responses. Two types of plasma membrane receptors for LTB4 have been described on human neutrophils. The high-affinity human leukocyte LTB4 receptor, BLT1 (or LTB4R) mediates aggregation, chemotaxis, chemokinesis, and increased adherence to surfaces, whereas the low-affinity receptor mediates degranulation and increased oxidative metabolism. BLT1 mRNA is expressed in leukocytes and to a lesser degree in spleen and thymus. Studies in mice with disrupted BLT1 gene indicate a major role for BLT1 in acute inflammation and immediate hypersensitivity, as well as in leukocyte functions such as chemotaxis and firm adhesion to endothelium in response to LTB4.