The monoclonal antibody 20/70 recognizes the mouse receptor for C5a (CD88). The mouse anaphylatoxin complement factor C5a, a split product of C5, is one of the most potent inflammatory chemoattractants and interacts with two C51 receptors, C5aR and C5L2. Most of the C5a effects are mediated through C5aR. C5aR is known to be crucial in the initiation of acute inflammatory responses. C5a receptor is a seven transmembrane GTP-binding-protein-coupled receptor that belongs to the rhodopsin supergene family. The 45 kDa C5a receptor is expressed by leukocytes such as neutrophils, eosinophils, basophils, monocytes, dendritic cells and mast cells. C5aR can be expressed on both immune and nonimmune cells. Nonimmune cells expressing C5aR include vascular endothelial cells, cardiomyocytes and bronchial epithelial cells. Signaling of C5aR involves intracellular calcium mobilization and activation of among others protein kinase- B (PKB) and protein phospholipase D (PLD) pathways. High C5aR expression levels have been associated with the pathogenesis of many inflammatory conditions, autoimmune and neurodegenerative diseases like rheumatoid arthritis, multiple sclerosis, atherosclerosis and inflammatory bowel diseases. In vivo blockade of C5aR greatly reduces inflammatory injury. The monoclonal antibody 20/70 has been used in functional studies and is capable to prevent the binding of C5a to its receptor.