Monoclonal antibody H4 recognizes mouse properdin. The complement system is the first line of defense against pathogens and facilitates elimination of apoptotic and damaged cells. Positive regulator plasma protein properdin is critical for the alternative pathway of complement. It is a single-chain glycoprotein (ca 53kDa) consisting of six TSR sequences. In the blood it exists as a mixture of preferably head-to-tail trimers, but also dimers and tetramers. The protein is produced by leukocytes, like activated neutrophils monocytes and T-lymphocytes, but also by eg. stressed endothelial cells.Properdin can both initiate and positively regulate the alternative pathway activity together with C3 and factors B, D, I and H. It binds to C3b where It stabilizes the labile C3bBb convertase which is deposited on immune complexes or foreign surfaces. Thereby enhancing the AP by stimulation of amplification of C3bBb-convertase formation in competition with catabolism of C3b by factor I, which uses factor H as a cofactor. The local amplification process leads to the creation of the alternative pathway C5 convertase, C3bBb3b, and initiates the terminal pathway of complement activation. The alternative pathway may account for ca 80% of the terminal pathway activity. Properdin has also been shown to directly limit factor H activity.Recent studies show that properdin is also a pattern-recognition receptor (PRR) able to bind directly to microbial surfaces, apoptotic and necrotic cells (dangerous nonself and altered self). Inappropriate activation or dysregulation of the alternative pathway is a critical factor in development of several autoimmune conditions. Targets opsonized with properdin are labeled for clearance by scavenger cells, even without complement. This makes it a potential therapeutic target in diseases. Recent studies has shown renewed interest in the evaluating role of properdin in disease pathogenesis, like Asthma, arthritis, septic shock, AMD and C3 glomerulopathy.