The monoclonal antibody WU13-15 recognizes a neoepitope on the 61 kDa complement componentC9, integrated in the terminal complement complex (TCC). The complement system is an ancientproinflammatory and microbial destruction system, that may be considered part of both the innate andadaptive immune systems. It consists of the classical, alternative, and lectin-binding pathways. Eachpathway is triggered in a distinct manner, yet all deposit C3 fragments on a target and engage acommon terminal sequence called TCC or the "membrane attack complex" (MAC). In contrast to theactivation pathways, which require enzymatic cleavage for activation, the terminal pathway relies onconformational changes induced by binding of the different subunits. TCC is composed of a complex offour complement proteins (C5b, C6, C7, and C8) which bind to the outer surface of the target plasmamembrane, and many copies of a fifth protein (C9) that hookup to one another, forming a ring in themembrane. The ring structure formed by C9 is a pore in the membrane that allows free diffusion ofmolecules in and out of the cell. If enough pores form, the cell is no longer able to survive. Themembrane attack complex is initiated when the complement protein, C5 convertase, cleaves C5 intoC5a and C5b. Binding of C6 facilitates binding of C7 which alters the conformation of the complex. Afterbinding of C8, a variable number of C9 molecules associate with the C5b678 complex, togetherconstituting TCC. The formation of TCC causes lysis of cells or can trigger a variety of cellular metabolicpathways resulting in the synthesis and release of inflammatory mediators. TCC contains neoantigensthat are absent in the individual native C9 proteins. Neoantigens are present both in the membranebound(MAC) and the fluid phase (SC5b-9) complex. TCC is present in normal human plasma andincreases upon complement activation.