Quantification of complement C5b-9 binding to cells by flow cytometry

Interaction of the complement system, directly or indirectly (e.g., via antibodies), with cells activates the early and late complement components and culminates in the deposition of a membrane-spanning C5b-9 complex on the cell surface. At a high copy number, this C5b-9 will activate cell death, whereas at a low copy number, it will transmit various signals into cells. Quantification of C5b-9 deposition is useful for assessments of the capacity of cells and antibodies to activate complement. By using an antibody that identifies a novel antigen of the C5b-9 complex, the amount of C5b-9 complexes on cells can be quantified by flow cytometry. The detailed protocol is described in this chapter.