The complement cascade and eculizumab. The complement cascade culminating in the production of the membrane attack complex that results in the lysis of the cell. The cleavage of C5 is the pivotal point of the pathway. Inherited deficiencies prior to C5 result in recurrent pyogenic infections and autoimmune disorders, whereas deficiencies after C5 have remarkably little effect except for an increased risk of infection by encapsulated organisms. The site of blockage of eculizumab is demonstrated.