These volumes, Cytolytic Lymphocytes and Complement: Effectors of the Immune System, originate from the realization that pathways of recognition and killings of foreign targets follow similar routes in the humoral and cellular part of the immune system. In particular, the homology of immunoglobins with the T-cell-MHC-antigen receptor at the beginning of the recognition sequence and the homology of complement component C9 with lymphocyte perforin 1 (P1) as pore formers at the end of the effector sequence are striking examples.