[The complement system]

After its discovery, about 100 years ago, complement was described as a cytotoxicity effector, the "armed hand" of antibodies. Since the description of an "alternate" pathway of activation, independent of antigen-antibody reactions, this restrictive concept has been modified. It has been progressively revealed to be a system that is composed of many proteins and our understanding of their biological roles has become much more developed. Nowadays, it is especially considered as a means for the body to generate peptides that regulate many functions: phagocytosis, antibody synthesis, inflammatory reactions, photosensitisation, coagulation, etc. Discovery of components, not only in the vehicles of serum or the other biological fluids but also associated with the cell membrane, has greatly increased the regions of action. Finally, progress in genetics, particularly in nucleic acid sequencing, has revealed families of proteins, a priori not apparent, that are analogues of the sequences and take part in the function of very varied areas. The roles of complement are thus not restricted only to immunological mechanisms, but they extend into many areas of biology and physiology.