The purpose of the immune system is to defend the host from constantly changing microbial pathogens. Autoimmune diseases develop as a consequence of the production of antibodies and/or cells that react with self-antigens, and may recruit other effector mechanisms that result in tissue damage. Thus, in this context, autoimmunity represents an immune response to self-antigens that is sufficient to cause disease. This article is specifically devoted to autoantibodies directed against complement components.