Another aspect of the innate immune system the complement system. Theme of pathogen recognition in absence of antibody also is involved in complement system, and how complement system can work together with antibodies to provide for elimination of pathogens. Immunize rabbits with cholera bacteria and took serum from immunized rabbits expect to contain antibodies. Found two things: the serum caused the bacteria to form aggregates; and these aggregates seemed to form granules. Take immune serum and heat it let it cool add it to bacteria observed clumping but no lysis, no granule formation conclude heat sensitive factor involved. Then took heated immune serum, then added normal rabbit serum clumping and granule formation. Normal serum does not contain antibodies does not contain clumping. Heated immune serum alone not enough to cause lysis normal rabbit serum alone not enough to cause lysis but together, were bacteriocidal . Second factor present in normal serum heat-sensitive and required to cause lysis.