Sentence:  This was later known to be the result of concentrating the minute amount of complete antibody found in these sera ; when the insoluble fraction was suspended in a volume of saline equal to that of the original serum sample , no complete <WSD>antibody</WSD> activity could be detected . 
Meaning of the word marked with <WSD> tags:  any of a large variety of proteins normally present in the body or produced in response to an antigen which it neutralizes, thus producing an immune response