Sentence:  The small amount of insoluble material which precipitated during dialysis was suspended in approximately 5 ml of starting buffer , centrifuged , resuspended in 2.5 ml of isotonic saline and tested for <WSD>antibody</WSD> activity . 
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