Sentence:  This was later known to be the result of concentrating the minute amount of complete antibody found in these <WSD>sera</WSD> ; when the insoluble fraction was suspended in a volume of saline equal to that of the original serum sample , no complete antibody activity could be detected . 
Meaning of the word marked with <WSD> tags:  an amber, watery fluid, rich in proteins, that separates out when blood coagulates