Sentence:  The cells were washed three times with saline , anti human <WSD>serum</WSD> was added , the cells were resuspended , and the mixture was centrifuged at 1000 rpm for 1 min and examined for agglutination . 
Meaning of the word marked with <WSD> tags:  an amber, watery fluid, rich in proteins, that separates out when blood coagulates