Sentence:  There appeared to be no difference in the distribution of anti - A and anti - B activity in group O <WSD>serum</WSD> , though in two group O donors ( J. F. and E. M. ) only one type of agglutinin was found in the regions of low anionic binding capacity ( Figs. 1 and 2 ) . 
Meaning of the word marked with <WSD> tags:  an amber, watery fluid, rich in proteins, that separates out when blood coagulates