Sentence:  In the present work whole <WSD>sera</WSD> have been fractionated by chromatography on DEAE-cellulose using single gradients similar to those described by Sober and Peterson , and certain chemical and serological properties of the fractions containing antibodies of the ABO and Rh systems have been described . 
Meaning of the word marked with <WSD> tags:  an amber, watery fluid, rich in proteins, that separates out when blood coagulates