Sentence:  In the present work whole sera 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 <WSD>systems</WSD> have been described . 
Meaning of the word marked with <WSD> tags:  a group of independent but interrelated elements comprising a unified whole