Sentence:  In fact agglutination of * * f <WSD>cells</WSD> in saline could be produced by the insoluble material from sera containing `` only '' incomplete antibody activity . 
Meaning of the word marked with <WSD> tags:  (biology) the basic structural and functional unit of all organisms; they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals