Sentence:  In all cases the activity against * * f cells was spread over a wider area than that with * * f <WSD>cells</WSD> , regardless of the type of test ( saline , albumin , indirect Coombs ) used for comparison . 
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