Sentence:  In all cases the activity against * * f cells was spread over a wider <WSD>area</WSD> than that with * * f cells , regardless of the type of test ( saline , albumin , indirect Coombs ) used for comparison . 
Meaning of the word marked with <WSD> tags:  a particular geographical region of indefinite boundary (usually serving some special purpose or distinguished by its people or culture or geography)