Sentence:  There appeared to be no difference in the distribution of anti - A and anti - B activity in group O serum , though in two group O donors ( J. F. and E. M. ) only one type of agglutinin was found in the regions of low anionic <WSD>binding</WSD> capacity ( Figs. 1 and 2 ) . 
Meaning of the word marked with <WSD> tags:  stick to firmly