Sentence:  The cells were washed three times with saline , anti human serum was <WSD>added</WSD> , the cells were resuspended , and the mixture was centrifuged at 1000 rpm for 1 min and examined for agglutination . 
Meaning of the word marked with <WSD> tags:  make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of