Sentence:  The specificity of staining in WTV tumors with * * f and * * f but not with NS and * * f or with antiserum to potato yellow-dwarf virus and * * f , and the absence of such staining in crown gall tumor tissue from sweet-clover , indicate that an <WSD>antigen</WSD> of WTV was being stained . 
Meaning of the word marked with <WSD> tags:  any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies)