Sentence:  The specific staining by both direct and indirect methods showed that WTV <WSD>antigen</WSD> was concentrated in the pseudophloem tissue and in a few thick walled cells in the xylem region , but was not detectable in any other tissues of the root and stem tumors . 
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)