Sentence:  The concentration and apparent localization of the WTV <WSD>antigen</WSD> in pseudophloem tissue of the tumor may indicate that the virus preferentially multiplies in the phloem and may need to be directly placed in this tissue in order to infect plants . 
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)