Sentence:  Many investigators feel that the double requirement for the antigen-recognition by cytotoxic T cells or DTH-reactive T cells may provide a <WSD>handle</WSD> for solving the T-cell receptor puzzle, and that anti-Id reagents are to be used in this approach. 
Meaning of the word marked with <WSD> tags:  An instrument for effecting a purpose (either literally or figuratively); a tool, or an opportunity or pretext.