It's a bad clue as both types exist in the immune system - B Cells attack the invading viruses and T Cells kill the body's own infected cells to stop the spread of the infection
I reckon we should all squeeze in both "B" & "T" to force them to accept either
Once again the MOS compiler has come up with a clue to which there are two possible answers. I am of the opinion that a crossword or any sort of puzzle should have only one positive answer! I may be wrong but I have opted for B Cell.