A T-cell is a type of blood cell. They have various roles in the immune system.
In 2370 while attempting to cure Lieutenant Reginald Barclay of Urodelan flu Dr. Beverly Crusher used a synthetic T-cell to activate the gene that would fight off the infection. The result was the mutation and activation of Barclay's dormant genes, including his latent introns, thereby creating Barclay's Protomorphosis Syndrome. (TNG: "Genesis")