A T-cell was a type of blood cell. They had various roles in the immune system.
In 2154, Doctor Arik Soong showed Jonathan Archer a DNA sequence for the modification of the Human T cell, guaranteed to make Sharat Syndrome history. According to Soong, his solution would be destroyed before he could test it. (ENT: "Borderland")
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")
Edit