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. However, the synthetic T-cell mutated, causing it to activate all of Barclay's dormant genes, including his latent introns, thereby creating Barclay's Protomorphosis Syndrome. (TNG: "Genesis")