DHFR and influenza: MTX—which inhibits dihydrofolate reductase, prevents the reduction of dihydrobiopterin (BH2) to tetrahydrobiopterin (BH4) and increases the sensitivity of T cells to apoptosis (diminishing immune responses)—is the drug that has been most extensively studied in the literature and it has been associated with a reduction in the humoral response generated by influenza and pneumococcal vaccines [6].