Effect of p-chlorophenylalanine and melatonin treatment on Delayed Type Hypersensitivity (DTH) response to oxazolone (in terms of ear thickness) of young adult and aged squirrels. Histogram represents Mean ± SE; n = 12 for each group. Con = Control, PCPA = para-chlorophenylalanine, PCPA + Mel = para-chlorophenylalanine + Melatonin, Mel = Melatonin. Significance of difference aged vs young adult; * = P < 0.05; ** = P, < 0.01. Significance of difference treated aged vs treated young adult; a = P < 0.05; b = P, < 0.01.