CD4 and Immunodeficiency: The total patient group was severely tryptophan depleted compared to the controls (24.36 ± 4.14 μmol/l vs. 43.57 ± 11.85 μmol/l; p < 0.0001) and the degree of tryptophan depletion correlated with the degree of immune deficiency (tryptophan vs. CD4: r = 0.341; p = 0.004).