It is unlikely that medications explain the difference in blood cell counts because both medication status and amount of medication have no more than a weak association with blood cell counts in PD subjects (Figure 5): when relating the amount of Levodopa (mg per day) to blood cell counts in PD subjects, we only found a weak marginally significant correlation with CD4+ T cells (r=−0.14, p=0.017 in Caucasians, Figure 5G). Here, CD4 is linked to Parkinson disease.