CD4 and Parkinson disease: Additionally, we found that increased resting CD4 regulatory T cell %CD4 + T cell counts were associated with a reduction in the risk of PD (with a 1 % genetically instrumented higher resting CD4 regulatory T cell %CD4 + T cell leading to a 4 % (95 % CI 0·93, 0·99) lower risk of PD, P= 0·038) (Fig. 6).