CD28 and Parkinson disease: The CD14+ CD16+ monocyte (OR = 1.091, 95%CI: 1.027–1.158, p = 0.005), HLA DR+ NK cell in CD3− lymphocytes (OR = 1.062, 95%CI: 1.008–1.119, p = 0.023), and CD28 on activated & secreting Treg (OR = 1.043, 95%CI: 1.004–1.083, p = 0.032) were identified as being associated with an increased risk of PD.