In Parkinson’s disease, research has found that CD4 cytotoxic T lymphocytes (CTLs) are recruited by SPP1 and secrete the cytokine IFNG, thereby activating endothelial cells (ECs), disrupting the blood-brain barrier (BBB), promoting the recruitment of immune cells, and ultimately advancing the progression of Parkinson’s disease (PD). Here, CD4 is linked to Parkinson disease.