PDCD1 and COVID-19: Furthermore, a machine-learning-assisted clinical characterization of the immune response of healthy volunteers, COVID-19 patients, and long COVID patients revealed significantly elevated B cell and CD14+ CD16+ CCR5+ monocyte levels, along with reduced T regulatory cells (Treg) and programmed cell death protein 1(PD-1)-expressing CD4+ and CD8+ T cells in the long COVID group compared to healthy controls (325).