CD40 and COVID-19: Of note, cluster of differentiation 40 ligand is involved in B-cell proliferation, and immunoglobulin class switching, antibody secretion, and CD40 agonists are currently being used to expand B cells for cancer immunotherapy.36 In the AILD group, the independent associations with breakthrough infection were older age and lower HDL-cholesterol levels; the association between HDL levels and susceptibility to COVID-19 has been previously observed.37