CD33 and Alzheimer disease: However, studies in humans showed that the CD33 AD-risk allele is linked to higher expression of CD33 on monocytes, and an expression quantitative trait locus (eQTL) study in patients with autoimmune or neurodegenerative diseases revealed that the AD susceptibility alleles are significant eQTLs only in monocytes, suggesting an involvement of this cell type in human AD pathology20,21.