CD33 and Alzheimer disease: Increased expression of CD33 in microglia has been observed in the AD brain, and the numbers of CD33-positive microglia were positively correlated with insoluble Aβ42 levels, which is reversed in APPSwe/PS1DE9/CD33−/− mice [87], suggesting the potential of CD33 inhibition in microglia as a treatment for AD, but the relationship between CD33 and microglia-synapse interactions remains elusive.