FCER1G and Alzheimer disease: However, they can traverse the blood brain barrier [242] (which is compromised in Alzheimer's disease [243]) and are also able to enter cells, essentially by hitching a ride on viruses, via high affinity IgG receptors (Fc gamma receptors) (FCER1G) in the case of the rhinovirus, or the SARS coronavirus, or via the tripartite motif protein, TRIM21, in the case of adenoviruses, where they are able to activate an intracellular immune attack.