Why do most of us only get dementia when we grow old? One suggestion is that in the young, some proteins interact with the species known to be involved in dementia preventing them from becoming toxic. This talk is interested in the behaviour of the cystatin C (HCC) protein, which normally has a useful but unrelated function, and it’s interaction with the amyloid beta peptide (Aβ) which is thought to cause Alzheimer’s disease.