The complement system, where C4 resides, is known to play a key role in synaptic pruning in the brain, which occurs with normal aging.

While they may be good at making people sleepy, it's also possible that they may interfere with the pruning required for forming memories.

Although C4 had long been known for its role in the immune system, the researchers found that it was also instructing this process of synaptic pruning.