Cerebral ischemia, i.e. oxygen deprivation due to lack of blood circulation after a stroke or during childbirth (asphyxia), causes great suffering for patients and their families and high costs to society. Brain cells in the ischemic tissue die and their functions are lost. Depending on the area of the brain affected, this can lead to difficulty controlling voluntary movements, loss of sensation, language and swallowing difficulties, epilepsy and more. Birth asphyxia with acute brain damage is an important cause of cerebral palsy. There is currently no effective way to repair damaged brain tissue and current research focuses on reducing the damage and improving functional recovery by stimulating the brain's plasticity that underpins our ability to learn and adapt. The complement system is part of the innate immune system and is best known for protecting us from dangerous bacteria. However, the complement system also has other important functions, not least in the brain. We have discovered that the complement system can protect brain cells from ischemia and stimulate the regeneration of neurons after a stroke. Our results show that treatment with a specific complement peptide increases brain plasticity and improves functional recovery after experimental stroke and birth asphyxia. Our goal is to build on these findings to develop new effective treatments for newborn children and adults with ischemic brain damage.