The systemic immune system has the ability to modulate multiple brain functions, including autonomic responses, glial reactivity following neural injuries, and neuronal excitability. Immune stimuli also influence microglia subpopulations originating from blood progenitors, and neuroprotective and reparative capacities of blood-derived microglia were recently described in mouse models of spinal cord injury and brain disorders. Furthermore, reparative roles for various immune subsets have been recognized, such as in inducing myelin repair. Nonetheless, uncontrolled and excessive activation of immune responses can be detrimental. The development of strategies to stimulate the systemic immune system safely to protect or repair brain disorders remains a major challenge ahead, but important inroads have been made. We discuss here some of the mechanisms underlying the neuroprotective and reparative effects of the systemic immune system and the most promising immunotherapies tested in mouse models of injuries and diseases, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis.
