The brain's immune response to new Alzheimer's antibody treatments reveals concerning cellular changes that may explain why these FDA-approved drugs cause dangerous side effects in some patients. Understanding this immediate immune reaction could help clinicians better predict and manage treatment risks. Researchers used single-cell sequencing to examine brain tissue three days after injecting anti-amyloid antibodies directly into mouse brains, capturing the earliest immune responses before chronic changes develop. The antibody treatment dramatically reduced populations of mobile microglia—the brain's primary immune cells—compared to control injections. More significantly, the treatment triggered widespread alterations in chemokine signaling networks, the communication system that coordinates immune responses throughout the brain. These signaling changes affected multiple microglia subtypes, including homeostatic microglia that maintain normal brain function and disease-associated microglia that respond to pathology. Surprisingly, the altered communication patterns primarily targeted healthy homeostatic microglia rather than the disease-associated cells typically implicated in Alzheimer's progression. This finding challenges assumptions about how these therapies work and suggests they may disrupt normal brain immune surveillance. The research provides crucial mechanistic insight into why anti-amyloid immunotherapies like aducanumab and lecanemab, while effective at clearing amyloid plaques, cause brain swelling and hemorrhage in roughly 40% of patients. The rapid onset of these immune changes—within just three days—indicates that adverse events may begin much earlier than previously recognized. This cellular-level understanding could inform dosing strategies and patient monitoring protocols, potentially making these breakthrough Alzheimer's treatments safer and more widely accessible.