"These diagrams of cell counts show how immune cell profiles differ between young mice and old mice. In particular, older mice have more cells that are implicated in runaway immune responses (neutrophils, orange), fewer ""helper"" and ""killer"" T-cells that can be trained to go after new threats (CD4 naive, purple; CD8 naive, pink), and fewer cells that clear away both viruses and inflammation (Alveolar macrophages (Mϕ) green). These trends carry over to humans, beginning to explain why older patients with respiratory viral infections such as influenza and COVID-19 are more likely to experience excessive and harmful inflammation. Credit: Wen group, University of Michigan."