Cytotoxic CD4+ T Cells in Aging, Both Protective and Harmful

The immune system is very complex, and immune cells exist in a continuum of states rather than being separated into clearly demarcated pigeonholes of behavior. Many of the categorizations applied to immune cells, largely based on cell surface marker differences, are conceptually useful, but also greatly oversimplify a complex reality. Sometimes one has little need of the deeper details, and it is…