Credit: Courtesy: National Institutes of Health: https://www.flickr.com/photos/nihgov/20673870162/in/album-72157656657569008/
Caption: Prolonged overactivity of killer T cells summoned to a tumor or infection site can render them useless to dispatch invaders, a cellular state immunologists call "exhaustion.” Image: Superresolution image of a group of killer T cells (green and red) surrounding a cancer cell (blue, center).