Helper T cells (TH cells) are essential in generating an immune response. They secrete signaling molecules called cytokines that activate antibody-producing B cells and other T cells (TC cells) that kill infected cells. Helper T cells thus play a key role coordinating the activity of these cell types in their response to both pathogens and vaccines. In fact, vaccines don't even work in the absence of TH cells, so making sure to activate them has been a priority in vaccine development.
But it is not really clear exactly how helper T cells contribute to immune protection. In order to try to figure that out, researchers made a vaccine that activated only TH cells (and not any other immune cells), immunized mice with it, and checked to see if the vaccine could protect the mice from infection. It didn't. Instead, it killed them.
When the mice were challenged with the virus they were immunized against, their helper T cells went completely berserk. The mice suffered from twenty percent weight loss, "catastrophic inflammation," and "immunopathology ... associated with a cytokine storm, generalized inflammation, and multi-organ system failure." As if that weren't enough, they also experienced hemorrhaging and "severe tissue destruction." Then ninety percent of them died. Mice treated with the standard vaccine averaged a ten percent weight loss, and then were fine.
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