The researchers studied helper T cells, a key component of the cellular immune system. When the supply of these cells dwindles, as in AIDS patients, the consequences are dire. Helper T cells patrol the body, checking for harmful microbes. Other parts of the immune system blow an invader's cover by posting fragments of its proteins on its surface or on the surface of a cell where it's hiding out. If the plate is foreign, they make the appropriate response.
Helper cells use receptors on their surface to read antigenic displays.
But instead of getting close enough to get a really good look, T cell receptors interact only weakly with antigens. Therefore scientists have wondered how the cells can respond so specifically to enormous numbers of antigens they've never seen before. The T cell receptor is a large collection of proteins. Those that stick out from the cell read the antigen and prompt inner parts to activate the cell. Scientists have suspected for some time that the activating signal involves the addition of phosphate groups to two long receptor components called zeta chains.