ng foreign entering the body," von Andrian explained. "They tell the T cells what to do, how to respond. We have known about some aspects of this process from tissue culture studies, but this has never been observed before in vivo. That has largely been a black box," he said.
His group has explored the first days after a T cell encounters the one antigen it will learn to recognize, von Andrian said. During this time interval, T cells first migrate rapidly and touch every dendritic cell for only a few minutes. This initial phase lasts about eight hours. Then the T cells pick one antigen-presenting dendritic cell and sticks to it for many hours.
After one day, the T cells disengage from the dendritic cell and begin to rapidly reproduce. About three days later, they depart from the lymph node on their search-and-destroy mission.
Homing in on the target
T cells patrol the body by hitching a ride in the blood, methodically stopping at each lymph node like a night security guard making the rounds and checking the doors. There are dendritic cells in the lymph nodes, waiting to present antigens in a way the T cells can recognize.
Each T cell recognizes only one antigen, but preserves that knowledge in its progeny. On average there are 6,000 T cells circulating in the human body that recognize a given antigen. Another 6,000 recognize a different antigen, etc. The human body is estimated to contain 25-100 million distinct T cell clones, he said.
When dendritic cells capture a foreign invader and bring it to the lymph node, many T cells stop by before one is able to recognize it. "The system, while cumbersome, works because of the amazing trafficking where they circle around and look for the presence of anything that might tickle their antigen receptor," von Andrian said.
If the T cells don't recognize the antigen, they hop back in the blood stream and head for the next lymph node. T cells and B cells can live for
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Source:American Physiological Society
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