The body responds much more rapidly to harmful invaders than previously thought, say UK researchers.
Dendrites detected bacteria within minutes
The Dundee University team uncovered a rapid phase of the immune response involving dendritic cells.
These cells became activated within minutes of sensing the presence of invaders, alerting other cells that the pathogen is friendly or dangerous.
The authors told Science the discovery could help scientists fine-tune vaccines to make them more powerful.
Scientists have been interested in dendritic cells for many years because they are one of the key cells involved with the body's defence against infection.
These sentinel-like cells are necessary for the body to be able to recognise and react to attack from viruses and bacteria.
Professor Colin Watts and his team looked in the laboratory at what happened when dendritric cells came into contact with microorganisms.
They found the dendritic cells sensed the presence of invading germs like viruses and bacteria with receptors on their surface called Toll Like Receptors (TLR).
Stimulating these receptors causes the dendritric cells to migrate out of the tissues that they have been patrolling to the lymph nodes, carrying with them fragments of the invader as evidence of the attack.
They did this by rapidly reorganising their internal structure, which allowed them to capture the foreign material and make it visible to other immune cells in the lymph nodes, called T cells, which could then attack the intruder.
Professor Watts said: "What amazed us was how quickly dendritic cells can respond to signals from pathogens.
"They respond damn quickly - within minutes," he said.
The whole process lasted only an hour or so, which is very short compared with the two-day time scale that most previous studies have shown.
"I believe we have discovered a new and very early chapter in the life story of these remarkable cells," said Professor Watts.
He said this could lead to new treatment avenues in the future.
"This understanding means we are better placed to manipulate the immune system."
For example, making vaccines more effective and turning off the immune system when it goes wrong and attacks the body in autoimmune diseases like rheumatoid arthritis, he said.
"We might be able to include in vaccines things that stimulate this dendritic response, something that says 'take this up quickly.'"
He said the next step was to see if the same happened with these cells in living tissue.
Professor Tim Elliott, an expert in immunity and cancer at the University of Southampton, said: "One puzzle in this series of events has been that stimulation of TLR was generally thought to switch off the ability of dendritic cells to acquire pieces of the invading germs to present to T cells which, if true, would make them less effective at raising the alarm.
"This study solves the puzzle," he said.
Very soon after dendritic cells encounter germs via their TLR the process of acquiring pieces of the germ is actually increased before it is switched off by the time the dendritic cell reaches a lymph node, he said.
"This information could be used to enhance the potency of vaccines against germs or even tumours if, for example, we could find a way to deliver them along with molecules that stimulate TLR to dendritic cells in the skin," he said.