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In the article, there is a statement which is:

Although DCs are remarkably efficient in evoking T cell responses with few antigen– MHC complexes (1–100 per DC) (1–3), they must first encounter a T cell with appropriate antigen specificity (one in 105 to 106).

What does the part "Although DCs are remarkably efficient in evoking T cell responses with few antigen– MHC complexes (1–100 per DC) (1–3)" mean? What is the significance of the value "1–100 per DC"?

What does evoking T cell responses with few antigen–MHC complexes mean?

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    $\begingroup$ I've replaced your title by something meaningful (in the light of @PeterCollngridge answer). I appreciate it's difficult to formulate a good title if you don't understand the meaning, but titles are important in allowing people to select questions of interest and for indexing. $\endgroup$ – David Feb 9 '17 at 13:40
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It means that a dendritic cell only needs between 1 and 100 antigen–MHC complexes on its surface in order to activate a T cell.

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A dendritic cell uses an extrinsic pathway to take whole foreign peptides and cut them into segments of 14-20+ residues that are loaded onto MHC-II molecules. The MHC-II interaction with the T cell receptor is what tells the T cell to activate along with some costimulatory signals from the DC.

The dendritic cell doesn't know how to cut the peptides, though. So when it engulfs a bacterial protein for example, and cuts it up, and presents these extracellularly to T cells, no single peptide loaded on MHC-II are exactly the same. There could be thousands of potential antigens.

T cells on the other hand are specific: Every single T cell receptor on a single T cell clone is specific for the same antigen. It's encoded in their genome during VDJ recombination! So the DC is only efficient in that they probably only need 100 or less unique peptide presentations to find a T cell specific for one of those antigens, which is quoted to be 1:100,000 to 1:1,000,000.

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