Hi there,

I was wondering if it's ever possible to run out of un-assigned t-lymphocyte cells in the sense that all of them could them could already be "assigned" to to certain pathogens?

Cheers.

Not really because T cells as all other cells are constantly being turned over.

agreed and in practice there is no real limit on the number of different epitopes they are"assigned"against.

I think that the input of new, naïve T cells into the lymphocyte pool from the thymus declines with age, that these cells are relatively long-lived compared to the effector and memory T cells, and that some memory T cells may actually return to a resting state. However, I see from this article - http://www.pnas.org/content/106/43/18333.full - that a “substantial naïve T cell pool is maintained even in aged animals”. I would imagine that the commitment (and number) of memory T cells will depend a lot on the type and degree of antigen exposure during your lifetime. For a review on human memory T cells see -http://www.nature.com/nri/journal/v14/n1/pdf/nri3567.pdf