system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a limit to the amount of diseases we can be immune to? Can our collection of memory T-cells run out of memory?

Say one was given a viable vaccine regimen to all of the current diseases in the world, and no new strains/diseases arose. Would one be able to remain immune to all disease in the world?

From my understanding, the majority of the memory T-cells in our body are naive, not yet exposed to pathogen. However, would the amount of memory T-cells in our body be able to efficiently maintain a database of all known diseases and maintain immunity?
gpt
This is a very complicated question, or at least the answer is very complicated; there's a great deal of ongoing research trying to understand the boundaries.

A short and simplified answer is that immune memory is not infinite. To some extent, generating memory to new antigens takes a bite out of the pre-existing memory pool.  

The complications start to come in when you ask how large the memory pool can be, how much of a bite gets taken out of it, how small individual memories can be and still be effective, and so on -- that is, how much practical impact this has. 

We know that elderly people can form new immune memories, and also that they maintain memory responses from their childhood.  (A paper a few years ago argued that people in the 2000s still retained immune memory to the 1918 influenza, 80-odd years after exposure.) So normal living, which entails ongoing exposure to lots of antigens, doesn't completely blow away all the old memory pools.

On the other hand, experiments in mice do show new memories pushing away some old memories. (I said this was complicated: Sometimes new memories enhance the old ones, too.) 

So there are upper and lower boundaries established -- it can happen, it doesn't always happen.  In a few years we might have a clear idea of what normally happens, and how biologically relevant this is.