Is it possible for us to have smaller cells than red blood cells that we don’t now?

Question: Is it possible for us to have smaller cells than red blood cells that we don't now?
- Keywords:
  cell,
  red blood cells,
  smaller
Asked by Peter to Carmen on 18 Mar 2015.
- Carmen Denman answered on 18 Mar 2015:
  
  Hi Peter,
  Thanks to our awesome microscopes these days we know a lot about the cells in our body, and all sorts can be smaller that RBCs. But that isn’t to say we know everything yet! 🙂

Question: Is it possible for us to have smaller cells than red blood cells that we don't now?