Low or high T-cells could indicate different problems affecting your immune system. 

High T-cells are a less common occurrence than having low T-cells and usually indicates that your body has increased T-cell production to fight an infection. It may also indicate the development of certain cancers.  

Some of the conditions that cause the body to increase production of T-cells include acute lymphocytic leukemia, multiple myeloma, infectious mononucleosis, and certain genetic disorders, such as in autoimmune lymphoproliferative syndrome.