The human immune system has about 2 trillion lymphocytes. About one trillion are t-cells T lymphocytes develop from precursor stem cells in fetal liver and bone marrow and differentiate into mature cell types during residence in the thymus. Mature T lymphocytes (antigen responding, response control, and response mediating cells) are present in thymus, spleen, lymph nodes, throughout skin and other lymphatic organs, and in the bloodstream.
In the recent anti-cancer success, T-cells were provided with receptors that were 100 times more effective in helping them target the cancer tumor.
Here is another online tutorial about how t-cells work
This site has a tool that shows the size of different cells
A t-cell is about 20 microns in size
Basic view of cells and receptors.
More detailed representation of t cell receptor