When someone proposes a theory, what are they doing?

Experimenting

In these instances,the Immune System uses cell-mediated immunity to destroy infected body cells.

T Cells are responsible for cell-mediated immunity,they assist with the elimination of infected body cells by releasing toxins into them and promoting apoptosis.

T Cells,are also called T Lymphocyte,type of leukocyte (White Blood Cell) that is an essential part of the immune system.