There are 2 main types of T-cells. They are classified based on their "CD" gene, which determines their function. The two types are CD4 or helper T-cells, and CD8 or killer T-cells.
When a CD4 T-cell's T-cell receptor (TCR) binds with it's antigen from a antigen presenting cell containing class 2 MHC, the T-cell activates macrophages and B-cells to find and destroy the original pathogen.
When a CD8 T-cell's TCR binds to an antigen presented by a cell displaying class 1 MHC, the T-cell binds to the presenting cell and destroys it. This is useful when normal body cells are invaded by viruses or become cancerous and display abnormal antigens which indicate they are not healthy cells.
When a CD4 T-cell's T-cell receptor (TCR) binds with it's antigen from a antigen presenting cell containing class 2 MHC, the T-cell activates macrophages and B-cells to find and destroy the original pathogen.
When a CD8 T-cell's TCR binds to an antigen presented by a cell displaying class 1 MHC, the T-cell binds to the presenting cell and destroys it. This is useful when normal body cells are invaded by viruses or become cancerous and display abnormal antigens which indicate they are not healthy cells.