There are a number of types of T cells involved in specific immunity.
T cells recognize their specific foreign protein or antigen only when it is presented on the surface of a body cell.
One type of T cell is the cytotoxic or killer T cell.  Its role is to destroy infected or cancerous body cells.
In this case a pathogen enters the cell on the left and that cell moves foreign protein to its surface.
A patrolling killer T cell recognizes and binds to that protein and releases perforin, a protein that ruptures the membrane of the infected cell.
A second type of T cell is the helper T cell.
One of the roles of a helper T cell is to activate B cells.
At the top of the screen a phagocytic white blood cell is presenting antigen from a pathogen it has engulfed.
The B cell that responds has antibodies for the same antigen that activates the helper T cell.
The T cell sends out chemical signals that affect various immune cells.
These chemicals stimulate the B cell to divide and produce a clone of antibody-releasing cells.