Costimulation. In some immune responses, a B cell or T cell becomes activated when an antigen or nonself cell binds to it. Activation then initiates proliferation. In most immune responses, however, activation requires the presence of a costimulator. Cytokines, released by helper T cells and APCs, act as costimulators.
Similarly one may ask, how are killer T cells activated?
When the perfectly shaped virus antigen on an infected cell fits into the Killer T-cell receptor, the T-cell releases perforin and cytotoxins. Perforin first makes a pore, or hole, in membrane of the infected cell. Cytotoxins go directly inside the cell through this pore, destroying it and any viruses inside.
What stimulates T cells?
An antigen is a molecule capable of stimulating an immune response, and is often produced by cancer cells or viruses. Antigens inside a cell are bound to class I MHC molecules, and brought to the surface of the cell by the class I MHC molecule, where they can be recognized by the T cell.