T-cells are white blood cells that can be taught to recognize foreign invaders such as bacteria, viruses, and even cancer cells. "T" is an abbreviation for thymus, which is one organ where these cells develop and mature. T-cells also can be formed in the bone marrow, the soft tissue inside bone cavities. Before these bone-marrow-derived T-cells mature, they move to the thymus too.
Once T-cells have been taught to recognize a particular invader, such as a certain strain of bacteria or virus, they have a memory of that invader. If that same invader makes its way into the body again, it can be destroyed more quickly than if it hadn't been seen before.
T-cells come in two general types, the helper T-cell and the killer T-cell. Helper T-cells primarily focus on activating B-cells and killer T-cells. The helper T-cell itself is first activated when a macrophage or dendritic cell brings information from a captured foreign invader, such as bacteria or viruses. Once activated, the helper T-cell divides to create more of itself and then produces substances to activate the B- and killer T-cells.
Killer T-cells attack cells of the body that have been infected with bacteria or viruses. The killer T will search each cell it encounters and if it senses infection, it will destroy the cell.