Read the passage below and then answer the following questions.
This final defense system, comprised of specialized cells and chemicals, is capable of attacking and remembering specific pathogens. At the heart of this system are white blood cells called T cells and B cells, and chemicals called antibodies.
T cells reside in the blood, lymph nodes, and spleen, where they play several roles. One type of T cell, called a T-helper cell, coordinates and stimulates the immune response. T-helper cells activate other T cells, B cells, and phagocytes—turning on the immune system in response to infections. T cells act as captains in the army, giving orders to other cells and assigning them defensive tasks. T cells also fight infections using their own weapons. Without the fighting leadership of T cells, the immune system becomes disorganized and ineffective.
Another type of T cell, the T-cytotoxic cell, attacks and kills cells in your body that have been infected with viruses. This stops cells from reproducing viruses and helps control viral infections. Some T cells kill tumor cells and fungi, too.