Built around white blood cells and certain molecules in the blood, the immune system helps an organism defend itself against life-threatening foreign invaders. It not only tries to kill external pathogens like bacteria and viruses but also the body's own renegade cells which become cancerous. There are three distinct immunity mechanisms which work in concert to this purpose. They generate a quick primary response to an invader as well as a more vigorous secondary response, in case the invader reappears. This is why people who get chicken pox once rarely catch it again. The first mechanism is called non-specific immunity, which involves macrophages, neutrophils and natural killer cells that roam the body's circulatory system on the lookout for new intruders. When they find such intruders or cancerous cells, macrophages and neutrophils eat them up in a process called phagocytosis while natural killer cells punch holes in them.

The second mechanism, humoral immunity, consists of B-cells, antibodies and complement proteins. Antibodies have the ability to bind to portions of foreign cells called antigens. Genes of B-cells can get rearranged very fast and as a result B-cells can produce billions of antibodies which are present at any time in the body to protect against. Antibodies, on binding to antigens, activate scavenger cells or complement proteins that kill the foreign cells.

The third mechanism, cell-mediated immunity, involves T-cells, which also have receptors that bind to antigens. Macrophages and B-cells, on encountering foreign cells, process their antigens and present them for inspection to T-cell receptors in a process called antigen presentation. After T-cell receptors bind to the antigen, specialised helper T-cells secrete lymphokines, chemical messengers that induce nearby macrophages to kill the foreign organism through a process called delayed-type hypersensitivity. In addition to cell-mediated immunity, lymphokines promote humoral immunity as well by inducing B-cells to produce a large number of antibodies directed towards the foreign organism. Another cell-mediated immunity mechanism, which works against viruses, involves cytotoxic T-cells. Viruses penetrate human cells and use their own genetic mechanism to reproduce themselves. But human cells can present viral antigens to T-cell receptors. When this happens, cytotoxic T-cells kill the human cell to prevent infection from spreading. At the same time, suppressor T-cells prevent excessive reaction lest too many of the body's own cells get killed. Cytotoxic and suppressor T-cells are together known as effector T-cells.