Level One:

The first level is simply physical barriers, such as the skin and mucous membranes.

Level Two:

When pathogens make it past the physical barriers, the second level of defense is called to action. This branch of immunity is called the Innate Immune System because of its ability to act without prior exposure to identify and destroy Cancers, microbial pathogens and other dangerous intruders. It is primarily comprised of the following components:

• Natural Killer (NK) cells: These cells are like rapid deployment Special Forces units which provide the first line of defense within the body. NK cells respond immediately to track down and eliminate Cancer, infected cells, and other damaging invaders. Natural Killer (NK) cells are specifically known to kill invasive tumor cells. They accomplish this task by binding themselves to cancer cell membranes and injecting powerful enzyme granules which cause the cancer cell to dissolve in death within five minutes.
• Macrophages: Literally translated as "large eaters," macrophages eat up pathogens and clean away cellular debris.
• Cytokines: These molecular messengers of the immune system enable the cellular defenses to communicate with each other.

Level Three:

When pathogens make it the second level of defense, the body launches the third and most critical level of defense – the Adaptive Immune System, which includes the following components:

• T cells: T cells are highly specialized white blood cells called lymphocytes. Killer T cells kill infected or abnormal cells, while helper T cells act as "immune system managers," telling other cells what to do. When T cells are severely compromised by infection with HIV, the cells develop AIDS.
• B cells: Like T cells, B cells are also white blood cells called lymphocytes. Their function is to make antibodies, which are highly specialized proteins that the immune system uses to identify and eradicate pathogens, harmful cells and foreign substances.