- The first-line of defense created by the immune system is a barrier that stops foreign organisms (pathogens) at the point of entry, keeping them from causing a full infection. A classic example of this is your skin, which keeps pathogens outside the body and actually secretes chemicals that can fight them if they begin to invade.
- If these first-line defense fail, a portion of the immune system makes antibodies to parts of the foreign organism, hopefully killing it. Antibodies actually provide the immune system with "memory," enhancing the immune response if the body's exposed to a specific pathogen a second time.
- In the final line of defense, certain immune system cells directly attack the foreign organism in an attempt to kill it.
When the immune system is healthy, it's able to fight off many foreign organisms, but there are times when the immune system doesn't work as well. This can occur if cells in the immune system aren't able to do their job or they "overdo" their job attacking the body's own cells, thinking they're foreign.