Your immune system is your body's defence against infection and illness. It recognises the cells that make up your body, and will try to get rid of anything unfamiliar. It destroys germs (bacteria and viruses) and parasites. But this defence system can also cause problems. It will attack donated organs or blood from another person, unless carefully matched. It may overreact to harmless invaders such as pollen grains, causing hayfever. In some cases (autoimmune diseases), the immune system turns against the body's own cells. The immune system is very complex. It's made up of several types of cells and proteins that have different jobs to do in fighting foreign invaders. Most organism have an immune system that uses phagocyte cells. Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
Every organism has a pre-programmed response to a certain pathogen or invading germ. There are two types of immunity. Innate immunity and Adaptive immunity. Innate immunity refers to nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen. Adaptive immunity refers to antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient. rganisms have two types of immune systems. When something dies, its immune system (along with everything else) shuts down. In a matter of hours, the body is invaded by all sorts of bacteria, microbes, parasites... None of these things are able to get in when your immune system is working, but the moment your immune system stops the door is wide open. Once you die it only takes a few weeks for these organisms to completely dismantle your body and carry it away, until all that's left is a skeleton. Obviously your immune system is doing something amazing to keep all of that dismantling from happening when you are alive.