The Immune System is a collection of parts (or a system) in the human body that helps to defend a person against foreign invaders. The invaders are often germs, like bacteria, viruses and parasites. Normally, when germs and other foreign objects try to invade the human body, the immune systems’ job is to either keep them out, or try to destroy them.
When the immune system doesn’t function properly, a person can develop serious conditions like HIV and AIDS, allergies and arthritis. In other words, with immune deficiency a person has less resistance to diseases that they would not ordinarily develop.