The immune system is a large network of organs, white blood cells and proteins that protects you from germs and other substances that can make you ill. It limits how much harm the invaders can do if they get inside your body, fights off infections and heals from injury. It also learns from past contact with certain germs and can offer protection against those germs in the future, even without direct exposure to them (called acquired immunity).
Innate immunity is protection you are born with – your first line of defense against germs. It works very quickly, such as destroying bacteria that enter the body through a small wound or killing viruses that attack healthy cells. But innate immunity offers no memory of past contacts with particular germs and does not stop new invaders from entering the body.
Your innate immune system also contains some of the proteins that help other parts of your immune system fight specific germs. These are called “immunoglobulins” or antibodies. Infants receive some of their mother’s antibodies through the placenta in the last few months of pregnancy, which gives them passive immunity to some diseases. Most vaccines work by turning on the immune system, allowing it to produce antibodies against a particular disease-causing germ, and then providing “memory” of this antigen so that the immune system can react more quickly if the germ invades the body again in the future.
Other parts of your adaptive immune system (the part that learns about specific germs and fights them) include B lymphocytes, which start out in bone marrow and then move to the thymus gland where they mature into T lymphocytes. T cells have features on their surfaces that lock onto matching features on the surface of a germ. When a T cell recognizes a germ, it sends signals to other immune cells to kill the germ and stop the infection.