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MEDICAL ANIMATION TRANSCRIPT: Normally, the organs and cells of your immune system defend your body from harmful germs, such as bacteria and viruses. Immune cells are constantly circulating through your body. They monitor certain substances on the surfaces of cells called antigens. Healthy cells have different antigens than diseased body cells or foreign invaders in the body. Immune cells usually ignore the antigens on healthy cells. But when immune cells come across antigens on germs, they destroy the germ. Afterward, the immune cell displays the germ's antigen on its surface. This activates other types of immune cells to help get rid of the infection. For example, some activated immune cells recognize the antigen on infected body cells and then destroy them. Other activated immune cells, called plasma cells, make molecules called antibodies. These antibodies travel through your body and lock on only to germs that have its specific antigen. This marks the germ for destruction. Then, other immune cells attack the germs that have these antibodies. Once the infection is gone, some of the immune cells that were exposed to the antigen become memory immune cells. In the future, if the same type of germ infects your body again, the memory immune cells will be ready to destroy it so you don't get sick. This is called natural immunity. In many cases, it will last your whole lifetime. more