This is all about the immune system, how it works, and what it does.

The function of the immune system is to defend the body against foreign invaders that may cause disease. The immune system is made up of cells, tissues, organs, and body systems that fight bacteria, viruses, harmful chemicals, and cancer cells. The skin functions as a defense against disease because of the millions of helpful bacteria that live on your skin. This is your first line of defense by killing many harmful types of bacteria. When …

…another pathogen. People with aids die from other diseases such as pneumonia, cancer or tuberculosis. Then the victim's body becomes defenseless. In the antigen-antibody response your body is fighting proteins and chemicals (antigens) that don't belong. Antibodies are formed. When your immune system recognizes the antigen, it forms specific antibodies. An antibody is a substance made by an animal in response to a specific antigen. The antibody binds up the antigen, which makes it harmless.