6.3 - B - Immunity
Phagocytes are white blood cells that provide the first layer of defense when pathogens enter the body. They are able to leave the capillaries and migrate towards the site of infection.
They engulf pathogens in a process called phagocytosis. Once engulfed, they digest the pathogens with enzymes stored in lysosomes. The digested remains are then secreted into the surrounds.
It is important to note that this process of phagocytosis is non-specific. This means, that the phagocytes will attack any pathogen that is not recognized as part of the host.
Antigens are proteins and molecules on the surface of foreign cells that the immune system is able to recognize. When a pathogen invades the body, the immune system responds by producing antibodies. These antibodies bind to the antigen on the pathogen.
Antibodies are produced by white blood cells called lymphocytes. However, each is only able to produce one type. When a new pathogen and its antigens are introduced to the body, the corresponding lymphocyte is stimulated to divide. A large clone of these cells is called a plasma cell. After a few days, when there are enough plasma cells, sufficient amounts of antibody are present to control the infection. As shown to the right, each antibody has a region that binds to the specific antigen.
After the pathogen has been dealt with and its antigens are no longer present, the lymphocytes cease antibody production and gradually die. However, some remain and become memory cells.
Memory cells live for long periods of time and remain inactive until the same pathogen is encountered again. When this happens, they are able to quickly divide and produce antibodies in response. As a result, these memory cells provide an immunity to the pathogen.