Immunity is the body’s ability to recognize and defend itself against germs that can make you sick. The immune system is made of cells, tissues and organs that work together to find and destroy disease-causing invaders. It also helps heal the damage caused by these invaders.
Certain types of white blood cells, called lymphocytes, are the backbone of your immune system. Lymphocytes begin in the bone marrow, thymus gland and lymph nodes and then move through the bloodstream and lymphatic vessels to other parts of the body. They mature into cells that produce antibodies, which attach to a specific antigen and help other immune cells destroy the germs. They also release chemicals, called cytokines, that control the immune response and help other cells do their job.
The enormous specificity of the immune response is due to clonal selection, in which the antibodies produced by individual lymphocytes are directed at only a single antigen. This results in the production of a large number of molecules, or clones of cells, each with only one antigen-binding site (Figure 7-1). The resulting clones differ from other clones in that they have identical regions of amino acids on their H and L chains, which define the binding site for an antigen.
The other major type of immunity is passive immunity, which occurs when you receive antibodies from someone else. For example, newborn babies have antibodies (MatAbs) from their mothers that pass through the placenta and enter the baby’s bloodstream. Passive immunity only protects against a specific disease and usually lasts for only a few weeks or months. Acquired immunity, on the other hand, is protection that you develop over time from exposure to a germ or by receiving a vaccine.