The basic connection between protein and immunity is that the body recognizes infectious agents, called pathogens, by their specific proteins. These proteins are called antigens and can be produced by viruses, bacteria, and cancer cells. Antibodies are a class of proteins that are produced by the body that can bind with antigens on the surface of bacteria, to give one example. When this happens, the bacteria are marked to be destroyed by white blood cells. Another important connection between protein and immunity is that the body can gain long-term immunity by remembering antigens associated with specific causes of infectious disease.
Immunity usually refers to what is called active natural immunity. This occurs when the body is naturally exposed to a pathogen, such as one that caused another person's cold. Active artificial immunity refers to the process of vaccination, in which a pathogenic antigen is introduced into the body by a vaccine.
Understanding the relationship between protein and immunity has allowed effective vaccines to be developed. Vaccines work by introducing antigens to produce an immune response. They are convenient because they stimulate an immune response in the body but do not produce the symptoms of disease. This is accomplished by methods that can include modifying a microorganism, or using viral proteins without introducing the actual virus.
The body can make use of the connection between protein and immunity to give itself long-term immunity against pathogens it has previously encountered. The first time it encounters an infectious pathogen, it takes a certain amount of time to produce effective antibodies, during which the foreign organism has had time produce the symptoms of sickness or disease. After this so-called primary response, certain cells will retain the ability to produce specific antibodies. When the antigen is encountered at a later time, the cells, called memory B cells, will divide quickly for a fast response. The body doesn't become sick again because the antibodies are produced much more quickly, and more of them are made.
The white bloods cells illustrate one somewhat complex connection between protein and immunity. The most important immune system cells are a type of white blood cell called lymphocytes, which have two main groups called T and B cells. T cells can attack a foreign cell like a bacterium first by recognizing the bacterium's specific antigens, then releasing proteins of their own that eventually cause the bacterium to die. Through a separate process, B cells eventually give rise to antibodies. Antibodies bind to antigens on bacteria which are then destroyed by phagocytes, another type of white blood cell.