Immunity is generally understood to mean protection from infectious disease. The greatest health achievement of this century has been the control of devastating, epidemic, infectious diseases by immunization (vaccination). Edward Jenner invented immune therapy when he inoculated an eight year old boy with scrapings of cowpox lesions. Jenner had noted the similarity of cowpox and small pox lesions, and was observant enough to notice that milkmaids, exposed to cow pox lesions on the teats of cows, did not get smallpox. Two hundred years later one major viral disease, smallpox, has been eradicated completely from the planet.
This masterful success of immunization was achieved by the World Health Organization (WHO) by the relentless vaccination of all people who came in contact with the disease, until the smallpox virus had no vulnerable hosts to infect. The smallpox virus is incapable of an independent existence. With no remaining human hosts, the virus is unable to reproduce. Diphtheria, whooping cough, tetanus, measles, mumps, typhoid, cholera, yellow fever, hepatitis, and polio are among the diseases now controlled by immunization.
The principal function of immune defense is protection against infection and invasion of the body space by foreign substances of all kinds. When we are ill with a viral infection such as a cold, we expect to get better, as a result of successful immune defense strategies. Immune defense stops infection with several subdivisions, specializing in attacking one of the many micro-organisms which threaten us - bacteria, fungi, viruses, and parasites. Another role of the immune system is the defense against molecules which invade body space from the outside.
Immune defense has at least two layers:
Innate immunity that depends on pre-programmed recognition of infectious agents by dendritic cells, macrophages, natural killer cells and antimicrobial peptides.
Acquired, adaptive immunity involves learning and memory to identify specific pathogens and to produce antibodies, signaling molecules and antigen-specific cytotoxic T lymphocytes that are capable of destroying infecting organisms and cells that have become abnormal.
The bone marrow is the major manufacturing area for immune cells. Some bone marrow cells migrate to the thymus gland to mature into T-lymphocytes. B lymphocytes produce antibodies which identify specific foreign molecules and cell-surface markers.
Immune defense covers all body surfaces exposed to the environment: skin, respiratory tract, gastrointestinal tract, and genitourinary tract. Immune cells are found circulating in the blood stream or patrolling in the intercellular spaces of all body tissues. Immune networks are also distributed in discrete organs of the lymphatic system which includes lymph nodes, tonsils, liver and spleen. Bone marrow is the major manufacturing area of immune cells. The thymus is the organ which develops T-lymphocytes and exerts regulatory influences on the immune system.
Immunity means that immune cells remember the identity of an antigen and initiate a defensive response. Immunizing injections contain antigens which belong to the infecting organisms. The first response to the injected antigen is the activation of antigen-specific B lymphocytes who make antibody and Killer T cells that can destroy cells that display the antigen. Memory cells can later identify the infecting organism. Several exposures to the vaccine boosts antibody production and leave an immune memory that can trigger a prompt defense against infecting organisms that reappear.