An animal’s immune system protects its body from intruders: bacteria, viruses, parasites, cancer cells, etc. An immune system is present in several animal groups, especially within the vertebrates. Animals have both non-specific and specific defense mechanisms to fight invaders. We will be focusing on the human immune system.
Non-specific defense mechanisms work against a wide variety of invaders. These defense mechanisms include the barrier formed by our skin; chemicals in perspiration, skin oil, saliva, tears, etc.; the hairs in our nostrils; the ciliary escalator (the cilia and mucus that clean out dust and debris from our lungs and trachea) in our respiratory tracts; the inflammatory response which is the dilation of blood vessels and accumulation of WBCs at the site of an injury (the signs of which are that the area is red, hot, and swollen); and fever, a raised body temperature to inhibit the growth of pathogens. Note that a fever is caused by your body to inhibit the growth of bacteria, etc., not by the “germs” themselves, per se.
Specific defense mechanisms are effective against specific pathogens. This involves various WBCs called lymphocytes or leukocytes. There are several kinds of WBCs involved in the immune system, all of which originate in the bone marrow. Leukemia is a cancer of the bone marrow, thus it typically is treated by killing all of the person’s bone marrow, and then replacing it with donor marrow. Unfortunately, this leaves the person with no immune system, so (s)he must be extremely careful during that time to avoid all possible pathogens. There are two main types of specific defense mechanisms involved in the immune system.
|The humoral immune system consists of B-cells which originate in the Bone marrow and stay there to develop.||The cell-mediated immune system consists of T-cells which originate in the bone marrow, but go to the Thymus to finish their development.|
|B-cells can produce antibodies, but need exposure to foreign antigens to do so. These antigens are cell surface oligosaccharides and proteins which the cell uses as “ID tags”.||T-cells are highly-specialized cells in the blood and lymph to fight bacteria, viruses, fungi, protozoans, cancer, etc. within host cells and react against foreign matter such as organ transplants.|
|Antibodies are proteins in blood plasma and lymph to fight bacteria and viruses in body fluids. All daughter cells of a B-cell will be able to produce the same antibodies as the mother cell. Antibodies bind to certain parts of an antigen to mark it for destruction (by the T-cells).||There are three kinds of T-cells. Cytotoxic T-cells directly kill invaders. Helper T-cells aid B and other T-cells to do their jobs, and HIV lives in and kills them. Suppressor T-cells suppress the activities of B- and other T-cells so they don’t overreact. Allergy injections are supposed to increase the number of supressor T-cells to make the person less sensitive to allergens.|
Here is an animation of a B-cell and a T-cell attacking an invader (If you have a slow connection, be forewarned that this involves a couple of 300 to 400 KB animated .gifs that could take a while to download.).
Immunity is the ability to “remember” foreign substances previously encountered and react again, promptly. There are two kinds of immunity: active immunity, when the body is stimulated to produce its own antibodies, and passive immunity, where the antibodies come from outside the person’s body. Active immunity is usually permanent, and can be induced due to actual illness or vaccination. Passive immunity is not permanent because the antibodies are introduced from outside the body, thus the B-cells never “learn” how to make them. Some examples of passive immunity include antibodies passed across the placenta and in milk from a mother to her baby, some travelers’ shots, and the Rhogam shots we discussed earlier this quarter. Because antibodies are only protein, they don’t last very long and must be replaced if the immunity is to continue.
A vaccination is a weakened or inactive form of a pathogen given to enable the immune system to respond and produce immunity to it. The first vaccination was when Edward Jenner purposely gave people cowpox, a mild disease, because he had figured out that could prevent them from getting smallpox, a very serious disease.
There are nutritional things a person can do to help strengthen his/her immune system. Vitamins A, C, and E are collectively known as the anticancer vitamins. The immune system needs these plus minerals like zinc (Zn) and selenium (Se) to do its job. Dark green leafy vegetables, especially cabbage relatives like broccili and kale and orange vegetables like carrots are high in ACE. In research where AIDS patients were given much larger than “normal” amounts of these nutrients, especially vitamin C, it was found that the people taking the “extra” nutrients had more and healthier helper T-cells. These nutrients also are valuable in fighting colds, flu, etc. Many knowledgable nutritionists feel that the RDA for vitamin C (currently 100 mg for non-smokers) is way too low, and often recommend anywhere from one to three grams a day.
AIDS stands for Acquired Immunodeficient Syndrome. This virus lives in and kills helper T-cells. With fewer helper T-cells, the person’s immune system can’t form any new antibodies against any new invaders, thus people with AIDS usually die from some secondary infection or unusual form of cancer. The AIDS virus is transmitted by direct blood-to-blood contact, such as in sexual contact where there is a tear in the tissue (more likely if anal tissue, not designed for this type of activity, is involved) or sharing the same needle to inject drugs intravenously (which usually also injects some of the first person’s blood into the second person’s arm. There have also been cases of young hemophiliac boys contracting AIDS from blood transfusions and babies born with AIDS because their fathers gave it to their mothers (it can cross the placenta). The AIDS virus is not spread via saliva nor skin contact. When a person has an AIDS test, what actually is being tested is the presence of anti-AIDS antibodies that would indicate that the person has been exposed to the virus.
In an autoimmune response, the immune system turns against the “self”, developing antibodies against its own antigens and destroying its own cells. For example, in myasthenia gravis, the person’s immune system destroys the acetylcholine needed to transfer nerve impulses across the synapses.
Interferon is a substance produced by virus-infected cells to “warn” neighboring cells thereby helping them to resist the virus.
In cancer cells, genetic changes cause changes in the cell-surface antigens such that the person’s immune system (hopefully) no longer recognizes them as “self” and destroys them. However, if the immune system is stressed and not functioning properly, a cancer cell may multiply before the immune system has a chance to kill it. Once a whole tumor has grown, it becomes difficult, if not impossible, for the immune system to fight it.
Allergic Reaction Allergy is an abnormal overreaction to a specific environmental antigen called an allergen. In an allergic reaction, antibodies bond to mast cells (instead of the antigens) which then produce histamine which attaches to other cells, such as those in the nose or skin, to cause an allergic reaction. An antihistamine is a chemical which competes with histamine for receptor sites on the nose/skin cells. Some people are concerned about this because the histamine is still present in the person’s body. More recently, mast cell inhibitors, such as cromolyn sodium (NasalCrom®), have been developed that stop the mast cells from even making histamine to begin with.