Every moment we are under attack by various disease-causing agents that could make us sick. Hence to stay alive and to thrive, the body is equipped with an immune system. The immune system consists of many parts including white blood cells (leukocytes), antibodies, bone marrow, lymph nodes, spleen, thymus and the tonsil. Here is a description of what they are and their function:
- White Blood Cells: Cells that move through the blood looking for foreign bodies called antigens to destroy e.g. bacteria and viruses
- Antibodies (immunoglobulins): Specialized proteins produced by white blood cells that circulate in the blood looking for antigens to trap
- Bone marrow: Spongy tissue inside the bone that produces red and white blood cells
- Lymph nodes: “Look out points” for the immune system. Here, lymph fluid is screened by white blood cells before it is returned to the blood
- Spleen: Organ sitting on top of the stomach that helps to filter the blood. It has a large concentration of white blood cells and acts like a giant lymph node. As blood passes through the spleen, they are exposed to white blood cells
- Thymus: Organ in the upper part of the body just below where a necklace might sit. This is where certain white blood cells called T-cells develop
- Tonsil: These are organs located at the back of the mouth that contains white blood cells. They serve to destroy antigens that may be in our food or in the air we breath. Hence, they prevent lung and throat infections
Other components of the immune system are fever and inflammation. The heat associated with fever creates an unfavorable condition for the growth of certain infections such as bacteria. Inflammation is a response to infection involving certain typical symptoms such as pain, swelling, soreness, fever, rash and fatigue. These uncomfortable symptoms send signals to the body that it is under attack. The body responds by activating the immune system and sending help to the area that is affected.
There are two major parts of the immune system, the innate or random immunity and the specific or adaptive immunity. The innate immune system randomly attacks and kills antigens without specificity. In other words, they are not like snipers picking out specific target, they kill just about any antigen in its path. The specific immune system on the other hand is very selective. It will kill only specific antigens. Let’s take a look at both of these and see how they work.
The Innate (Random Immune System)
The innate system consists of the skin, mucous membranes, complementary proteins, natural killer cells, interferon proteins and white blood cells called phagocytes. The skin and mucous provides a natural barrier, preventing antigens from getting into the body. Phagocytes are white blood cells that engulf and destroy antigens. They may come in different forms such as neutrophils, monocytes, macrophages, mast cells, and dendritic cells. Dendritic cells are called “antigen-presenting cells” since they wear the proteins of the antigens they destroy on their surfaces and present them to T-cells (more about that in the next section). Complementary proteins provide backup to the phagocytes by lysing (bursting) pathogens and making them inactive. They also signal more phagocytes to come to the area to provide help. Natural Killer Cells command infected and cancer cells in the body to commit suicide so that the disease does not spread. Interferon proteins provides added protection to cells that are not yet infected, and enhance the activity of natural killer cells and phagocytes.
The Adaptive Immune System
The adaptive immune cells consists of B and T white blood cells called lymphocytes since they concentrate in the lymph node. B cells destroy antigens that are external to the cells and are trying to get into the cells while T-cells destroy antigens already in the cell such as viruses, tumors or body transplants that the body rejects. When B cells comes into contact with an antigen they are converted to “plasma cells”. These are cells that produce specific antibodies (also known as immunoglobulins) to fight against that specific antigen. Antibodies bind with the antigens preventing them from attacking body cells and making it easier for phagocytes to destroy them. Plasma cells can live only a few days (4-5) and so B cells also produce memory cells which has the ability to last your entire life. That means if you are infected by that antigen again, you will quickly be able to destroy it. Hence we say that you are “immune” to the infection.
When a certain type of T cells called Helper T or CD4 cells comes into contact with dendritic cells, they ID the antigen on the surface of the cell and produce clones of themselves along with memory cells. The clones stimulate production of antibodies by plasma cells and also activate phagocyte and natural killer cell activity. Like the memory B cells, the memory T cells survive for many years providing life-time immunity. Exposure to dendritic cells also stimulate another type of T-cells called CD8 cells to clone themselves and also produce memory T-cells. CD8 cells are called cytotoxic cells and are primarily responsible for destroying cancer and infected cells.
Fig. Summary of how the adaptive immune system works.
Age and the Immune System
As we approach old age, our immune system does not work as well as it did in our youth. The thymus atrophies reducing the number of T Cells. This cause a reduction in antibodies and lower activity of phagocytes that as we learned earlier are stimulated when T cells multiply. This result in greater risk for disease including cancer cell development.
The immune system can be boosted by getting vaccinated against certain diseases. In a vaccination, a microorganism e.g. bacteria or virus in a weakened state or their protein toxin is injected in the body. This activates the adaptive immune system to produce antibodies and memory cells in response. Therefore if you are exposed to the antigen again, the body is able to quickly mount an attack. Common vaccine-preventable childhood infectious diseases include measles, mumps, rubella, whooping cough, diphtheria, tetanus and chickenpox, poliomyelitis and meningitis.
We have learned so far that when antigens attack the body, antibodies are produced to destroy them. Generally, antigens are non-self-agents. That is, they are not a part of the body. However, there are times when the immune system will mistakenly attack its own self believing it is an antigen. This is due to the development of autoantibodies. These are antibodies that attack self rather than non-self. Autoimmune diseases cannot be cured but their conditions can generally be controlled to reduce inflammation symptoms. Examples of autoimmune diseases cause by autoantibodies include:
- Celiac Disease: The immune system attacks the lining of the small intestine in response to eating gluten proteins
- Type 1 Diabetes: The immune system attacks the pancreas, preventing production of insulin
- Rheumatoid Arthritis: The immune system attacks joint tissues causing swelling and pain
- Lupus: The immune system attacks healthy tissue including joints, skin, kidneys, blood cells, heart, lungs and the brain
- Scleroderma: The immune system attacks healthy tissue resulting in overproduction of collagen and thickening and scarring of tissue
- Sjogren’s Syndrome: The immune system attacks glands that make tears and saliva
Hypersensitivity of the immune system can result in much discomfort and may even be fatal. The immune system is hypersensitive when it overreacts to exposures that are normally harmless such as food, pollen, bees sting, latex and other substances in the environment. There are 4 types of hypersensitivity.
Type I: Immediate hypersensitivity due to exposure to the allergen. For example, the person may start swelling or rash may appear soon after exposure. This is due to activation of mast cells which releases histamine. Histamine cause blood vessels to become more porous, hence releasing more fluid to the area along with white blood cells. In serious causes, a person reacting to an allergen may have a sudden drop in blood pressure and they may not be able to breath properly. This may require immediate injection with an EpiPen. EpiPen contains the hormone epinephrine also known as adrenalin to increase blood pressure and open airways in the lungs.
Type II: This occurs when the antibodies destroy cells. Hence it is called cytotoxic hypersensitivity. This can happen when you receive a transfusion of the wrong blood type.
Type III: This type of hypersensitivity is called immune complex hypersensitivity. It occurs when antibody-antigen complexes are not adequately removed by phagocytes. Hence they end up attaching themselves to blood cell walls and other body tissues. This attracts phagocytes to the area to try to remove them. Because they are attached to body cells, those cells become casualties as the phagocytes try to clear up the complex.
Type IV: This type of hypersensitivity is called delayed hypersensitivity since there is a long delay between exposure and reaction. This is because T Cells takes time to clone itself before it can start its attack. An example of this is exposure to pison ivy. Rashes as a result of the exposure may take typically anywhere between 12-48 hours. Another example is the tuberculosis test, also called TB test. In the test the antigen tuberculin, a protein from tuberculosis bacterium is injected under the skin. If the person was ever been exposed to tuberculosis, antibodies would already be in the body. This would cause a reaction resulting in raised and thickened skin known as an induration. If there was never any exposure, there would be no antibody present to cause the reaction and so there would be no induration.