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As one can surmise, the immune system is very complex, and entire books can be devoted to just that. The intention here is to provide the general basics of how the immune system components work together to “fight” invasive pathogens. By understanding the basics of the immune system, choices that strengthen immunity against new pathogens can be made.
Historically the subject of immunity is ancient, and evolved from the study of resistance to infection. Recent contributions to immunology have come from related branches of science such as biochemistry, anatomy, developmental biology, genetics, pharmacology, pathology, and of late, bioengineering. In addition, the study of clinical entities such as allergies, infectious diseases, organ transplants, rheumatology, immune deficiency diseases, holistic medicine, complimentary medicine, and oncology, have also contributed to new understandings of the complexities associated with immunity.
Less than 50 years ago, surprisingly, immunology was barely mentioned; it was not considered to be of importance to most clinicians. At that time, clinicians were mainly physicians and nurses, without the many clinical specialties of today. In fact, the study of holistic medicine or the study of stress and its effects on physical health was given little or no credence.
Today, immunology is gaining greater momentum as the need to fight against infectious diseases is becoming more apparent. With the rise of new pathogenic strains, strong immunity might make the difference between life and death. Therefore, it is important to understand the basics of the immune system and to teach others how to strengthen it in an effort to help create a healthy population.
The white blood cells are probably the most important part of the immune system. White blood cells are a collection of different cells that work together to destroy bacteria and viruses. A normal white blood cell count is in the range of 4,000 to 11,000 cells per microliter of blood. There are different subtypes of white blood cells and each type has specific functions or roles in the immune system. Here are the different names of white blood cell types:
White blood cells are known officially as leukocytes. White blood cells act like independent, living single-cell organisms able to move and capture things on their own. White blood cells behave very much like amoeba in their movements and are able to engulf other cells and bacteria. Many white blood cells cannot divide and reproduce on their own, but instead are produced in the bone marrow.
There are two basic types of leukocytes. The phagocytes are cells that chew up invading organisms and the lymphocytes are cells that allow the body to remember and recognize previous invaders.
White blood cells start in bone marrow as stem cells. Stem cells are generic cells that can form into many different types of leukocytes as they mature. They are called “stem cells” because they can branch off and become many different types of cells - they are precursors to different cell types.
A “bone marrow transplant” is accomplished simply by injecting stem cells from a donor into the blood stream of a recipient. The stem cells find their way, almost magically, into the marrow and make their home there. Stem cells change into actual, specific types of white blood cells.
For example, you can take a mouse, irradiate it to kill off its bone marrow’s ability to produce new blood cells, and then inject stem cells into the mouse’s blood stream, the stem cells will divide and differentiate into all different types of white blood cells and regenerate.
Each of the different types of white blood cells has a special role in the immune system, and many are able to transform themselves in different ways. The following descriptions help to understand the roles of the different cells.
Leukocytes are divided into three classes:
Macrophages are the first line of defense in the lymphatic system. Of all blood cells, macrophages are the biggest. Monocytes are released by the bone marrow, float in the bloodstream, enter tissues then become macrophages. Macrophages engulf foreign particles called phagocytosis.
Phagocytes such as neutrophils and macrophages are thus the primary weapons in the immunological war engulfing and digesting microorganisms and other antigenic particles. Macrophages are vital to life as they permeate the body to swallow up toxins.
A stressful unhealthy lifestyle can hinder macrophage function. In time, macrophage dysfunction creates an increase in metabolic residues that accumulate in the blood, lymph and tissues and can become chronic disease. Macrophages swim freely, clean up dead neutrophils, and engulf pus in the healing process.
Lymphoid organs include the bone marrow and the thymus, as well as lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue in the small intestine, known as Peyer’s patches. The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs.
The lymph system is the keystone of immunology. It originates primarily in the bone marrow and liver producing white blood cells (leukocytes). The lymph system is composed of millions of tiny ducts where white blood cells, nutrients, fluid and oxygen flow through the bloodstream and tissues. More than a billion cells, they are responsible for specific resistance to disease.
Lymph nodes are small bean shaped structures lying along the course of the lymphatics. Lymph nodes can be as small as the head of a pin, or as big as a grape. There are 400-700 lymph nodes in the body, half of which are located in the abdomen, and about 170 to 200 in the neck.
Lymph nodes serve largely as a barrier to the spread of infection. The main function of the lymph glands is to trap infection and foreign material by acting like a sieve. Each lymph node has a capsule and an internal mass of lymphoid tissue. Several lymph vessels carry lymphatic fluid into the node, with a single large vessel carrying the lymphatic fluid back out.
Again, all of the immune system cells are derived from stem cells in the bone marrow. The bone marrow is the site of origin of red blood cells, white cells, and platelets.
There are mainly two sub-types of lymphocytes – “B” lymphocytes (B cells) that are produced in the bone marrow and “T” lymphocytes (T cells) that are produced in the thymus. These are the two most important lymphocytes.
T-cells and B-cells each have different functions. B cells are responsible for making antibodies. (*Antibodies are special molecules, tailor-made to help eliminate foreign material, bacteria, and viruses that enter the body). Portions of these cells become “memory” cells and produce the antibody again, giving resistance or immunity.
The T lymphocytes have two functions — first they assist the B cells in producing antibodies, and secondly they recognize and eliminate cells that seem foreign to the body. Hence T cells are responsible when an organ transplant is rejected. T cells are also responsible for eliminating otherwise normal cells that have been infected with a virus.
T-cells (also called T4, T-helper, or CD4 cells) are the first cells to respond to a “non self” (virus, bacteria, parasite, etc.) There are two major types:
Cytotoxic T Cells, also known as Natural Killer (NK) cells are the defense line against viruses and cancer. They destroy body cells infected with a virus or bacteria. Memory T cells remain in the body awaiting the reintroduction of antigens (an antigen is a molecule that stimulates the production of antibodies).
These cells recognize infected cells by using T cell receptors releasing a substance (granzymes) that kills that particular cell. They are the scavengers of the immune system, killing anything that appears to be foreign. Cytotoxic T-cells multiply and can attack invader cells directly. (In the case of tuberculosis, however, the tuberculosis bacillus can disable the NK-cells, especially if the immune system is weakened). NK cells destroy foreign cells by recognizing the viral antigens and attaching to that cell’s plasma membrane. The T cells secrete proteins that punch holes in the infected cell’s plasma membrane. The infected cell’s cytoplasm leaks out, the cell dies, and is removed by phagocytes. Killer T cells may also bind to cells of transplanted organs.
Helper T cells are actually very important and interesting. Helper T Cells are cells that activate macrophages (cells that ingest) and produce cytokines (interleukins) that trigger the proliferation of B and T cells. These chemicals activate B cells so that they produce antibodies. The complexity and level of interaction between neutrophils, macrophages, T cells and B cells is amazing and is a book in itself.
B-cells are able to produce specific antibodies in search of specific antigens. Antibodies are “Y” shaped with claw-like arms and a long leg at the bottom. When the “specific memory” or code is missing on the exterior, antibodies can mistake it as a “non-self” or foreign cell, which becomes an autoimmune disorder.
B-cells may develop and mature in the blood marrow, intestines, fetus, and the liver. B cells divide, forming plasma cells and B memory cells. *(Stimulated mature B cells respond to antigens and change into centrocytes and then centroblasts. The centroblasts leave the follicle then pass to the paracortex and medullary sinuses, where they become immunoblasts. The immunoblasts divide to give rise to plasma cells or memory B cells which are ready for their next encounter with specific antigen.)
Plasma cells make and release between 2000 and 20,000 antibody molecules per second into the blood for the next four or five days. B memory cells live for months or years, and are part of the immune memory system. Thus, plasma cells are the product of B- cells, producing antibodies immediately. Antibodies (immunoglobulins) are proteins, not cells.
Therefore, a specific B cell is tuned to a specific “invader “ (germ). When the “germ” is present in the body the B cell self clones to produce millions of antibodies designed to eliminate the germ or substance identified by the T-cell. Basically, antibodies act like bullets, which will only strike particular targets.
Secondary immunity, the resistance to certain diseases after having had them once, results from production of Memory B and T cells during the first exposure to the antigen. A second exposure to the same antigen produces a more massive and faster response. The secondary response is the basis for vaccination, (please see Chapter Five).
Neutrophils are by far the most common form of white blood cells. Granulocytes are part of the innate immune system and have somewhat nonspecific, broad-based activity. They do not respond exclusively to specific antigens, as do B-cells and T-cells.
Granulocytes include neutrophils - polymorphonuclear leukocytes basophils and, eosinophils. Neutrophils are attracted to foreign material, inflammation and bacteria. A process called chemotaxis attracts neutrophils.
Once a neutrophil finds a foreign particle or a bacteria it will absorb it. Then it releases enzymes, hydrogen peroxide and other chemicals from its granules to kill the bacteria. In a site of serious infection, pus will form. Pus is composed of dead neutrophils and other cellular debris.
From the immune system’s standpoint inflammation is a good thing. It brings in more blood and it dilates capillary walls so that more immune system cells can get to the site of infection. Bone marrow produces trillions per day releasing them into the bloodstream, although their lifespan is typically less than 24 hours.
Also known as polymorphonuclear leukocytes, eosinophils and basophils, are also phagocytes and are far less common than neutrophils. Eosinophils seem focused on parasites in the skin and the lungs, while Basophils carry histamine and are, therefore important (along with mast cells) to causing inflammation. Mast cells are basophiles responsible for releasing histamine and other compounds involved in allergic reactions.
The mast cell is a non-circulating counterpart of the basophil. Located in the lungs, skin, tongue, and linings of the nose and intestinal tract, the mast cell is responsible for the symptoms of allergy. Platelets, too, contain granules. In addition to promoting blood clotting and wound repair, platelets release substances that activate components of the immune system.
The spleen also provides a meeting ground for immune defenses. A fist-sized
Organ, it is located in the upper left of the abdomen. The spleen contains two main types of tissue: the red pulp that disposes of worn-out blood cells, and the white pulp that contains lymphoid tissue.