The Immune System?
An immune system is a collection of mechanisms within an organism that protects against disease by identifying and killing pathogens and tumor cells.
What is the
The immune system protects you from dying from infection with layered defenses of increasing specificity.
Physical barriers prevent pathogens such as bacteria and viruses from entering the body.
If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response.
A pathogen which had been eliminated, will be recorded in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.
Both innate and adaptive immunity depend on the ability of the immune system to distinguish between known and foreign molecules.
Several barriers protect organisms from infection, including mechanical, chemical and biological barriers. There are several types of surface barriers.
- Mechanical Barrier
As organisms cannot be completely protected against their environments; other systems act to protect body openings such as the lungs, intestines, and the genitourinary tract.
In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory tract.
The flushing action of tears and urine also mechanically expels pathogens.
Mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.
- Chemical Barrier
Chemical barriers also protect against infection. In the stomach, gastric acid and proteases serve as powerful chemical defenses against ingested pathogens. The skin and respiratory tract secrete antimicrobial peptides also help to kill microorganisms.
- Biological Barrier
Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as pH or available iron. This reduces the probability that pathogens will be able to reach sufficient numbers to cause illness. There is good evidence that re-introduction of probiotic flora, helps restore a healthy balance of microbial populations in intestinal infections in children and encouraging preliminary data in studies on bacterial gastroenteritis, inflammatory bowel diseases, urinary tract infection and post-surgical infections.
Will there be
Yes. Failures of host defense do occur, however, and fall into three broad categories: immunodeficiencies, autoimmunity, and hypersensitivities.
Immunodeficiencies occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the elderly, with immune responses beginning to decline at around 50 years of age due to immunosenescence. In developed countries, obesity, alcoholism, and illegal drug abuse are common causes of poor immune function. However, malnutrition is the most common cause of immunodeficiency in developing countries. Deficiency of single nutrients such as iron; copper; zinc; selenium; vitamins A, C, E, and B6; and folic acid (vitamin B9) also reduces immune responses.
Overactive immune responses comprise the other end of immune dysfunction, particularly the autoimmune disorders. Here, the immune system fails to properly distinguish between self and non-self, and attacks part of the body. One of the functions of specialized cells (located in the thymus and bone marrow) is to present young lymphocytes with self antigens produced throughout the body and to eliminate those cells that recognize self-antigens, preventing autoimmunity.
Hypersensitivity is an immune response that damages the body's own tissues. They are divided into four classes (Type I - IV) based on the mechanisms involved and the time course of the hypersensitive reaction. Symptoms can range from mild discomfort to death. Type I hypersensitivity is mediated by IgE released from mast cells and basophils.
Type II hypersensitivity occurs when antibodies bind to antigens on the patient's own cells. This is also called antibody-dependent hypersensitivity, and is mediated by IgG and IgM antibodies.
Type III hypersensitivity reactions trigger when immune complexes (aggregations of antigens, complement proteins, and IgG and IgM antibodies) are deposited in various tissues.
Type IV hypersensitivity usually takes between two and three days to develop. Type IV reactions are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis (poison ivy). These reactions are mediated by T cells, monocytes, and macrophages.
Another important role of the immune system is to identify and eliminate tumors. The transformed cells of tumors express antigens that are not found on normal cells. To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells. The main response of the immune system to tumors is to destroy the abnormal cells. Clearly, some tumors evade the immune system and go on to become cancers. Immunological tolerance may develop against tumor antigens, so the immune system no longer attacks the tumor cells.
Paradoxically, macrophages can promote tumor growth when tumor cells send out cytokines that attract macrophages which then generate cytokines and growth factors that nurture tumor development. In addition, a combination of hypoxia in the tumor and a cytokine produced by macrophages induces tumor cells to decrease production of a protein that blocks metastasis and thereby assists spread of cancer cells.
The immune system is enhanced by sleep and rest, and is impaired by stress. Diet may affect the immune system; foods rich in certain fatty acids may foster a healthy immune system. Likewise, fetal undernourishment can cause a lifelong impairment of the immune system. In traditional medicine, some herbs are believed to stimulate the immune system. Studies have suggested that such herbs can indeed stimulate the immune system, although their mode of action is complex and difficult to characterize.