THE IMMUNE SYSTEM

The human body is one of the greatest wonders of nature and the immune system is designed to defend the body against foreign antigens that invade it.

An Antigen is a substance that can elicit an immune response through association to specific T and B receptors. These antigens may be contained within or on bacteria, viruses, other microorganisms, or cancer cells. Antigens may also exist on their own- for example, as pollen or food molecules. A normal immune response consists of recognizing a foreign antigen, mobilizing forces to defend against it, and attacking it. There are many substances, which can bind with antibodies or lymphocytes. The bigger and more complex an antigen is, the more likely it is to be capable of starting the specific immune response (which makes it an immunogen). All known biochemical families of compounds- such as carbohydrates, lipids, proteins, and nucleic acids, as well as drugs, antibiotics, food additives, cosmetics and small synthetic peptides can be antigens. Proteins, being the largest and most complex, are most immunogenic. Carbohydrates (polysaccharides) are second most immunogenic. Others such as lipids, drugs, antibiotics and nucleic acids are poor immunogens, but can become immunogenic if bound to a carrier protein.

How Does The Body Recognize That The Antigen Is Foreign?

Glycoproteins in the cell membrane of the body's cells are necessary for self-recognition; if the proteins are not self, the body then assumes that they are foreign and will attack them.

Major Organs Of The Immune:

1. Thymus- Organ located in the upper chest. Immature lymphocytes leave the bone marrow and find their way to the thymus where they are "educated" to become mature T-lymphocytes.
2. Liver- Major organ responsible for synthesizing proteins of the complement system. In addition, it contains large numbers of phagocytic cells which ingest bacteria in the blood as it passes through the liver. Detoxification of chemical elements whether ingested or inhaled.
3. Bone Marrow- Is the location where all cells of the immune system begin their development from primitive stem cells.
4. Tonsils- Collections of lymphocytes in the throat.
5. Lymph Nodes- Collections of B-lymphocytes and T-lymphocytes throughout the body. Lymph nodes are one of the major sites of antibody formation.
6. Spleen- Collection of T-lymphocytes, B-lymphocytes, and monocytes located mid-stream in the blood.
7. Blood- Circulatory system that carries cells and proteins of the immune system from one part of the body to the other.

The normal immune system has two "arms" non-specific ( innate) immune response to initial infection and specific (adaptive) immune response or specific to a particular antigen or pathogen. Together, these arms work to maintain normal host function and resistance to infection. Disruption of any part of this immune response can result in an inability to control infection and subsequent illness. Foreign invaders get into our bodies through our skin, airways and digestive system.

Immune Response: Non-Specific Immunity (Innate or Natural Immunity)

You are born with Innate immunity. It is active or capable of being activated at all times. The innate immunity is the barriers that keep harmful materials from entering the body and is the first line of defence in the immune response. Innate immunity does not require prior exposure to activate a response and does not involve memory or recognition. Examples of Non-Specific protection or barriers are: Skin and mucous membranes (traps microorganisms and small particles) cough and sneeze reflex, pH of body secretions, antimicrobial enzymes, complement proteins, and cilia in lungs to sweep out bacteria or particles. Inflammation is the result of actions taken by the Non-specific immune system- neutrophils, monocytes, mast cells, basophils, and macrophages- (macrophages are monocytes which are matured into organ specific cells fixed in one location). Some examples of macrophages are Kupffer cells in the liver, Alveolar macrophages in the lungs, and histiocytes in lymphoid tissue. If an antigen gets past the external barriers, it is attacked and destroyed by other parts of the immune system.

Specific Immune System: (Adaptive or Acquired Immune Response)

You develop specific immunity throughout life. Specific immunity is the mechanism that provides protection against specific types of bacterial or toxins and involves recognition and memory. Can tell self from non-self. The cells involved are Lymphocytes, which produce antibodies and lymphokines and requires a prior exposure to an antigen. It takes about 10 days or so before we make effective antibodies. In the secondary response, we quickly develop lots of antibodies and kill the organism without getting sick. The response developed is specific for just that antigen, and does not work against any other antigen. The specific immune system is more advanced that the non-specific system. It is made up of two major components:

Cellular (Cell-Mediated-T Cells) and Humoral (Antibody-Mediated-B-Cells)

Cell-Mediated- This response requires the T-Cells that are produced in bone marrow but mature in the Thymus. Cell-Mediated immunity defence activities are carried out by specialized T-Cells that circulate through the body. They defend against: bacteria, fungi and protozoans, own cancer cells and tissue transplant. Delayed type hypersensitivity reactions are antigen-specific, cell-mediated immune response which, depending on the antigen involved; mediate beneficial (resistance to viruses, bacteria, fungi, and tumors) or harmful (allergic dermatitis, autoimmunity) aspects of the immune function.

Humoral Immunity- The humoral immune responses require B-Cells which are also produced in bone marrow, however unlike T-Cells they under go maturity in the bone marrow. Humoral immunity results in the production of antibodies that circulate around the body in blood and lymph (humors) and defend against free bacteria and viruses. B-Cells respond to antigen by producing antibodies.

The Cells Of The Immune Network:

Immune cells circulating in the blood are called White Blood Cells this group includes: lymphocytes, neutrophils, eosinophils, basophils, and monocytes.

The above synopsis provides a basic overview, but for further reading here are some interesting links:
The Immune system Explained
The Immune System - Wiki
Immunity in more detail

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