Download The Immune System The immune system allows the body to defend

The Immune System
The immune system allows the body to defend against disease-causing agent. This system recognizes
and destroys “foreign” substances, such as bacteria, fungi, protozoa, viruses, toxins and cancer cells.
The body has two ways to defend against these substances nonspecific and specific resistance or
immunity.
I. Nonspecific Immunity
Nonspecific immunity involves mechanisms which are general in nature and protect against many
types of disease causing agents (pathogens). The protective mechanisms function in the same way
against any invading organism. Specific immunity involves mechanisms that are very precise,
targeting specific pathogens. Specialized lymphocytes recognize foreign molecules (antigens) in the
body and respond to them by executing specific defense mechanisms.
A. Mechanisms of Nonspecific Immunity
1. Mechanical barriers – The skin and mucous membranes lining the ducts of various organs.
2. Chemical barriers – enzymes, gastric juice and tears.
3. Inflammation – A localized protective tissue response to injury or infection which prevents the
spread of the infectious agent. Inflammation is designed to confine, dilute and destroy the
infecting agent or injured tissue.
4. Phagocytosis – Neutrophils and moncytes (macrophages) leave the blood by emigration
(diapedesis). Emigration is a process by which the WBCs squeeze between the endothelial cells
of the capillary walls into the surrounding interstitial spaces. Chemicals released from the injured
tissues attract these cells by chemotaxis. The phagocytic activities of the WBCs remove the
foreign substances before the interstitial fluid is returned by the lymphatic system to the blood.
II. Specific Immunity
Specific immunity is resistance to specific foreign substances (antigens).
Lymphocytes and macrophages recognize specific foreign substances and execute immune responses.
Antigens are a variety of large complex molecules, most often proteins, located on the foreign cells
surface.
A. Types of Specific Immunity
There are two types of lymphocytes, T-cells and B-cells. They respond to antigens in different
ways. The T- and B-cells can recognize the specific antigens because of cell membrane receptors
on the T- and B-cells. The antigen receptors on the cell membrane of specific T- and B-cell respond
only to a specific antigen.
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1. Cell Mediated Immunity (CMI)
T-cells attach and interact directly with the foreign antigen, i.e. cell to cell contact,
to inactivate and destroy the antigen.
2. Antibody Mediated (Humoral) Immunity (AMI)
B-cells differentiate into plasma cells which produce antibodies. The antibodies are soluble
globulin, glycoproteins (immunoglobulins) that make up the gamma globulin fraction of the
plasma proteins. The antibodies are carried in the body fluids. When coming upon a specific
antigen the antibody attaches to the specific antigen to inactivate and destroys it. One example
of antibody action is agglutinating and precipitating of the antigen. Because antibodies have
sites for binding antigens, the antigen-antibody reaction can cross link pathogens to each other.
This causes agglutination (clumping together) of the pathogens. The macrophages then can
phagocytize the agglutinated pathogens more readily. Also soluble antigens when cross-linked
by antibodies precipitate out of solution and are more easily phagocytized by the macrophages.
B. Types Acquired Specific Immunity
1. Natural Acquired Active Immunity
The person is exposed to a pathogen in the environment and develops the symptoms of the
disease. The person produces an immune response and becomes resistant as a result of
activation of T- and B-cells upon encounter with the pathogen.
2. Artificially Acquired Active Immunity
The person is given a vaccine or a toxoid to a pathogen and stimulates an immune response by
activation of T- and B-cells.
A vaccine consists of killed or weakened pathogen so that it cannot cause a serious infection. A
toxoid is a modified toxin from the pathogen that is chemically altered to destroy its toxic effects.
3. Artificially Acquired Passive Immunity
The person is given an injection of ready made antibodies or gamma globulins against the
pathogen. This is passive immunity because the person does not produce the antibodies but
receives them without activation of the person’s T- and B-cells.
4. Natural Acquired Passive Immunity
During pregnancy antibodies pass from the mother to the fetus, if the mother has the antibodies
against the pathogen. The fetus and the newborn acquires short-term immunity against the
pathogen without activation of the fetus’ T- and B-cells.
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C. Types of Immune Responses
1. Primary Immune Response
Initial activation of T- and B-cells after an encounter with an antigen constitutes a primary
immune response.
2. Secondary Immune Response
After a primary immune response some T- and B-cells produced during the primary response
remain dormant as memory cells. If an identical antigen is encountered again, these memory
cells divide to form more and rapidly respond to the antigen to which they were previous
exposed. This produces a secondary immune response.
A primary immune response usually produces a response within 5-10 days following an antigen
exposure. A secondary immune response produces a much quicker and much greater response
within 1-2 days on exposure to the antigen. The memory cells live a long time and the ability to
produce a secondary immune response can be long lasting.
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