Animal body is always exposed to the invasion of countless infectious microorganisms such as virus, bacteria. Due to the defense mechanisms evolved by the animals, such invasions in number of cases are overcome. The ability of the body to resist microorganisms, their toxins if any, foreign cells and abnormal cells of the body is termed as immunity.
Immune system:
Immunity is conferred to animals through the activities of the immune system which combats infectious agents. The study of functioning and disorders of the immune system is termed as immunology.
Immune system is a collection of cells and proteins that work to protect the body from potentially harmful, infectious microorganisms. It also plays role in the control of cancer, allergy, hypersensitivity and rejection problems when organs or tissues are transplanted. The immune system can be divided into two functional divisions.
(1) Innate immune system and (2) Adaptive immune system.
(1) Innate immune system:
It is responsible for natural immunity which is non specific in nature since it combats all microorganisms. It consist of physical (e.g. skin mucous membrane) and chemical (e.g. lysozyme, gastric juice etc) barriers against infectious microorganisms. Skin and mucous membrane with their secretions act as first line of defense. The intact skin provides an impenetrable barrier to the vast majority of infectious agents, most of which can enter only through the mucus membranes that lines the digestive, respiratory and urinogenital tracts. These areas are protected by movement of mucus and secretions (e.g. lysozyme in tears) to destroy many microbes.
Most of the microorganisms present in food or trapped in swallowed mucus from the upper respiratory tract are destroyed by highly acidic gastric juice of stomach. If some how microorganisms are able to penetrate the outer layer of the skin or mucous membrane, they encounter second line of defense offered by the innate immune system. Phagocytes are certain type of WBC which can digest and destroy the particles including agents. Short lived phagocytic cells called neutrophils ingest bacteria actively. Another group of WBC, the natural killer cells (NK cells) destroy virally infected own cells of the body. They also attack abnormal cells (cancerous cells).
Inflammation (to set on fire) is body’s reaction to an injury or by the entry of microorganisms. A cascade of chemical reactions takes place during inflammatory response. It is characterized by redness, heat, swelling and pain in the injured tissue. When injured basophils and most cells release a substance called histamine which causes increase in the permeability of the adjacent capillaries, local vasodilation and also make capillaries leakier. Due to chemotaxis phagocytes and macrophages are attracted at the injured site. Thus phagocytes literally eat up microorganisms, dirt, cell debris etc forming pins.
In case of warm blooded animals a number of microorganisms who escape away from the inflammatory response to infect some large part of the body, trigger fever. It is usually caused by certain WBC that release substance called pyrogen. It sets the temperature of the body higher than the normal. It inhibits the growth of some microorganisms, facilitates phagocytosis, increases the production of interferon and may speed up repair of damaged tissue.
(2) Adaptive immune system:
It is externally complex. It produces specific immune response against a range of different invading organisms, toxins, transplanted tissues and tumour cells. This is the third live of defense which comes into play simultaneously with the second line of non specific defense.
The responses of the adaptive immune system are provided chiefly by two types of lymphocytes called B cells and T cells. Depending upon their migration and maturity during early development in either bone marrow or thymus, they are designated as B and T cells. Although B cells and T cells play quite different roles in the immune system, yet they share the basic key features of the immune response.
In Order to develop immune response, the immune system must recognize the invading organisms or foreign proteins from its self tissues and proteins. A foreign substance that elicits immune response is called antigen. The immune system responds to an antigen by activating lymphocytes and producing specific, soluble proteins called antibodies. The antibody combines with the antigen and helps to eliminate it from the body. The immune system of a vertebrate has virtually unlimited capacity to generate different antibodies which recognize and bind millions of potential antigens or foreign molecules.
The immune system has also the ability to memorize antigens it has encountered. Thus upon subsequent response to the same pathogen, it respond, very quickly and effectively. The adaptive immune system mounts two types of attacks termed as humoral immunity and cell mediated immunity (CMT) on invading microorganisms.
Humoral immunity:
Immunity provided by antibodies secreted in circulatory system by B cells is termed as humoral immunity. It is helpful in bacterial invasion.
Cell mediated immunity (CMI): It is contributed by the second family of lymphocytes called T cells which do not secrete antibodies. They mediate immunity by killing infected cells and aiding in inflammation.
Active immunity: Immunity acquired by own immune response is called active immunity. It is a consequence of natural infection it is said to be Natural Active Immunity, and is acquired by artificial active immunity.
Passive immunity: It depends upon the antibodies transported from another person or even an animal. It could be natural passive immunity.
Passive immunity can be transferred artificially by introducing antibodies derived from animals or human beings who are already immune to that disease. This is termed as Artificial Passive Immunity.
Immunization: It is the process of introducing immunity as a preventive measure against certain infectious diseases. The incidence of number of diseases e.g. diphtheria measles etc has declined dramatically since the introduction of effective immunization programmes once thought to be the dreadful diseases like tuberculosis etc is now under control through immunization and treatment.
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