The most important characteristic of the immune system is that it recognises 
self and nonself. The formation of antibodies in response to the antigens as described above is also a part of the immune system. The immune system has two components.
1. Humoral immune system: Humoral immune system is made up of various types of specific antibodies which are present in the blood plasma and lymph. This immune system defends the body against specific bacteria and viruses that enter the body fluids like blood and lymph. The term humors is used for body fluids. B – Lymphocytes (B-cells) are responsible for the humoral immune system.
2. Cell—mediated immune system: Cell—mediated immune system refers to highly specialized cells of the immune system that work not only against pathogens but also against fungi and protistans that invade the host cells. These cells recognise the body's own cells and react against the transplants also. This immune system also protects the body from its own cells if they become cancerous. The cell – mediated immune system is formed by the T – lymphocytes (T-cells).
Cells of the Immune System
The lymphocytes – a type of agranular W.B.C's are the main cells of immune system. These immune system are of two types: B cells and T cells
(i) Both need an exposure to antigens to trigger them into action.
(ii) Both types of cells develop from the precursor cells i.e. stem cells in the bone marrow.
Response of B – cells and the humoral immunity
When B – cells in the lymph nodes come into contact with a specific antigen, the receptors on their membrane recognise the antigen.
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The B – cells are stimulated and divide repeatedly to give rise to a clone of plasma cells and memory cells. Clone is a population of cells derived from a single original cell.
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All the plasma cells are genetically identical and hence they produce numerous identical antibodies which are releases into the lymph which eventually enter the blood. Most plasma cells can produce 2000 molecules of antibodies per second. They live for a few days only. The memory cells produced however remain in the lymph nodes for years and provide a rapid response during any second encounter with the same antigen.
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The antibodies either stick to the surface of the bacteria and speed up phagocytosis or neutralise the toxins produced by them. Various types of B – cells, millions of these, each being specific to a particular antigen is present in the body at birth. Number of these B – cells which react with the antigens of one's own body (during embryonic development) are rendered inactive and are killed and this is the reason why antibodies are not formed to the individual's own antigens.
Response of T – cells and the cell – mediated immunity
The T – cells also have receptors on the membrane that can recognise a specific antigen. On stimulation, they also produce a clone of T cells and produce specific antibodies but the way they react is different. The T cells instead of releasing the antibodies into the bloodstream, hold on to them and the entire cell attacks the specific antigen. The life of T – cells is about 4-6 years. The body contains specific T-cell for every antigen. They are various types of T cells which perform different functions.
(i) The killer T – cells directly attack and destroy the antigens. They get attached to the specific antigen like a key fits into a lock and then destroy it. About 80% of the total lymphocyte population of human blood is of this variety only. They produce chemicals to attach other T –cells also.
(ii) Helper T – cells stimulate B cells to mature and increase their antibody production.
(iii) Suppressor T – cells suppress the total immune reaction.
Like humoral immunity some T –cells that become memory cells remain in the lymph nodes and persist even after the antigen has disappeared. They can respond to any future attack by the same antigen. The cellular immunity can also last for a life time.
Allergies
An allergy is an immune response of the body to an antigen. Some antigens may be harmless and generally do not show any reaction in most of the people. These weak antigens that cause allergy in some people are called as allergens. The allergens could be on pollen grains, dust, feathers, fur, fugal spores, etc. The symptoms of allergic reactions vary from person to person. The most common are rashes on the skin, sneezing or breathing difficulties but in severe cases it needs immediate hospitalization as it may lead to choking or anaphylactic shock. Many a times a single bee sting or a dose of penicillin may be fatal. The allergic reactions are usually very rapid. During allergy the antigen – ant
ibody reactions occur on the membrane of the tissue cells especially the mast cells which become permeable and burst. They release histamines, which cause dilation of capillaries, redness, itching in the skin and evoke an inflammatory response. Generally the first exposure to an allergen does not cause allergy and helps in the formation of antibodies only. The second or subsequent exposures cause strong reactions and allergies. The antihistamine drugs nullify the effect of histamines. Eosinophills also increase in number during allergic reactions and are thought to have anti – inflammatory response by absorbing histamines.
Immunity
Immunity is the capacity of the body to defend against the disease. Immunity can also be defined as the ability of the body to resist the disease. The immunity can be of two types:
(i) Natural or in – born and
(ii) Acquired
self and nonself. The formation of antibodies in response to the antigens as described above is also a part of the immune system. The immune system has two components.1. Humoral immune system: Humoral immune system is made up of various types of specific antibodies which are present in the blood plasma and lymph. This immune system defends the body against specific bacteria and viruses that enter the body fluids like blood and lymph. The term humors is used for body fluids. B – Lymphocytes (B-cells) are responsible for the humoral immune system.
2. Cell—mediated immune system: Cell—mediated immune system refers to highly specialized cells of the immune system that work not only against pathogens but also against fungi and protistans that invade the host cells. These cells recognise the body's own cells and react against the transplants also. This immune system also protects the body from its own cells if they become cancerous. The cell – mediated immune system is formed by the T – lymphocytes (T-cells).
Cells of the Immune System
The lymphocytes – a type of agranular W.B.C's are the main cells of immune system. These immune system are of two types: B cells and T cells
(i) Both need an exposure to antigens to trigger them into action.
(ii) Both types of cells develop from the precursor cells i.e. stem cells in the bone marrow.
Response of B – cells and the humoral immunity
When B – cells in the lymph nodes come into contact with a specific antigen, the receptors on their membrane recognise the antigen.
↓
The B – cells are stimulated and divide repeatedly to give rise to a clone of plasma cells and memory cells. Clone is a population of cells derived from a single original cell.
↓
All the plasma cells are genetically identical and hence they produce numerous identical antibodies which are releases into the lymph which eventually enter the blood. Most plasma cells can produce 2000 molecules of antibodies per second. They live for a few days only. The memory cells produced however remain in the lymph nodes for years and provide a rapid response during any second encounter with the same antigen.
↓
The antibodies either stick to the surface of the bacteria and speed up phagocytosis or neutralise the toxins produced by them. Various types of B – cells, millions of these, each being specific to a particular antigen is present in the body at birth. Number of these B – cells which react with the antigens of one's own body (during embryonic development) are rendered inactive and are killed and this is the reason why antibodies are not formed to the individual's own antigens.
Response of T – cells and the cell – mediated immunity
The T – cells also have receptors on the membrane that can recognise a specific antigen. On stimulation, they also produce a clone of T cells and produce specific antibodies but the way they react is different. The T cells instead of releasing the antibodies into the bloodstream, hold on to them and the entire cell attacks the specific antigen. The life of T – cells is about 4-6 years. The body contains specific T-cell for every antigen. They are various types of T cells which perform different functions.
(i) The killer T – cells directly attack and destroy the antigens. They get attached to the specific antigen like a key fits into a lock and then destroy it. About 80% of the total lymphocyte population of human blood is of this variety only. They produce chemicals to attach other T –cells also.
(ii) Helper T – cells stimulate B cells to mature and increase their antibody production.
(iii) Suppressor T – cells suppress the total immune reaction.
Like humoral immunity some T –cells that become memory cells remain in the lymph nodes and persist even after the antigen has disappeared. They can respond to any future attack by the same antigen. The cellular immunity can also last for a life time.
Allergies
An allergy is an immune response of the body to an antigen. Some antigens may be harmless and generally do not show any reaction in most of the people. These weak antigens that cause allergy in some people are called as allergens. The allergens could be on pollen grains, dust, feathers, fur, fugal spores, etc. The symptoms of allergic reactions vary from person to person. The most common are rashes on the skin, sneezing or breathing difficulties but in severe cases it needs immediate hospitalization as it may lead to choking or anaphylactic shock. Many a times a single bee sting or a dose of penicillin may be fatal. The allergic reactions are usually very rapid. During allergy the antigen – ant
ibody reactions occur on the membrane of the tissue cells especially the mast cells which become permeable and burst. They release histamines, which cause dilation of capillaries, redness, itching in the skin and evoke an inflammatory response. Generally the first exposure to an allergen does not cause allergy and helps in the formation of antibodies only. The second or subsequent exposures cause strong reactions and allergies. The antihistamine drugs nullify the effect of histamines. Eosinophills also increase in number during allergic reactions and are thought to have anti – inflammatory response by absorbing histamines.Immunity
Immunity is the capacity of the body to defend against the disease. Immunity can also be defined as the ability of the body to resist the disease. The immunity can be of two types:
(i) Natural or in – born and
(ii) Acquired
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