Immune Disorders

Immunodefeciency Disorders

Immunodefeciency means deficiency of the immune system. The children who are born without either T cells or B cells lack the ability to fight against the diseases and so are highly prone to infections.

Severe Combined Immuno defeciency (SCID): In this disorder both T cells and B cells are absent at birth. Since the body lacks the immune system even a small infection can be fatal. The children have been kept alive by keeping them in the isolation suits which resemble the space suits, in the developed countries. The air inside the suits is almost sterile, without germs so that the child does not catch any infection.

Acquired Immune Defeciency Syndrome – AIDS
Cause of AIDS – Acquired Immune Defeciency Syndrome is a disease caused by a virus that can be passed from one person to another chiefly during sexual contact or through sharing of intravenous drug needles and syringes. Other names: Scientists have named the AIDS virus HIV or HTLV – III or LAV also. These abbreviations stand for information denoting a virus that attacks white blood cells (T-lymphocyte) in the human blood.
HIV – Human Immuno defeciency Virus
HTLV – III – Human T-Lymphotropic Virus Type III
LAV – Lymphadenopathy Associated Virus

Sign and Symptoms: People infected with AIDS virus remain apparently well even after infection. They may not show any physical symptoms of illness for a long time. When the AIDS virus enters the blood stream, it begins to attack certain white blood cells (T-lymphocytes) and substances called antibodies are produced in the body. These antibodies can be detected by a specific blood test usually two weeks to three months after infection.
Once an individual is infected, the body may respond in many ways:

- In some people, the protective immune system may be destroyed by the virus and then other germs (like bacteria, fungi, protozoans) and cancers that ordinarily do not attack, cause opportunistic diseases (meaning using the opportunity of lowered resistance) to infect and destroy the body. Some of the common diseases are pneumonia, tuberculosis and certain cancers.
- AIDS virus may attack nervous system causing damage to the brain.
- Some may develop AIDS – Related complex (ARC) condition.
- Some early signs may be persistent cough and fever associated with difficulty in breathing.
- In some, AIDS may take years to develop and may show up as memory loss, indifference, loss of coordination, partial paralysis or mental disorder.

Symptoms of ARC (AIDS – Related complex)
ARC is a condition caused by the AIDS virus which has a specific set of clinical symptoms.
(i) Swollen lymph nodes and fever.
(ii) Night sweats and tiredness.
(iii) Weight loss, diarrhoea, loss of appetite and lack of resistance to the infection.

Mode of Infection: The infection from AIDS virus occurs only:
- When a person has sexual contact with an infected person.
- By using contaminated needles. Even the smallest amount of infected blood left in a used needle or syringe can contain live AIDS virus which can be passed on the next user.
- By the blood to blood contact as in blood transfusion.

Currently in many countries as in U.S.A., all blood donors are initially screened for AIDS virus and proper precautions are taken before blood transfusion.
Although the AIDS virus has been found in tears and saliva, no instance of transmission from these body fluids has been reported. There is no danger of infection with AIDS virus by casual social contacts or by sharing cups, towels or food. Also there are no known cases of AIDS transmission by insects like mosquitoes or pets.

High Risk groups
It is now absolutely clear that AIDS is not a black or white disease though it may be confined to a specific region. AID is not a male disease, it is found in women and children as well. But the high risk groups who become infected with the AIDS virus are found mainly among:
- Homosexual men.
- Bisexual men (who have both homosexual and heterosexual partners)
- Male and female intravenous drug users.
- Recipient of blood products.
- Sexual partners of these high risk individuals.
- Children born to women who carry the virus.

If a woman is infected with the AIDS virus and becomes pregnant, she can pass the AIDS virus to her unborn child.
AIDS is a life threatening disease. At present there is no vaccine to prevent and there is no cure. However, AIDS is preventable. It can be controlled by changes in personal behaviour. AIDS is no longer the concern of any one segment of society but is the concern of all. The need of the hour is to provide care for those infected with AIDS and inform and educate others as how to prevent it.
AIDS was first recognised in USA in 1981. Although in 1984 the American and the French scientists independently identified the agent as a virus, AIDS is still a mysterious disease in many ways. We still do not know why the AIDS virus is not destroyed by man's immune system.
Organ Transplants and Immunosuppression
Transplantation means the replacement of a diseased organ or a tissue of an individual with the living healthy organ or tissue from another individual. The transplanted organs contain about forty antigens which can trigger the production of antibodies in the recipient (the person in which organ is transplanted). The recipient body recognises the transplanted organ as the foreign body or antigen and hence produces antibodies against it which cause the destruction of the transplanted organ (also called as rejection).

Ways to prevent rejection of Transplanted organ or Tissue
(i) By use of immunosuppressants. These drugs counteract the immune system and stop the production of antibodies against the transplanted organ. These drugs are not only expensive but also inhibit the body's normal immune response to disease causing microorganisms. The patient becomes prone to all kinds of infections. The research is still on to develop the immunosuppressive drugs which only stop the rejection response and do not affect the general immune system.
(ii) By taking transplanted organ from the siblings. It has been seen that if the transplanted organ like kidney is taken from the sibling (i.e. from brother or a sister) or identical twins the chances of acceptance are more as they have a comparatively similar genetic constitution.

(iii) By taking skin grafts in case of burns from one part of the body to another of the same individual.

(iv) By the use of X – irradiation on bone marrow and lymph tissues. This suppresses the blood cell production and slows the phenomenon of rejection.

Defence Mechanisms of the body

The body of an organism is surrounded by the environment that carries numerous types of bacteria and other organisms that can cause diseases. The body has a natural defence mechanism that stops the invasion of these at various stages of attack. There are two lines of defence in the body.

(i) Non – specific defence mechanisms which try to prevent the entry of micro organisms.
(ii) Specific defence mechanisms are active processes which destroy the microorganisms once they manage to get in the body. It is also called as immune system.

1. Non – specific defence mechanisms
These mechanisms guard against the entry of micro-organisms in the body.

a. Skin: Skin forms the first line of defence. It forms the hard, keratinised outer layer of the body which covers all external parts and forms an effective barrier for most bacteria and viruses. The secretions of oil and sweat glands make the surface pH acidic (pH 3 to 5) which does not let microorganisms to survive for long. Certain friendly bacteria also release acids and other metabolic wastes which also inhibit the multiplication of pathogens. The salts and fatty acids present in perspiration contain lysozymes that destroy the bacterial cell walls. Tears secreted by tear glands also contain lysozymes which prevent the eye infection. Saliva also contains the lysozymes that kill the bacteria present in the foods and drinks. Any bacteria left are killed by the secretion of the stomach. The respiratory tract is lined by the mucous membrane. First the cilia and fine hair filter out the undesired particles in the nostrils and then mucus – the slimy substance traps the bacteria. In the trachea or bronchi, the movements of cilia sweep bacteria towards the throat from where they are expelled by coughing or sneezing.

b. Phagocytic cells or W.B.C.: Phagocytic cells form the second line of defence and work when the first line of defence is broken and bacteria do gain entry through abrasion or wound in the skin. At the place of injury, clotting not only prevents bleeding but also blocks the entry of bacteria. Dilation of small blood vessels at injury increases the blood flow. The increased permeability of capillaries lets phagocytes pass out into the intercellular spaces and engulf the invading bacteria. Phagocytosis is generally carried out by neutrophils. The pus formed at he infected wound is actually the debris of the battle i.e. dead cells, digested bacteria and broken phagocytes in the blood serum. Inflammatory response: At times the phagocytosis results in a hot red, swollen inflamed area at the site of infection. Redness is due to increases flow of blood. The lymph helps by carrying bacteria and phagocytes that have engulfed bacteria to the lymph nodes where they are filtered out. The inflammatory response is due to a chemical histamine released by the lymphocytes, and the damaged tissues which causes dilation of the blood capillaries. These reactions are generally localized i.e. confined around the wound. Many a time the fight between microorganisms and W.B.C. results in rising of temperature. Such an inflammatory response is wide spread and is called a systemic reaction. High fever not only inhibits the growth of germs but also activates the body defenses and is actually beneficial if not too high and uncomfortable. The fever is caused by either the toxins released by the pathogens or by the chemicals called pyrogens (the fever producing substances) released by W.B.C. that set the body's thermostat at a higher temperature.

2. Specific defence mechanism
The first and the second line of defence were nonspecific as they acted on any type of bacteria but this third line of defence by antibodies is very specific. An antibody act against a specific bacteria or antigen only. An antigen is any foreign protein or a polysaccharide that is not found in the organism's body. Viruses, bacteria or their toxins and the cells of other person, all contain proteins not recognised in the body and hence act as antigens. An antigen stimulates the body to produce a specific antibody. The word antigen is a shorten form of antibody generating. The antibodies interact with the antigen and either destroy them or make them inactive. The antibodies are globular proteins and are called as immunoglobulins (or Ig in short form). They are produced in the lymph nodes, spleen and by the thymus gland. Entry of an antigen in the form bacteria or virus triggers a reaction in the body as a result of which the antibodies produced defend the body in various ways and try to neutralise the antigens. Antigens are not only parts of microorganisms but certain blood cells, pollen, eggs white, fruits or vegetables may also act as antigens and stimulate the immune system of the body.

The Immune System

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 – antibody 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