2. Non-specific immunity
A. The first line of defence
An intact skin acts has a barrier against entry by micro-organisms. A cut or abrasion will allow entry of bacteria or viruses.
Mucus secreted by the cells lining your respiratory tract traps bacteria which are then swept upward to the back of the throat by the action of cilia which line much of the respiratory tract.
Many secretions of the body contain bactericidal agents. Tears and saliva contain lysozyme, an enzyme that causes bacteria to lyse or burst.
Many different bacteria are normally found on the skin, in the gut and ( in females) in the vagina. These bacteria are natural flora of the body and are generally non-pathogenic in those areas.
B. The second line of defence
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Cells called phagocytes move to the point of entry of the bacteria, they are white blood cells that destroy micro-organisms and other foreign materials.
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Neutrophils are the most common of the white blood cells.
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Monocytes are the largest of the white blood cells
- Macrophages are particularly active against micro-organisms that can live inside of the person they infect
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Some white blood cells that kill virus-infected body cells are natural killer cells (NK). They help destroy larger blood parasites such as worms that are to large to be engulfed.
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Complement proteins assist the second line of defence in a number of ways: stick to invading micro-organisms, stimulate phagocytes to become more active, attract phagocytes to the site of infection, complement proteins destroy the membranes of invading micro-organisms
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Interferons are secreted by some cells when they are infected by virus particles. These interferons act on uninfected cells making them resistant to the virus.
- Interferons are particularly important if a virus hasn’t far to travel. This is the case with cold and influenza viruses
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Inflammation is a reaction to the infection and occurs when arterioles in the areas around the cut dilate, resulting in an increased blood supply to the area.
- Figure 8.9
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Phagocytes that arrive early at the scene of the injury release chemicals such as histamines that attract more and more phagocytes to the infection.
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Pus contains white blood cells that are dead as a result of the many bacteria they have engulfed. It also contains living white blood cells as well as other cell debris
3. Specific Immunity
A. Cells involved in specific immunity
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Two mains groups of lymphocytes are involved in specific immunity.
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All lymphocytes are produced in the bane marrow. Some mature in the bone marrow in to B lymphocytes or B cells. Others leave the bone marrow before they are fully developed and travel to the thymus gland where they differentiate into mature T lymphocytes or T cells.
- Figure 8.10
B. How do B cells and T cells identify foreign materials?
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Proteins on cell membranes are determined by genes. These genes are called the major histocompatibility complex(MHC)
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Class 1 markers are found on all cells of the body except red blood cells
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Class 2 markers are found only on T cells, B cells and some macrophages
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The material that triggers a response from a B cell or a T cell is called an antigen. Antigens are usually proteins, but can also include carbohydrate.
C. Many kinds of B cells
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Immunoglobulins are proteins that identify antigens. Immunoglobulins of each B cell have a specific structure and recognise only one kind of antigen
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When a B cell identifies an antigen, it replicates rapidly to produce large numbers of special cells called plasma cells which produce antibodies and release them into body fluids.
D. The clonal-selection theory of antibody production
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The B cell selected by the antigen reproduces rapidly to give rise to a number of identical cells. Each of these cells also reproduces rapidly to produce a large clone of cells.
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Some of the B cells produced differentiate into other cells called B-memory cells. B memory cells also have the same antibody-antigen specificity as the parent B cell. Plasma cells survive for only a few days but memory cells can survive for several years.
- Figure 8.14
E. How much antibody is produced?
- On a second infection the concentration of antibodies rises much more rapidly then during the primary response and many more antibodies are released.
- Figure 8.15
F. Structure of an antibody
- An antibody molecule had four polypeptide chains, two long heavy chains and two shorter light chains joined together
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Immunity involving antibodies in body fluids is called humoral immunity
- Table 8.2
G. Different kinds of antibodies
- IgM and IgG antibodies activate macrophages and the complement system and are particularly active against bacteria and their toxins. IgA antibodies play an important role in surfaces that are vulnerable to infection and are present in the mast cells which release histamines.
H. T cells
- After encountering their specific antigens, T cells reproduce rapidly in the same way as B cells and T-memory cells also form.
- T cells do not make antibodies
- Phagocytes that have ingested foreign material carry some of the foreign antigen on their surfaces as well as their usual class 2 marker proteins.
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Helper T cells recognises these antigens and stimulates B cells
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Cytotoxic T cells kills body cells that have been infected with a virus
- They can only kill the virus when it is inside a cell
I. T cells and B cells travel around the body
- B cells and T cells develop from primary lymphoid tissues: bone marrow thymus respectively. They enter the bloodstream, then leave it and move around the body.
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Lymphatic vessels contain lymph which drains from nearby tissues. Antigens that enter the body are carried in lymph to a lymphatic organ where there is a high concentration of white cells
- Figure 8.20
4. Acquiring Specific immunity
A. Active specific immunity
- Active immunisation involves the production of antibodies within a person in response to exposure to a particular antigen.
- Active immunity can be acquired in two ways: naturally or induced
B. Natural active immunity
- When a person comes into contact with a particular disease-causing organism for the first time, no antibodies against the organism will be present. It takes a few days for the appropriate plasma cells and antibodies to form.
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The antibodies formed have identical sites for binding to the foreign material. As the amount of antibody increases, the infecting micro-organisms begin to be destroyed and the person starts to recover. This type of immunity is called naturally acquired active immunity.
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Sub-clinical infection results in antibody formation in the same way as an obvious infection
C. Induced active immunity
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Vaccines which contain dead or treated living micro organisms are used to activate the immune system to produce antibodies against specific disease-causing organisms without actually causing the disease.
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When a vaccine is injected into a person the immune system shows a primary antibody response. A second injection of vaccine produces a secondary antibody response.
- If a person comes into contact with the live organisms at a future date, memory cells and antibodies will be ready to act and the person is immune to the infection.
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Some micro-organisms cause disease by secreting toxins or poisonous substances. These toxins can be treated to form toxoids. Toxoids are made of the same material as the toxin, and so act as antigens, but are unable to cause disease.
5. Eradicating a disease by vaccination
A. why do we continue to get colds and flu?
- Cold and flu viruses are continually changing to give new strains of the viruses.
B. Passive immunity
- Antibodies produces in one person and introduced into another can react with antigens to provide immunity
- Passive immunity can be acquire naturally or induced
C. Natural passive immunity
- A developing fetus receives maternal antibodies across the placenta
- A baby also acquires antibodies through the mother’s milk
D. Induced passive immunity
- If you immediately receive an injection of antibodies specific for hepatitis A then infection may be avoided. The antibodies used in such injections are obtained from blood collected from voluntary donors by the Australian Red Cross Society.
- Figure 8.34
E. Rabbits as antibody-making machines
- Antivenom is produced in rabbits which receive injections of venom
- At an appropriate time, blood is taken from the rabbits; the antivenom is extracted and used in vaccines for people and other animals that are bitten by snakes.
6. Adverse events associated with immunity
A. Allergies
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Mast cells are immune cells involved in allergic responses. Mast cells are fixed cells found around blood vessels, in connective tissue, near the lining of the gut and in the lungs
- IgE antibodies are made against antigens such as dust, pollen and plant spores
- Histamine also causes blood vessels to dilate allowing cells and serum to move into the surrounding tissue causing swelling and inflammation.
- Some allergies can also be caused by T cells
B. How does a fetus survive the mother’s immune system?
- Half the genetic material of a fetus is inherited from the father and so many of the compounds made by a fetus are foreign to the mother and are capable of acting as antigens.
- The placentas allows only some antibodies to cross from the mother into the fetus
C. Rejection of transplanted organs
- Transplanted donor tissue will not match that of the recipient perfectly and the immune system of the recipient will react against the non-self material. T cells are particularly important in tissue transplants. Helper T cells identify unmatched tissue and attack the grafted organ.
- The better the match of MHC markers between donor and recipient, then the higher the chance that the transplant will be successful.
D. Immune deficiency diseases
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Immune deficiency is a malfunction or a deficiency in one or more components of the immune system. Immune deficiency disorders can be inherited or may develop as a result of some other disease.
E. Acquired immune deficiency syndrome (AIDS)
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It is generally accepted that acquired immune deficiency (AIDS) is caused by the human immunodeficiency virus (HIV). This virus only infects cells carrying a particular protein marker. The marker is found mainly on mature helper T cells and to a lesser extent on macrophages.
- Figure 8.38
- The development of the disease after infection with HIV is variable
- Once the serious symptoms of AIDS appear, death usually follows within about two years.
F. Do plants have an immune system?
- Most plants are resistant to pathogens
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Mechanical barriers- The silicon content of some leaves makes them particularly resistant to the degrading enzymes of pathogens. If a disease-causing organism penetrates the outer layers, and the stomata in particular can be a point of entry; layers of thickened cells form in many cases. This gives rise to abnormal swellings at the infection site. These swellings are called galls.
- Galls are caused by pathogenic organisms, including insects and nematode worms.
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Chemical barriers- Some plants resist disease by producing chemicals that act as antibiotics
- Chemicals that have a defence role in plants include: resins, tannins and phenolic substances