8.1.1 Second Line of Defence
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A Search For Better Health Topic 8: Second Line of Defence Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s) identify antigens as molecules that trigger the immune
response explain why organ transplants should trigger an immune response identify defence adaptations, including: inflammation response phagocytosis lymph system
cell death to seal off pathogen
Antigens An antigen is any molecule the body recognises as foreign and that triggers the immune response. On the surface of cells in the body, there are ‘marker’ molecules that identify the cell as belonging to the body (‘self’). This protects the cells in the body from attack by its own immune system.
Antigens When pathogens enter the body they have chemical markers, called antigens, on their surface; the immune system recognises these as not belonging to the body (‘non-self’). The presence of these antigens causes the immune response to be activated to destroy the foreign organisms.
Antigens It is not only pathogens that have antigens on their surface. Any foreign cell, cell fragment, protein debris or toxin produced by bacteria can also contain antigens. The venom of poisonous snakes contains a number of antigens. In all these cases the immune response will be activated because the body recognises all these antigens as foreign molecules (non-self).
Immune Response: Organ Transplant When a person has an organ transplant, the new organ has antigens that are different to the antigens on their own cells. The transplanted organ is therefore identified as foreign and the immune response is activated to attack the organ in order to defend the body.
Immune Response: Organ Transplant To try to prevent this from happening, the tissue type of the donor is matched to the recipient as closely as possible so that there is a high number of matching ‘marker’ molecules. This will mean that there are fewer foreign (antigen) molecules on the surface, a situation which may lead to a less ‘violent’ immune response.
Immune Response: Organ Transplant The patient can also be treated with immunosuppressant drugs, which will also lessen the immune response so that the transplanted organ is not attacked. This has the disadvantage of making the patient more susceptible to infection from pathogens and they must take precautions (such as isolation) to reduce their potential exposure to any infections.
Second Line of Defence When pathogens are successful in penetrating the barriers that are in place to prevent their entry into the organism (the first line of defence), non-specific responses that are the ‘second line of defence’ are quickly activated to try to destroy the invaders before they can cause any damage to the body.
Second Line of Defence These non-specific defence adaptations include: the inflammation response phagocytosis the lymph system cell death to seal off the pathogens. These processes work together, ahead of, and with the third line of defence, if required, in order to defend the body against attack. Students to complete the worksheet instead of taking notes on the following section.
Inflammation Response The inflammation response is a non-specific defence
mechanism and occurs at the site of infection. When the cells are infected or injured in some way, they release chemical alarm signals such as histamines and prostaglandins.
Inflammation Response These chemicals cause the blood vessels to dilate, increasing the blood flow to the site of infection or injury and causing the area to become red, hot and swollen.
Inflammation Response These chemicals also increase the permeability of the blood vessels; this allows the movement of phagocytes from the blood into the tissues so they can attack the invading pathogens. Phagocytes are a special type of white blood cell and are described in more detail in the next section.
Inflammation Response Plasma also moves into the tissues, bringing more phagocytes and producing swelling in the area of the infection, forcing tissue fluid into the lymph and taking debris and pathogens with it.
Inflammation Response Chemicals that increase the temperature are released. This inhibits the growth rate of pathogens, inactivates some enzymes and toxins made by the pathogens and increases the rate at which the biochemical reactions occur in the body. When the pathogens are destroyed, they are removed along with any toxins and the tissues are repaired.
Phagocytosis Phagocytosis is the process by which phagocytes change their shape so that they can surround a foreign particle, such as a bacterium, and completely enclose it within their cell. Once it is inside the cell, enzymes are released to destroy the foreign material.
Phagocytosis Phagocytes are specialised white blood cells or leucocytes. The two main types of phagocytes are: Neutrophils Macrophages.
Phagocytosis Neutrophils are the first to be called upon and move to the site of the infection, inactivating pathogens. Neutrophils are short-acting and then selfdestruct after a few days. They are used by the body to fight acute (short, severe) infections.
Phagocytosis Macrophages are long-lasting phagocytes that can either stay in the tissues or travel from the blood vessels into the infected tissues. They are used by the body to fight chronic (longlasting) infections.
Phagocytosis After the macrophage has destroyed the foreign particle, parts of the antigen are displayed on the surface of the macrophage. We will see why this is important later when we learn about the bodies third line of defence. Phagocytosis is not always successful as the pathogens can sometimes repel the phagocytes and may escape before being completely destroyed.
Lymph System As the blood circulates around the body, some of the plasma moves out of the capillaries into the tissues and becomes part of the tissue fluid. This tissue fluid then moves into a system of vessels that is known as the lymphatic system. The lymphatic system consists of lymph (a milky fluid), lymph nodes, lymph vessels, thymus, spleen, tonsils and adenoids.
Lymph System The lymph vessels form a one way drainage system from all parts of the body back to a point near the heart where the cleansed lymph fluid is drained back into the blood. The muscles that surround the vessels squeeze the fluid in a one-way direction and the presence of valves prevents the fluid moving backwards.
Lymph System At different points along the lymph vessels, there are structures called lymph nodes that play an important part in the body’s defence system.
Lymph System If there is an infection in the tissues the foreign particles, along with dead cells and other debris, move with the tissue fluid into the lymph vessels. When they get to a lymph node, the waste particles are filtered off and any foreign particles are destroyed by macrophages. Swollen lymph nodes (glands) are a good indicator of infection. The lymph nodes also play an important role in the third line of defence, as we will learn more about later..
Cell Death Sometimes cells die to seal off an area of tissue that is infected and is not being successfully defended by the body. If the infected cells are surrounded by a wall of dead cells it prevents the infection from spreading to other areas and infecting them. www.pathguy.com
Cell Death This wall of dead cells forms a capsule (known as a granuloma) or cyst. The cells inside will then die, causing the destruction of the pathogens that are infecting them. The debris inside the granuloma or cyst will be destroyed by the macrophages that had also surrounded the walled-off area. Granulomas form when a person is suffering from tuberculosis or leprosy.
Other Secretions The body also produces special proteins that assist in the second line of defence. Interferons are secreted by some cells when they are infected with viruses. Interferons can cause nearby noninfected cells to produce their own antiviral chemicals, which inhibit the spread of the virus. These interferons are non-specific and are most effective in short-term viral infections such as colds and influenza.
Other Secretions The complement system is a group of 20 proteins that assist other defence mechanisms. These complement proteins can be involved in the destruction of pathogens by stimulating phagocytes to become more active, attracting phagocytes to the site of the infection or destroying the membranes of the invading pathogen.
Homework -Students to complete close passage at the bottom of 8.1.3 Second Line of Defence Worksheet.