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Outcomes 6.3 Defence Against Infectious Disease IB Biology 6.3.1 Define pathogen. 6.3.2 Explain why antibiotics are effective against bacteria but not against viruses. 6.3.3 Outline the role of skin and mucous membranes in defence against pathogens. 6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues. 6.3.5 Distinguish between antigens and antibodies. 6.3.6 Explain antibody production. 6.3.7 Outline the effects of HIV on the immune system. 6.3.8 Discuss the cause, transmission and social implications of AIDS. Pathogens A pathogen can be defined as an organism or virus that causes a disease. A pathogen enters the body of a host and causes damage. Pathogens can be viruses, bacteria, protozoa, fungi, and various types of worms. Antibiotics Many disease caused by bacteria can be effectively treated with antibiotics because they block or interfere with the metabolic pathways of the bacterium that is causing the disease without having the same effect on human cells. This is because bacteria are prokaryotic cells and human cells are eukaryotic. Viruses Viruses are not composed of cells and they reproduce using the metabolic pathways and cell contents of its host therefore the antibiotics do not have any effect on the virus. In other words, the viruses use the same metabolic pathway as the eukaryotic cells. Antibiotics take advantage of the fact that prokaryotic cells are different from eukaryotic cells in structure and in function. Some antibiotics interfere with protein synthesis in bacteria. Others focus on preventing the bacterium from building new cell walls therefore blocking the ability of the bacterium to grow and divide. Defence! The Skin Our skin and mucus membranes act as our primary line of defence against invading pathogens! The skin is a barrier that covers that entire body and prevents pathogens from entering the body. Your skin has two layers. A very tough outer layer known as the epidermis that is constantly being replaced by new cells from the dermis layer underneath. Mucus Membranes Skin cont’d… As long as the epidermis stays intact it is an excellent line of defence against pathogens. The skin itself is a tough outer barrier but it also contains defences within. The sebaceous glands in the pores of our skin secrete lactic acids and fatty acids. These acids create an unwelcoming environment on the surface of our skin that prevents the growth of most bacteria. Mucous membranes are located in and around openings in the body with areas of soft skin such as the vagina, urethra, nose, and trachea. The mucous contains an enzyme called lysozyme that can kill many bacteria that try to enter. The trachea also contains cilia which can sweep or push the mucus and trapped bacteria out of the trachea. More protection… Although the human body has these outer defences some pathogens will still enter the body so internal defences are needed. Acids in the stomach are one of those defences. If a pathogen enters the body in food or water the acidic environment of the stomach often kills the pathogen. Leukocytes… Leukocytes Human blood contains leukocytes also known as white blood cells. There are several different types of blood cells and they are involved in the body’s defence system or immune system. These cells have the ability to recognize foreign invaders in our system using the protein molecules that are part of the surface of every cell and virus. They determine if a substance or cell is part of the body and recognize it as “self” or they may determine it is not part of the body and therefore “not self”. Anything that is determined to be “not self” is flagged as something that should be removed. Phagocytes One type of blood cell known as phagocytes will consume or ingest pathogens that enter the body. There are lysosomes inside the phagocytes that contain digestive enzymes which destroy the ingested pathogen. Phagocytic Leukocytes… Phagocytes also have the ability to slip through the spaces between the cells in the walls of the capillaries and enter the body tissues. From here they can target areas of infection and engulf the pathogens that are responsible for the infection. When there is a large amount of phagocytes at an infection site they form pus. A Phagocyte animation! http://www.mydr.com .au/healthimages/animationphagocytosis Antigens Antigens are foreign substances that stimulate an immune response in the body. Different parts of pathogens can be antigens that will stimulate the response such as cell walls of bacteria and fungi or the protein coats of viruses. Antigens are molecules that our immune system recognizes as “not self”. Unfortunately not all pathogens can be stopped by phagocytes and general (nonspecific) protection so again, more and more specific/targeted defences are needed. Antibodies Antibodies are composed of proteins and they target certain antigens. For example, in human blood types, anti-A antibodies target A antigens on the surface of red blood cells. Most antibodies have a specific antigen that they target and they are produced in response to that specific antigen. Pathogens Antibody Production Pathogens contain proteins on their outer surface that the immune system recognizes as antigens and these non-self proteins trigger and immune response. Most pathogens contain more than one protein or antigen on their surface so this triggers the production of more than Lymphocytes, a type of leukocytes are responsible for making antibodies. Each lymphocyte can only produce one type of antibody so a tremendous number of different lymphocytes are needed. one type of antibody. Antigen-Antibody recognition Antibodies… Although there are many different types of antibodies their structure is quite similar. They are Y shaped proteins that each contain a binding site at the end of the Y where the antibody will attach to the The cell membrane of each lymphocyte contains some of the antibody that it is responsible for producing. They also contain antigen-combining or bonding sites that project out from the cell membrane surface. antigen. B Cells produce antibodies… B Lymphocytes Leucocytes that produce antibodies are known as B lymphocytes. Humans contain many different types of B cells because each B lymphocyte can only produce one type of antibody. A problem arises because we do not contain high numbers of each type of B cell and they can only produce a small number of antibodies which are not high enough in numbers for the invasion that is occurring. Despite this hurdle our immune system has a way of producing many antibodies. Typical Immune Response 4. The newly formed B cells being the production of antibodies which circulate in the bloodstream in search of the matching antigen they were produced for. 5. The antibodies use several different methods to eliminate the pathogen from the body. 6. When the pathogen has been eliminated some of the B cells will remain in the blood stream to provide immunity from a second invasion of that pathogen. These cells are known as memory cells. 1. Immune Response Let’s see this in action! Once a pathogen enters the body, the specific antigen of the pathogen is identified. 2. The B lymphocyte is identified that matches the antigen and can produce the antibody to bind to the antigen. 3. That B cell and several others begin rapid cell division by mitosis to increase the numbers of that B cell. AIDS AIDS is a syndrome that is caused by the HIV virus. AIDS is categorized as a syndrome and not a disease because syndromes are a group of symptoms that occur together. This syndrome attacks the body’s defences and destroys the immune system leaving the body weak and defenceless. http://www.youtube.com/watch?feature= player_embedded&v=lrYlZJiuf18 HIV The human immunodeficiency virus (HIV) infects white blood cells known as lymphocytes. The virus targets the lymphocyte known as the helper T-cells which play a very important role in producing antibodies. The Virus HIV Over a period of time the virus will destroy the lymphocytes and antibodies can no longer be produced. This destroys the body’s immune system and the infected individual is very susceptible to pathogens that cause disease that would be easy to control under normal circumstances. The HIV virus cannot survive long outside of a host and it cannot pass easily across the skin. Transmission of the virus involves an exchange of fluids between an infected person and an uninfected person. The most common methods of transmission are: HIV http://highered.mcgrawhill.com/sites/0072495855/student_view 0/chapter24/animation__hiv_replication. html Method of Transmission The virus can be transmitted during sexual intercourse through small cuts or tears in the vagina, penis, mouth or intestines. Through the sharing of hypodermic needles with traces of blood. The virus can be transmitted from mother to child across the placenta, through cuts during childbirth, or in breast milk. The virus can be transmitted during a blood transfusion using infected blood. Social/Ethical Implications The social implication of living with HIV can be devastating. The infected individual will suffer injustices associated with the disease such as loss of job, income, insurance, and housing. Their families and friends will suffer grief and they may lose both friends and family. We love Ted! (10 minutes) http://www.ted.com/talks/lang/en/hans_r osling_the_truth_about_hiv.html