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Defence against pathogens
Biology SL
IB
High School - US
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Pathogens ➢ Any living organism or virus that is capable of causing a disease is called apathogen ○ They include viruses, bacteria, protozoa, fungi andworms ➢ Only a very, very small percentage of these are pathogenic to humans and in fact the vast majority of the bacteria is very useful ○ We’re too well defended for most pathogens to enterour bodies and if they manage, we have often previously developed immunityto the pathogen ○ There are antibiotics too ➢ The best way to stay healthy is to stay away from sources of infections or to isolate people with a pathogen
Skin and mucous ➢ Theprimary defence systemis to keep pathogens out of the body and it is formed by our skin and mucous ➢ The underneath layer is calleddermis ○ It contains sweat glands, capillaries, sensory receptors and dermal cells ■ Dermal cells give structure and strength to the skin ➢ The layer on top of this is calledepidermis ○ This epidermal layer is constantly being replaced as the underlying dermal cells die and are moved upward ○ This layer of mainly dead cells forms a good barrier against most pathogens, because it is not alive ➢ Pathogens can enter the body at a few points that aren’t covered by skin ○ These entry points are lined with tissue cells thatform a mucous membrane ■ Cells of mucous membranes produce and secrete a lining of sticky mucous where pathogens will get stuck ➢ Some mucous membrane tissues are lined with cilia ○ Ciliaare hair-like structures capable of a wave-like movement which can transport trapped pathogens up and out of mucous-lined tissues
Area with a mucous membrane What it is and does Trachea The tube carrying air to and from the lungs
Nasal passages Tubes that allow air to enter the nse and then the trachea
Urethra Tube that carries urine from the bladder to the outside Vagina The reproductive tract leading from the uterus to the outside
Blood clotting ➢ When small blood vessels are broken, blood escapes from the closed circulatory system and so pathogens have a way to gain access to the body, as these blood vessels are often in the skin ○ The body responds creating aclotthat seals the damaged blood vessel preventing excessive blood loss and helping preventpathogens from entering the body ➢ Circulating in the blood plasma are a variety of molecules calledplasma proteins some of which are involved in clotting ○ Two of them areprothrombinandfibrinogen ➢ Also circulating in the bloodstream are cell fragments known asplatelets ○ They form in the bone marrow along with erythrocytes and leukocytes ○ They don’t remain as entire cells but instead, one very large cell breaks down into many fragments and each of the fragments becomes a platelet ○ They don’t have a nucleus but they have a short cellular life span of about 8-10 days ➢ Steps ○ The damaged cells of the blood vessel release chemicals that stimulate platelets to adhere to the damaged area ○ The damaged tissue and platelets release chemicals calledclotting factors that convert prothrombin into thrombin ■ Thrombinis an active enzyme that catalyses the conversion of soluble fibrinogen into the relatively insoluble fibrin ○ Fibrin,a fibrous protein, forms a mesh-like network that helps to stabilize the platelet plug ○ More and more cellular debris becomes trapped in the fibrin mesh and soon a stable clot has formed
Antibodies ➢ Antibodiesare Y-shaped proteins that are produced by the body and that can bind to the surface of pathogens ➢ Each type of antibody is dierent, because each type has been produced in response to a dierent pathogen ➢ Each pathogen is made up of either cells with cell membranes or in case of a virus, a protein coat calledcapsid ○ The cellular invaders have proteins that are embedded in their outer surface calledantigens ■ Non-specific proteins = antigens ➢ Each antibody at the end of the forks of the Y is a binding site ○ Thebinding siteis where an antibody attaches itself to an antigen, meaning that it attaches to the pathogen ➢ The leukocytes that produce the antibodies are a type of cell calledplasma cells ○ Each of has has many dierent types of antibody-producing plasma cells and each type can produce only one type of antibody ○ Each cell produces a relatively small number of antibodies but our immune response has a way of producing many when they are needed ➢ Steps ○ A specific antigen type/pathogen is detected and identified by a white blood cell ○ A specific plasma cell, calledlymphocyte, that can produce an antibody that will bind to the antigen is identified ○ The specific plasma cell type clones itself by mitosis to increase rapidly in number ○ The specific plasma cells begin to produce antibodies ○ The antibodies circulate in the bloodstream and eventually find their antigen match ○ Using various mechanisms, the antibodies help eliminate the pathogen ○ Some of the plasma cells remain in the bloodstream and provide immunity against a second infection by the same pathogen ■ These long-lived cells are calledmemory cells ○ Memory plasma cells of this type respond quickly ifthe same antigen is encountered again ➢ Vaccinesare weakened or non-pathogenic forms of pathogens that cause a primary immune response producing the same lymphocytes as the actual disease
Viruses ➢ Avirusis a semi-living particle ○ Doesn’t have its own metabolism ○ Reproduce by infecting the cells of other organisms ■ They reprogram a cell to produce viruses instead ofproteins by injecting their DNA into the cell ➢ They usually specialize on one type of cells in an organism ➢ The Ebola and HIV are two problematic cases ○ Ebola specifically infects and destroys epithelium cells and other cells of other layers leading to severe bleeding and necrosis in large body parts ○ The Human Immunodeficiency Virus (HIV) infects and destroys plasma cells/lymphocytes which makes the body unable to produce antibodies against new pathogens ■ It takes many years until someone loses the specificimmune response capabilities ■ When this happens, the resulting immune disease is calledAIDS (Acquired Immune Deficiency Syndrome) ➢ Individuals with AIDS are very susceptible to pathogens and usually die because of a secondary infection ➢ No cure for AIDS exists today, but there’s a variety of treatments that prolong the time period between infection and the onset of the symptoms ➢ HIV can be transmitted by: ○ Having unprotected sex ○ Using a hypodermic needle that has been used by a HIV positive person ○ An HIV positive mother to the child during pregnancy, labour, delivery or breastfeeding ○ Through blood transfusion ○ Some medical treatments, such as injections for treating haemophilia
Antibiotics ➢ Bacteria are prokaryotic cells ○ They have some structural and biochemical dierences from eukaryotic cells ■ Dierent protein synthesis (similar but not the same) ■ Have a cell wall ■ And more
Defence against pathogens
Subject: Biology SL
IB
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