Defence Mechanisms

Melissa Lamb
Note by Melissa Lamb, updated more than 1 year ago
Melissa Lamb
Created by Melissa Lamb almost 4 years ago


AS - Level Biology (Cell Recognition & The Immune System) Note on Defence Mechanisms, created by Melissa Lamb on 07/06/2017.

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Defence MechanismsNon-Specific: response is immediate and the same for all pathogens. Physical barrier, e.g. skin Phagocytosis Specific: response is slower and specific to each pathogen. Cell-mediated response (T-lymphocytes) Humoral response (B-lymphocytes)

Microbes can enter the body through: Ears, e.g. piercings Eyes Nose Mouth Tattoos Insect bites Cuts / open wounds in the skin Belly button piercings Reproductive system Groin area Urinary opening

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An antigen is a protein, glycoprotein or polysaccharide which stimulates an immune response. Found on surface of cells, including body cells. Antigens not normally found in body referred to as foreign or non-self antigens. To defend the body from invasion by foreign material, lymphocytes must: Recognise the difference between self and non-self material. Recognise antigens on the surface of cells. Antigens allow the immune system to identify: Pathogens - on the surface of a virus or bacterial cell. Abnormal body cells - on a cancerous cell or a cell infected with a virus so has viral proteins on its surface or on a phagocyte that has ingested a pathogen. Toxins - poisons released from bacteria. Cells from other individuals of the same species - on the surface of cells of a transplanted organ. Purpose of an antigen is for cell communication - cells from different individuals have different antigens whereas cells of the same individual have the same antigens. Close relatives have more similar antigens than unrelated individuals.

Pathogens: Organisms that cause disease, e.g. bacteria, viruses and fungi. Have antigens on their surface which are identified as foreign by immune system cells - respond by destroying pathogen. Abnormal Cells: Cancerous or pathogens-infected cell. Have abnormal antigens on surface which trigger an immune response. Toxins: Poisons - they are molecules, not cells. Most toxins produced by bacteria. Immune system can respond to toxin and the pathogens that release them. Cells from other individuals of the same species: Cells received from another person during a blood transfusion / organ transplant will have some antigens that are different (unless donor is genetically identical). Foreign antigens trigger an immune response. Response leads to rejection of transplanted organs if drugs not taken to suppress recipient's immune system.

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How do lymphocytes (white blood cells) recognise cells belonging to their body?  About 10 million different lymphocytes present at any time, each recognising a different chemical shape.  In foetus, lymphocytes constantly colliding with other cells.  Infection in foetus is rare - protected from outside world by mother and placenta.  Lymphocytes collide almost exclusively with body's own cells.  Some lymphocytes will have receptors that fit those of body's own cells - these are suppressed or die.  Remaining lymphocytes respond only to foreign / non-self material.  Lymphocytes produced in bone marrow initially only encounter self antigens - any lymphocyte showing immune response to self antigens undergo apoptosis (cell death).

The Skin: Body's first line of defence. Acts as barrier, prevents entry of microorganisms. Outer most layers make up epidermis - consists of dead cells, strengthened by the protein keratin. Sebaceous glands of lower dermis produce water proofing oils (sebum) - keep hair follicles relatively free of bacteria. Sweat glands produce sweat - contains enzyme lysozyme, destroys bacterial cell walls, results in lysis of bacteria. Bacteria naturally living on skin compete with pathogens. Respiratory System: Surface of organs exposed to outside world (respiratory, digestive and urinary system) act as physical barriers to microbe attack. Surfaces covered with epithelial tissue - epithelial cells tightly packed and sit on a basement membrane. Mucus produced in respiratory tract traps bacteria and other microbes - contains lysozyme. Epithelial cells in upper parts of respiratory system have cilia on outside surface. Cilia beat steadily, causing net flow of mucus (with entrapped microbes) up bronchioles and trachea towards throat where it is then swallowed.

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Digestive System: Concentrated hydrochloric acid of stomach denatures enzymes of microorganisms. Enzyme pepsin in stomach hydrolyses them. Eyes: Glands around eyes release tears. Tears contain lysozyme which kills microbes.

Should first line of defence fail, next line of defence is white blood cells - phagocytes and lymphocytes.Phagocyte (e.g. macrophage) carries out phagocytosis (engulfment of pathogens).Found in blood and tissues - first cells to respond to immune system trigger inside the body. Bacteria releases chemicals which attract phagocytes. Phagocyte moves to site of infection and recognises antigens on bacteria are foreign. Phagocyte engulfs bacteria. Bacteria is enclosed in phagocytic vacuole. Lysosome fuses with vacuole (forming a phagosome). Releases lysozyme into vacuole to digest / hydrolyse bacteria. Phagocyte displays pathogen's antigens on surface to alert T and B-lymphocytes that body has been invaded by a particular pathogen.

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