Topic 4 Response to Infection PDF

Topic 4: Response to infection Pathogens 1. Infectious (aka communicable) diseases are caused by pathogens. 2. Pathogens are disease-causing microorganisms. 3. There are 2 common groups of pathogens: a) Viruses b) Bacteria Viruses 1. Viruses don’t have cellular structures. 2. They contain...

Topic 4: Response to infection Pathogens 1. Infectious (aka communicable) diseases are caused by pathogens. 2. Pathogens are disease-causing microorganisms. 3. There are 2 common groups of pathogens: a) Viruses b) Bacteria Viruses 1. Viruses don’t have cellular structures. 2. They contain only nucleic acid & proteins. 3. Viruses possess protein coats which surround the nucleic acid core, either DNA / RNA. 4. Some viruses (eg. HIV) have external phospholipid envelope which contains proteins. 5. The viral envelope is modified & derived from the host cell membrane. 6. Viruses vary in shape. 7. Some are rod-shaped while others are polyhedral with / without tails. 8. Viruses depend on other organisms for reproduction. 9. Hence, they are known as obligate endoparasites. 10. They make use of the biochemistry machinery of the host cell to make new virus particles. Bacteriophages = viruses that invade bacteria 1 Bacteria 1. They are microorganisms that are classified as prokaryotes that lack a nucleus & other organelles. 2. All bacteria have: a) a peptidoglycan cell wall b) a cell membrane c) cytoplasm d) a circular chromosome e) small ribosomes 3. Some have: a) capsule / slime layer b) flagella c) pili d) plasmids 4. Bacteria have different shapes: a) cocci b) bacilli c) spirilla d) vibrio 5. These cell shapes are used to classify them. 6. Bacteria reproduce by binary fission, a type of asexual reproduction. 7. It involves splitting of a parental cell into two identical daughter cells. 2 Binary fission of Mycobacterium tuberculosis 3 Transmission of pathogens 1. Transmission is the passing of an infectious disease, caused by pathogens, from an infected individual to another individual. 2. Pathogens are transferred through certain means of transmission: a) Physical contact eg. skin infections & sexually transmitted infections (STIs) i. Direct o By touching an infected person. ii. Indirect o By touching an object that has been touched by an infected person. b) Transfer of body fluids eg. HIV/AIDS & hepatitis B virus/ hepatitis B By blood / other body fluids from an infected person. Requires a break in the skin / by a needle. c) Disease-specific vectors eg. dengue virus/dengue fever by mosquitoes (vector). Other vectors include ticks & fleas. A vector is a living organism that transmits pathogens from one host to another. d) Food & water eg. Salmonella/food poisoning & Vibrio cholerae/cholera By contaminated food & water. External defence mechanisms against pathogens 1. External defences are non-specific defences. 2. Some of the external defences are as follows: a) Skin an impenetrable barrier toughened by keratin. has oil & sweat glands that give an acidic pH to inhibit the growth of pathogens. a home to non-pathogenic bacteria that prevents the growth of pathogens by outcompeting them. b) Mucous membranes They are specialised epithelial cells that line the digestive tract, urogenital tract & respiratory system. They secrete mucus that trap pathogens. Digestive tract also produces acidic stomach juices that kill pathogens. Urogenital tract excretes urine that washes away (flushing action) the pathogens. Respiratory system contains cilia that constantly move mucus, containing trapped particles & pathogens, out of system. 4  c) Ears produce acidic ear wax (cerumen) that contains lysozyme, an enzyme that kills bacteria. d) Eyes protected by flushing action of tears that contain lysozyme. 3. Lysozyme is also found in sweat, mucus & saliva. Non-specific immune responses 1. aka internal non-specific defences. 2. These responses include: a) inflammatory response b) fever 3. This line of defence is triggered when the pathogens enter the body. 4. It involves phagocytes & other cells. 5. Phagocytes are cells that engulf & digest invading pathogens & cell debris in the process called phagocytosis. 6. Common type: macrophages. 5  Phagocytes attacking larva of a parasitic worm Inflammatory response 1. Induced by any damage to the tissues. 2. Its purpose is to: a) reduce the spread, destroy & prevent the entry of pathogens. b) remove cell debris. c) repair of damaged tissues. 3. Characterised by redness, warm, swelling & pain. 6 A series of events in the inflammatory response 7 4. The phagocytes can be seen as pus. 5. Pus is dead bacteria & phagocytes. 6. Repair of damaged tissues takes place through mitosis. Fever 1. The leucocytes release chemical substances called pyrogens in response to pathogens. 2. Pyrogens act on the hypothalamus by resetting the body’s thermostat to a higher set level. 3. Advantages of fever: a) Inhibit the growth of pathogens. b) Speed up the rate of chemical reactions, therefore enhance the tissue repair. 4. Disadvantages of high fever (> 40oC): a) Enzymes denature. b) Permanent tissue damage. c) Cause sweating that leads to dehydration. Specific immune responses 1. Provide protection against a specific foreign antigen. 2. 2 responses: a) Antibody-mediated response (aka humoral response) involves B-cells that produce antibodies, proteins that circulate around the body & attack invading agents. Maturity of these lymphocytes happens in bone marrow. b) Cell-mediated response involves T-cells that destroy invading agents. Maturity of these lymphocytes happens in thymus. 3. Both lymphocytes can be found in the lymphoid tissue in the lymph nodes. Antigens 1. Any foreign substance that triggers a specific immune response is known as a non-self antigen. 2. Antigens can be: a) chemicals such as proteins, lipids, carbohydrates / nucleic acids. b) toxins made by bacteria. c) whole microorganisms (bacterial cell / virus). d) part of a bacterium (eg. flagella, cell wall / capsule). e) transplanted tissues / organs. f) allergens. Antibodies 1. aka immunoglobulins. 2. Specific Y-shaped proteins secreted by B-cells. 3. Made up of 4 polypeptide chains held together by disulfide bonds. 4. They have antigen-binding sites that are complementary with the antigens. 5. Hence, they are able to recognise antigens, bind to them & deactivate them. 8 Antibody-mediated response 1. Lymphoid tissue contains thousands of types of B-cells. 2. Each type is responding to a specific antigen. 3. When a specific antigen enters the body, one type of B-cell is activated. 4. This specific B-cell enlarges & divides into a group of cells called a clone. 5. Most of the clone differentiate into plasma cells which produce & release the specific antibody into circulation (lymph & blood). 6. The antibody molecules bind to antigens & deactivate them by: a) combining with antigens, hence preventing them entering cells. Enhance phagocytes b) causing agglutination (clumping together). activity c) reacting with soluble antigens to make them insoluble (precipitation). d) working as antitoxins (neutralisation). e) lysing the bacterial cells. 9 7. Some cells of the clone remain as memory cells. 8. They are long-lived cells which initiate a rapid response during re-infection by the same antigen. 10 11 Primary vs. secondary immune response 1. Primary response is a response when the pathogen invades the body for the first time. 2. It takes several days to produce large amounts of antibodies (slow response). 3. In contrast, secondary response is a more rapid response due to the presence of memory cells. 4. It happens when the same pathogen re-infects the body. 5. The pathogen is destroyed before symptoms appear. 6. Antibody production increases dramatically in a shorter time period. Cell-mediated response 1. This response is effective for intracellular infections, fungi, parasites, cancer cells & transplanted tissues / organs. 2. Like B-cells, there are thousands of types of T-cells in the lymphoid tissue. 3. Each type is responding to a specific antigen. 4. When a foreign antigen enters the body, one type of T-cell is activated. 5. This specific T-cell enlarges & divides into a group of cells called a clone. 6. Some cells of the clone remain as memory cells that initiate a rapid response during re- infection by the same antigen. 7. Most of the clone differentiate into 3 different types of T-cell: a) Killer T-cells (Cytotoxic T-cells) kill cells that display antigen. b) Helper T-cells assist both humoral & cell-mediated response. c) Suppressor T-cells (Regulatory T-cells) stop immune response when all antigens have been destroyed. 12 13  Summary of immune responses Treatment of pathogen induced infections 1. There are 2 treatments: a) Antiviral drugs b) Antibiotic drugs Antivirals 1. Specifically for treating viral infections. 2. They inhibit the viral development. 3. Some common eg: a) zidovudine aka azidothymidine (AZT). used to treat AIDS caused by HIV. inhibits reverse transcriptase. Hence, affect the replication of HIV. b) acyclovir used to treat chickenpox & shingles caused by varicella zoster virus. inhibits DNA polymerase. Hence, prevents viral DNA synthesis. 14 Antibiotics 1. Antibiotics are chemical substances produced by microorganisms, mainly bacteria & also some fungi, that are capable of killing / inhibiting the growth of an infective microorganism, particularly bacteria. 2. They are ineffective against viral infections. 3. If the antibiotic kills the microorganisms, the action is described as microbicidal (bactericidal). 4. It kills the bacteria by: a) preventing the formation of cell wall (eg. penicillin & cephalosporin). b) changing the structure of cell membrane (eg. polymyxin). c) disrupting the action of enzymes such as RNA polymerase (eg. rifampicin). 5. If the effect is inhibition, then the action of this antibiotic is expressed as microbiostatic (bacteriostatic). 6. It inhibits the protein synthesis by binding to ribosomes (eg. streptomycin), hence prevent the bacterial reproduction. 7. Antibiotics that act on a wide range of bacteria are known as broad-spectrum antibiotics whereas those target a limited range of bacteria are called narrow-spectrum antibiotics. Prevention of pathogen induced infections 1. Vaccination is the administration of antigens that have been made harmless (weakened) to produce immunity to diseases. 2. This is called artificial active immunity. 3. Vaccines are commonly administered either by injection / oral. 4. A vaccine is a preparation containing harmless antigenic materials: a) Dead / inactivated microorganisms. b) Attenuated microorganisms (living but modified, therefore harmless). c) Toxoids (harmless form of toxins). d) Preparation of harmless antigen (aka sub-unit vaccine): i. Fragments of outer coats of viruses. ii. Purified polysaccharides from bacterial cell walls. iii. DNA segments. 5. The process of vaccination manipulates the immune system so that an individual is immune to a disease before they become infected. 15 16 An overview of vaccine production using recombinant DNA technology 1. Modern vaccines are now developed using recombinant DNA technology. 2. In this method, DNA sequence from pathogen is inserted into harmless bacterial cells. 3. Antigens of the pathogen are produced. 4. Vaccination with these harmless bacteria results in immunity against the pathogen. Immunity 1. It is resistance to infection by pathogens. 2. There are 4 types of immunity: a) Natural passive immunity b) Natural active immunity c) Artificial passive immunity d) Artificial active immunity 17 Herd immunity 1. Some individuals cannot be vaccinated due to some reasons such as allergy. 2. In order to protect this risk group of people, a large number of people are vaccinated so that those who are vulnerable will never encounter the pathogens. 3. Disease spread cannot occur as there are too few susceptible individuals. 4. This effect is called herd immunity. 18 Participation 1. Participation in an immunisation program may be influenced by social, economic / cultural background. 2. Several factors that influence decision-making by individuals about vaccination: a) Low income / expensive vaccines. b) Poor parental education / awareness on the benefits of vaccination. c) Religious belief. d) More confidence on traditional medicines. e) Unreliable information from Internet & other media. f) Complementary & alternative medicine (CAM) belief. eg. acupuncture & diets have greater benefits than risks compared to vaccines. Some practitioners involved with CAM advise against vaccination. 3. Consequences: a) Low herd immunity. b) High possibility of outbreaks. 19