4.1 communicable diseases

• Bacteria and viruses are the main disease causing pathogens in humans
• Bacteria are prokaryotic cells with no membrane-bound organelles and their genetic information is stored in a circular strand of DNA
• Viruses consist of nucleic acid enclosed in a protein coat and their genetic material can be DNA or RNA
• Bacteria do not require a host to survive, while viruses are entirely dependent on their hosts
• Viruses are significantly smaller than bacteria
• Bacteria have a cell membrane, cell wall, cytoplasm, ribosomes, plasmids, flagellum, and pili, while viruses do not possess such structures
• An example of a bacterial disease is tuberculosis (TB) caused by Mycobacterium tuberculosis infecting phagocytes in the lungs
• The first infection of TB is symptomless as infected phagocytes are sealed in tubercles due to an inflammatory response in the lungs
• TB bacteria lie dormant inside tubercles, covered with a thick waxy coat, until the immune system weakens, leading to breathing problems, coughing, weight loss, and fever
• Meningitis can be caused by bacterial infection of the meninges
• An example of a viral infection is HIV (Human Immunodeficiency Virus) causing AIDS
• First symptoms of HIV include fevers, tiredness, and headaches
• HIV antibodies appear in blood, making a person HIV positive, and symptoms reappear when the immune system weakens, leading to AIDS
• Influenza virus infects the ciliated epithelial cells of the gas exchange system
• Athlete’s foot in humans is caused by a fungus spread by direct contact with spores
• Malaria is transmitted indirectly via a vector, a female Anopheles mosquito
• Plants have physical and chemical defences against pathogens
• Physical barriers include cellulose cell walls, lignin layer, waxy cuticles, and blocking of old vascular tissue
• Mechanisms like closing stomata, thickening cell walls, callose deposits, necrosis, and canker help prevent the spread of infection in plants
• Chemical defences in plants include menthols, phenols, alkaloids, defensins, and hydrolytic enzymes
• Primary non-specific defences in animals include skin, stomach acid, gut and skin flora, inflammation, lysozyme action, interferon, phagocytosis, and blood clotting
• Phagocytes present antigens on their surface after destroying pathogens, stimulating the immune response if the antigen is recognized as foreign
• The specific immune response is antigen-specific and relies on lymphocytes produced in the bone marrow
• B cells mature in the bone marrow and are involved in the humoral response
• T cells mature in the thymus gland and are involved in the cell-mediated response
• Antibodies are globular protein molecules produced by lymphocytes
• Antibodies attach to specific antigens, inhibiting their action through neutralization, facilitating phagocyte binding, agglutination, and neutralization of toxins
• Antibodies have constant and variable regions, hinge regions for flexibility, and are composed of four polypeptide chains linked by disulphide bridges
• Immunity can be active or passive, natural or artificial
• Active immunity results from the immune system producing antibodies in response to an antigen, while passive immunity results from the introduction of antibodies from another source
• Natural active immunity comes from exposure to an antigen, while natural passive immunity is from maternal antibodies crossing the placenta or in breast milk
• Artificial active immunity is acquired through vaccinations, and passive artificial immunity involves injecting antibodies
• Antibiotics can be bactericidal (kill bacteria by destroying their cell wall) or bacteriostatic (inhibit bacterial growth)
• Bacteria can become resistant to antibiotics through natural selection, leading to antibiotic-resistant bacterial infections like MRSA
• Hospitals control the spread of antibiotic-resistant infections by screening new patients, completing antibiotic courses, and following strict hygiene regimes