Communicable Diseases

• Types of pathogens

  - Bacteria

  - Viruses

  - Protoctista

  - Fungi

  
• What are the 2 bacterial diseases?

  - Tuberculosis

  - Ring rot
• Tuberculosis

  Infects humans, deers, cows, pigs, badgers

  Causes harm by damaging lung tissue and suppressing the immune system

  Cured using antibiotics and preventing through vaccinations

  
• Ring rot

  Infects potatoes and tomatoes

  Damages leaves, tubers and the fruit itself

  
• Structure of viruses

  Consists of DNA/RNA, a capsid and attachment proteins

  
• Structure of HIV

  - Capsid: Outer protein coat

  - Core: Genetic material (RNA) and the enzyme reverse transcriptase which is needed for viral replication

  - Envelope: An outer extra layer made of lipids taken from the host's membrane

  - Protein attachment: Enables the virus to attach to the host's helper T cell

  
• HIV

  HIV is transported in the blood until it attaches to a CD4 protein on a helper T cell. The HIV protein capsule and then fuses with the helper T cell membrane, enabling the RNA and enzymes from HIV to enter. The HIV enzyme reverse transcriptase copies the viral RNA and moves to the helper T cell nucleus. mRNA is transcribed and the helper T cell starts to create viral proteins. AIDS is when the replicating viruses in the helper T cells interfere with the normal functioning of the immune system. It is unable to produce an adequate immune response to other pathogens (left vulnerable to infections and cancer).

  Destruction of immune system —> Death.
• Influenza

  Viral infection of the ciliated epithelial cells in the gas exchange system

  Leaves the airways open for secondary infection.

  Can be fatal to young children/older people/people with chronic diseases

  
• Tobacco Mosaic Virus

  Virus that infects tobacco plants, including tomatoes

  Damages leaves, flowers and fruit, stunts growth, resulting in a mosaic pattern and reduces yield.

  Resistant crop strains are available, but there is no cure.

  
• What is a protoctista?

  Eukaryotes that exist as a single-celled organism

  
• Malaria

  The parasite reproduces both sexually (in mosquitoes) and asexually (in human hosts). Passes from mosquitoes to humans when female mosquitoes bite and take blood from humans. Infects red blood cells, the liver and brain.

  There are some preventative medicine, but there is no cure.

  The vector can be controlled using insecticides, mosquito nets and removing the breeding sites of mosquitoes

  
• Potato blight

  Causes potato blight and tomato late blight

  Has hyphae which enters the plant and causes damage to the leaves and fruit

  There is no cure, but resistant strains have been developed

  
• Fungi

  Eukaryotes that can be multicellular or unicellular

  
• Black sigatoka (banana disease)

  Infects bananas

  The hyphae penetrates and digests cells, turning them black and preventing plant growth

  Fungicides can be used to kill the fungus and resistant strains have been developed

  
• Athlete's foot

  Infects only humans and is a type of ringworm

  Thrives in warm, moist regions between the toes

  Causes the skin to crack and causes itchiness and discomfort

  Cured using antifungal creams

  
• What are the modes of transmission?

  Direct

  Indirect

  Hot climates

  Social Factors

  
• Hot climates

  Increased heat provides kinetic energy for chemical reactions and reproduction
• Social Factors (poor/developing countries)

  It could result in poorer sewage infrastructure, lack of fresh water, poorer sanitation

  Medicines and vaccinations are also less readily available
• Direct transmission

  - Direct contact: Touching, contact with cuts on skin, sexual contact

  - Ingestion: Drinking and eating contaminated food

  - Inoculation: Sharing needles, animal bites

  
• Indirect transmission

  - Vectors: Animals that pass pathogens to humans, e.g. mosquitoes transmitting malaria

  - Droplets: Pathogens transmitted in droplets of water, e.g. saliva/mucus being expelled when sneezing

  
• Modes of transmission - plants

  Direct transmission: Direct contact with other plants, e.g. ring rot/Tobacco mosaic virus

  Indirect transmission: Contaminated soil (pathogens and their spores can remain in the soil and infects the roots of subsequent plants) and vectors (wind, water, animals carry pathogens and their spores from one plant to another)

  
• Plant defences

  - Barriers to prevent entry of pathogens, e.g. waxy cuticle/bark

  - Antibacterial chemicals as defence against bacterial infections. It can repel insects and kill pathogens

  - Physical defences such as the production of callose

  
• Primary non-specific defences against pathogens in animals

  - Skin: Serves as a physical barrier (dead cells prevent penetration of pathogens) and contains skin flora which competes with the pathogens for space and resources

  - Mucous membranes: Lines many body tracts. The mucus traps pathogens and cilia wafts the mucus away

  - Expulsive reflexes: Sneezing, coughing, vomiting forces pathogens out of the body

  - Inflammation: Happens in localised areas where cells have been damaged. It causes the area to be hot, itchy and swollen. When the cells are damaged, this triggers mast cells to release histamines and cytokines. Histamines cause blood vessels to dilate so more blood flows to the area and the temperature kills the pathogens. Furthermore, it makes the walls of blood vessels more permeable, so more white blood cells are delivered to the site of damage

  Blood clotting: When the skin is cut it activates platelets which trigger blood clotting. The platelets form a plug over the damaged area and release a range of substance and via a cascade of events results in the formation of insoluble fibrin. The insoluble fibrin forms a network, trapping the platelets and forming a clot

  
• What is an antigen?

  A protein molecule on the cell-surface of a membrane of non-self cells
• What are phagocytes (macrophages and neutrophils)?


  Travel in the blood and engulf and digest pathogens

  
• Process of phagocytosis (non-specific response)

  1) Pathogens release cell-signalling chemicals (cytokines) which attract phagocytes to the site of infection

  2) An opsonin protein attaches to pathogens to mark them. This makes it easier for neutrophils and macrophages to engulf them

  3) The phagocyte them engulfs the pathogen into a vesicle to create a phagosome. Within the phagocytes are lysozymes, which contain hydrolytic enzymes

  4) Lysozyme fuses with the phagosome (becoming a phagolysosome). The lysozyme hydrolyses the pathogen and any soluble, useful molecules are absorbed into the cytoplasm of the phagocyte

  5) The phagocyte present the antigen of the digested pathogen on their surface - they are called antigen-presenting cells.

  
• When does the second line of defence happen?

  If the first line of defence does not destroy the pathogen or prevent it from entering
• What is the second line of defence (specific response)?

  - B lymphocytes (B cells)

  - T lymphocytes (T cells)
• Structure of B lymphocytes

  
• Structure of T lymphocytes

  Receptors on T cells bind to antigens on antigen-presenting cells (APCs). This causes the T cell to divide rapidly by mitosis (clonal expansion)

  
• Where are B cells and T cells produced?

  Both B cells and T cells are created by bone marrow stem cells. But B cells mature in the bone marrow, whereas T cells mature in the thymus

  
• What are antigen-presenting cells (APCs)?

  Cells that present a non-self antigen on their surface

  
• Examples of antigen-presenting cells (APCs)

  - Infected body cells: Presenting viral antigens on their surface

  - Macrophage

  - Cells of a transplanted organ: Will have different shaped antigens on their surface compared to self-cell antigens

  - Cancer cells: Have abnormal-shaped self antigens

  
• Cell-mediated response (T cells)

  1) Once the pathogen has been engulfed and destroyed by the phagocyte, the antigens are positioned on the cell surface. It is now called an antigen-presenting cell (APC)

  2) T helper cells have receptors on their surface which attach to the antigen on the APC

  3) Once attached, interleukins are produced which activate T helper cells to divide by mitosis to replicate and produce a large number of clones

  4) Cloned T helper cells can differentiate into different cells

  
• What cells can cloned T helper cells differentiate into?

  - T memory cells (for the shaped antigen)

  - T killer cells: Destroy abnormal or infected cells. It releases a protein which embeds in the cell surface membrane, creating a pore. This allows any substances to leave and enter the cell and this causes cell death

  - T regulator cells: Supresses the immune system to make sure that cell-mediated response only happens when a pathogen is detected

  - T helper cells and produce interleukins to activate B lymphocytes

  - Some produce more interleukins which stimulate macrophages to perform more phagocytosis
• Humoral Response (Involves antibodies)

  T helper cells produce interleukins which stimulate the production of B cells. This initiates the humoral response which involves antibodies

  
• Structure of antibodies

  Antibodies are globular proteins that have a binding site complementary to antigens

  Made of 4 polypeptide chains: 2 heavy polypeptide chains and 2 light polypeptide chains

  The binding site is the variable region in which it is complementary to the antigen

  The rest of the antibody is the constant region

  
• What happens when an antigen binds to an antibody?

  An antigen-antibody complex forms
• What are the 3 ways in which antibodies work?

  - Agglutination

  - Marking pathogens

  - Acting as anti-toxins
• Agglutination

  Agglutination is the clumping of pathogens together which make it easier for phagocytes to locate and engulf them