Module 4: Communicable Diseases Flashcards
Define the term pathogen
Pathogens are organisms that cause a disease.
- Take nutrition from host as energy source.
Bacteria (prokaryotes) ‐ e.g. tuberculosis
Fungi e.g. athlete’s foot
Viruses e.g. HIV
Protoctista e.g. malaria
Describe the 4 main groups of pathogens including examples of each
Bacteria (Prokaryote Kingdom) • Smaller the Eukaryote cells • Reproduce rapidly • Damage cells through release of toxins • e.g. TB & Ring Rot in potatoes and tomatoes
Fungi
• Fungus often lives in skin, hyphae form a mycelium
• Reproductive hyphae grow into the skin & release spores (redness to skin)
• In plants lives in vascular tissue to gain nutrients
• Hyphae release extracellular digestive enzymes to break down cellulose. (decays plant)
• e.g. Black sigatoka, ringworm
Protoctista
• Enter host cell & feed on the contents of the cell
• Malaria parasite Plasmodium has immature forms
which feed on haemoglobin
• e.g. Potato Blight
Viruses
• Viruses invade cells and take over genetic
machinery & other organelles
• Causes the cell to manufacture more copies of the virus
• Host cell eventually bursts, releasing new viruses to
invade new host cells
• e.g. HIV, Influenza, tobacco mosaic virus
What are the 2 main methods of disease transfer?
Direct transfer:
Transmission via Physical contact, Faecal transmission,
Droplet infection, Spores transmission
Indirect transfer:
Transmission via vectors e.g. malaria via female anopheles mosquitos and Dutch elm disease is caused by a beetle vector.
What factors affect disease transfer?
Poor diet Poor health Overcrowding Migration Poor ventilation Sex
Explain disease transfer in plants
Can be spread directly or indirectly. Most common infection of plants is through the roots, particularly if there is any damage around the roots.
This is an example of direct transmission.
Airborne transmission ‐ Fungi releasing spores which get carried by the wind.
Once a pathogen has infected it may infect all vascular tissue, when the leaves are shed the pathogen will be spread to the soil where it can infect other plants.
What are the 2 main types of plant defences?
Physical defences (active or passive) Chemical defences
Why do plants have defences?
As plant provide a rich source of nutrients for many organisms they need to have defences in order to protect themselves. Plants do not have an immune system so they require defences in order to prevent extensive damage.
What are passive defences?
Passive defences are defences present prior to infection, their role is to prevent spread and infection of the pathogen. They can be split into physical and chemical defences.
Examples:
Cellulose Cell Wall ‐ Physical barrier waterproofed by lignin, contains tannins ‐ chemical defences which are activated if a pathogen is detected.
Waxy Cuticle ‐ Prevents water collecting which can contain pathogens.
Bark ‐ Physical barrier, contains chemicals which will work against pathogens.
Tylose formation ‐ Tylose is a balloon like projection which fills the xylem, acts a plug preventing the xylem from carrying water, prevents the spread of pathogens. Tylose contains high levels of terpenes which are toxic to many pathogens.
Callose ‐ Callose is a large polysaccharide molecule which is deposited within sieve tubes when a pathogen is detected. It blocks the flow in the sieve tube and at plasmodesmata preventing the spread of the pathogen into cells.
What are chemical defences?
Terpenoids, Phenols, Alkaloids, and hydrolytic enzymes can be found in plant tissues which have anti‐pathogenic properties.
Tyloses and tannins are found in bark before infection, however they use a lot of energy to create many chemicals which are not made until infection is detected.
What are active defences?
When a plant becomes infected with a pathogen, proteins in the cell wall of the plant detect and they plant can respond:
These include‐
• Cellulose cell walls thicken with more cellulose
• Callose deposition
• Oxidative bursts that produce highly reactive oxygen molecules ‐damage the pathogen
• Necrosis ‐ deliberate death of infected cells to stop spread
• Increase in the production of chemicals (terpenoids, phenols, alkaloids, defensins, hydrolytic enzymes)
What are primary defences?
Defences that prevent a pathogen from entering the body.
E.g.
- Lysozyme in tears- kills pathogens in the eyes.
- Mucus traps pathogens in airways.
- Blood clots prevents pathogens entering the blood.
Describe primary defences and outline their importance
1) Mucous membranes- specialised epithelial tissue covered in mucous:
At exchange surfaces, the diffusion distances into the blood are small ‐ this makes them more susceptible to infection from pathogens. Mucous membranes can coat these surfaces as a primary defence.
2) Coughing, sneezing and vomiting:
Coughing, sneezing and vomiting are expulsive reflexes ‐ the irritation caused by microbes or their toxins causes the expulsion which will carry microbes with it .
3) Inflammation- swelling and redness of tissue caused by infection:
• Microbes detected by mast cells which release histamine.
• Histamine causes vasodilation ‐ makes capillaries more permeable so more WBCs can leave.
• More tissue fluid forms because more plasma leaves.
• This causes swelling (oedema).
• Tissue fluid can drain into the lymph vessels so that pathogens may come into contact with lymphocytes (WBCs) and cause a specific immune response.
Describe how the blood clots
• Damage to a blood vessel ‐ platelets bind to exposed collagen to form a temporary platelet plug.
• Platelets also release clotting factors which activate an enzyme cascade.
• Enzymes cause fibrinogen to form insoluble fibres which attach to the plug.
• RBCs are also trapped ‐ this forms a clot.
• Clot dries and forms a scab which pulls the skin closer together.
• Under the skin collagen is deposited.
• Stem cells in epidermis divide by mitosis and differentiate to form new skin cells at the
edge of the cut.
• New blood vessels form.
• When edges of the cut are drawn together the repair is complete.
Define a secondary defence
Secondary defences attempt to kill a pathogen after it has entered the body.
What is a phagocyte?
Phagocytes attempt to kill pathogens before they can reproduce and cause any symptoms ‐ they are non‐specific.
Describe the 3 types of phagocyte
• Neutrophils:
‐ most common
‐ travel in blood
‐ pass into tissue fluid (possible because they can change shape and have a multilobed nucleus, allowing them to fit between gaps in endothelium)
• Macrophages:
‐ larger
‐ travel in blood as monocytes
‐ settle in lymph nodes and become macrophages
‐ initiate immune response (antibody production)
• Antigen Presenting Cells:
‐ when in contact with antigens can mount a full immune response
‐ activates T & B Lymphocytes (clonal selection)
‐ cells signal using hormone like chemicals (cytokines)
‐ stimulates production of B & T cells and Macrophage
How does a phagocyte engulf and digest a pathogen?
- receptor on phagocyte’s cell surface membrane binds to antigen on pathogen’s cell surface membrane.
- pathogen engulfed by endocytosis.
- this produces a phagosome (phagocytic vesicle).
- lysosomes fuse with phagosome, releasing enzymes (lysins) into it.
- the pathogen is digested into amino acids and fatty acids etc.
- products are absorbed into cytoplasm by diffusion.
The whole series of events is co ordinated by hormone like chemicals knows as cytokines. Cytokines stimulate B cells, T cells and Macrophages.
What are B and T lymphocytes?
White blood cells are made in the bone marrow when cells differentiate into phagocytes and lymphocytes.
- B lymphocytes mature in the bone marrow
- T lymphocytes mature in the thymus
Both are cells with large nuclei and specialised receptors on their cell surface membranes.
Mature B and T lymphocytes circulate around the blood and lymph.
Triggering an immune response
• The antigens on the pathogen’s surface communicate to our body’s cells that it is foreign.
• To initiate an immune response, the pathogens have to be detected by B and T lymphocytes with the correct complementary receptors to the pathogen’s antigens. The
white blood cells communicate using signalling molecules called interleukins.
- Infected cells sometimes get the pathogen’s antigens on their surfaces ‐ this helps to select the right B and T lymphocytes.
- Macrophages in the lymph nodes engulf and digest pathogens. They separate the pathogen’s antigens and incorporate them into their own cell surface membrane. They are now antigen presenting cells ‐ they increase the chances of the correct T lymphocytes locating the foreign antigens.
- The selection of the correct lymphocytes with receptors complementary in shape to the antigens is call clonal selection.
- More of these lymphocytes are needed to fight the pathogens so they divide by mitosis in clonal expansion.
Describe 3 examples of cell signalling in the immune response
- Pathogens antigens communicate to body cells that they are foreign.
- Infected with foreign antigens on surface communicate to lymphocytes to be selected in clonal selection and to T killer cells that they need to be killed.
- Macrophages engulf and digest pathogens and incorporate the pathogen’s antigens on their cell surface membrane ‐ communicates to T lymphocytes to be selected in clonal selection.
- T helper cells release cytokines ‐ bind to receptors on B cells and stimulate them divide by mitosis and differentiate.
The changes and roles of T lymphocytes in the immune response
- Differentiate into 4 types of cell
- T helper cells (T h) release cytokines with specific shapes which bind to complementary receptors on the cell surface membrane of B lymphyocytes, stimulating them to divide by mitosis and differentiate. They also stimulate macrophages to carry out more phagocytosis.
- T killer cells (T k) search for and kill infected host cells by secreting protease enzymes into them.
• T memory cells (T m) which stay in the blood in case there is a second invasion by the same pathogen. They allow a faster secondary response because they recognise the antigen and can make clones and change to form new T cells more quickly than in the primary
response.
• T regulator cells (T r) shut down the immune response once the pathogen has been removed
The changes and roles of B lymphocytes in the immune response
- Differentiate into two types of cells involved in the humoral response :
- Plasma cells which produce and secrete antibodies which are complementary in shape to the antigen.
- B memory cells which stay in the blood in case there is a second invasion by the same pathogen. They allow a faster secondary response because they recognise the antigen and can make clones and change to form new plasma cells and so antibodies are made more quickly than in the primary response.
Define antigens and antibodies
Antigens ‐ protein/glycoprotein found on the cell surface membrane of cells. Foreign antigens stimulate an immune response.
Antibodies ‐ proteins which identify and neutralise antigens. Each antibodies is specific to a particular antigen due to the complementary shapes of the antigen and variable region of the antibody.
- Antibodies are made by B lymphocytes
- They are released in response to infection
- 1 specific antibody per antigen
Describe how the structure of antibodies are related to their function
4 polypeptide chains held together by disulphide bonds‐ Y
shaped molecule.
Constant region - for binding to phagocytes
Variable region - for binding to antigens
More than one variable region - allows attachment to more than one antigen (hence attaching to more than one pathogen)
Hinge region - Allows flexibility for the branches of the Y shaped molecule to move closer/further apart to bind to more than one antigen
