4.1 - communicable diseases

communicable diseases

Diseases that can be passed from one organism to another

what are communicable diseases caused by?

Pathogens

Types of pathogens

• bacteria

• Viruses

• Protoctists

• Fungi

Bacteria as a pathogen

Produce toxins that damage body cells

Viruses

Use host cells to replicate before bursting out and destroying cells

Protoctists as pathogens

Take over cells and break them open

Fungi as pathogens

Digest living cells to destroy them. Some also produce toxins

Ways of direct transmission

• Direct contact

• Airborne droplets - Coughing or sneezing tiny droplets of mucus or saliva onto someone

examples of indirect transmission

• Food and drinking water - Ingestion of contaminated food or water can cause disease.

• Vectors - These transmit pathogens from one host to another

• Contaminated objects - Pathogens from infected individuals can live on objects for a short time and infect others.

how is risk of communicable diseases increase?

• overcrowded areas increase the risk of direct transmission

• climate

• lack of health education and healthcare systems increase the risk of communicable disease

tuberculosis

• pathogen - bacterium

• damages lungs and suppresses immune system

• transmitted by airborne droplets

bacterial meningitis

• bacterium

• damages membranes of the brain and can cause blood poisoning

• transmitted by airborne droplets

HIV/AIDS

• virus

• gradually destroys immune system

• transmitted by exchange of bodily fluids

Athlete’s foot

• fungus

• causes a circular, red rash

• transmitted by direct contact

malaria

• protoctist

• damages RBCs, liver and the brain

• transmitted by vectors

direct transmission of plants

healthy plants comes into direct contact with any part of an infected plant

indirect transmission of plants

can take place via soil contamination when infected plants leave pathogens or spores in the soil ready to infect other plants

examples of plant vectors

• wind - carries spores to uninfected plants

• water

• animals - carry pathogens or spores from one plant to another

• humans

how risk of communicable disease can be increased among plants

• crop variety - some crops are more susceptible to disease than others

• overcrowding - increases the likelihood of direct contact

• poor nutrition - reduces resistance of plants

• climate change - increased rain and wind increases the spread of disease

ring rot

• bacterium

• potatoes and tomatoes affected

• damages leaves, tubers and fruit

tobacco mosaic virus

• virus

• affects tobacco, tomato etc.

• damages leaves, flowers and fruit

black sigatoka

• fungus

• affects banana plants

• attacks and destroys leaves, turning them black

potato blight

• protoctists

• affects potatoes and tomatoes

• destroys leaves, tubers and fruit

physical plant defences

• waxy cuticle - provide a physical barrier against pathogens

• cell walls

• production of callose - when plants are attacked by pathogens they produce callose, which is deposited between the cell wall and cell-surface membrane to make it harder for pathogens to enter cells

chemical plant defences

• insect repellents - reduce the number of insects feeding on plants to prevent them from transmitting pathogens

• insecticides - kill insects to prevent them from transmitting pathogens

• antibacterial substances are produced to kill bacteria or inhibit their growth

non-specific defences

defence mechanisms that act quickly to defend the body, but respond in the same way for all pathogens

specific defences

defence mechanisms that are slower to defend the body but produce a specific response for each pathogen

how does the skin act as a chemical and physical barrier?

• physical - blocks pathogens from entering the body

• chemical barrier by producing sebum

how do mucous membranes acts a barrier?

secrete mucus to trap pathogens and use lysozymes to destroy them

expulsive reflexes

• coughing and sneezing expel foreign objects from the gas exchange system

• vomiting and diarrhoea expel the contents of the gut along with any pathogens present'

how do clotting and wound repair act as barriers?

• blood clots seal wounds. when the clot dries a scab forms that blocks entry to the body

• after a scab has formed, the skin repairs itself to reform its physical barrier

how does inflammation act as a chemical barrier?

• damaged tissues release chemicals that dilates blood vessels to increase blood flow to the area, making it hotter to prevent pathogens from reproducing

• blood vessels become more permeable and leak tissue fluid, causing swelling and isolating any pathogens

antigens

unique molecules on the surface of cells that can trigger an immune response

• they allow the immune system to distinguish between the body’s own cells and foreign cells

phagocytosis

a type of non-specific defence

phagocytes

a type of WBC that engulf and destroy pathogens (phagocytosis)

two main types of phagocytes

neutrophils - rapidly engulf and destroy pathogens at the site of infection

macrophages - engulf and digest pathogens but also prevent the pathogen’s antigens on its cell surface to activate other cells in the immune system

process of phagocytosis

• pathogen releases chemicals that attract a phagocyte

• phagocyte recognises the pathogen’s antigens as non-self, which causes the phagocyte to bind to the pathogen

• phagocyte engulfs the pathogen

• pathogen is now contained within a vesicle known as a phagosome

• the lysosome fuses with the phagosome to form a phagolysosome

• lysozymes digest and destroy the pathogen

• the phagocyte presents the pathogen's antigens on its surface to activate other cells in the immune system (antigen-presenting cell)

cytokines

chemicals released by phagocytes that have engulfed a pathogen. They act as cell-signalling molecules to trigger the movement of other phagocytes to the site of infection

opsonins

chemicals that bind to pathogens to make them easily recognisable by phagocytes.

how are phagocytes adapted to their function?

they contain receptors on their cell-surface which bind to common opsonins, making it easier for the phagocyte to bind to the pathogen and destroy it

two types of lymphocyte

• T lymphocytes

• B lymphocytes

T lymphocytes

cells that are involved in the cellular response where they respond to antigens presented on body cells

• mature in the thymus gland

B lymphocytes

cells that are involved in the humoral response where they produce antibodies found in body fluids

• mature in the bone marrow

T helper cells

• have receptors on their cell-surface that bind to complimentary antigens on antigen presenting cells

• produce interleukins (a type of cytokine), which stimulate B cells or phagocytes

T killer cells

kill abnormal and foreign cells by producing perforin (a protein that makes holes in the cell-surface membrane, causing it to become freely permeable and causing cell death)

T regulator cells

cells that suppress the immune system after pathogens have been destroyed. This helps to prevent the immune system from mistakenly attacking body cells

T memory cells

cells that provide long-term immunity against specific pathogens. They provide a rapid response if the body is re-infected by the same pathogen

stages of the cellular response

• macrophages engulf pathogens and display their antigens on the cell-surface (APCs)

• T helper cells with complementary receptors bind to these antigens

• After binding, the T helper cell is activated to divide by mitosis to form genetically identical clones

functions of cloned T cells

• develop into memory cells - provide LT immunity

• Develop T killer cells

• Stimulate phagocytosis

• stimulate division of B cells

humoral response

a specific defence mechanism what involves the use of B lymphocytes

B cells

• have antibodies on their cell-surface membrane that bind to complementary antigens

• Once activated, B cells can divide into plasma cells and memory cells

plasma cells

• types of B cell that can produce and secrete antibodies against a specific antigen

memory B cells

• types of B cell that provide long-term immunity against specific pathogens

• They rapidly divide into plasma cells if the body is re-infected by the same pathogen

Helper T cells

cells that bind to antigen-presenting cells to activate the division of B cells

stages of the humoral response

• B cell with a complementary antibody binds to the antigens on a pathogen

• B cell engulfs the pathogen and presents its antigens on the cell-surface membrane (APC)

• Clonal selection - Activated T helper cells bind to the B cell, causing activation of this B cell

• Clonal expansion - The activated B cell divides by mitosis to form plasma and memory cell clones

• The cloned plasma cells produce and secrete the specific antibody which is complementary to the antigen on the pathogen's surface. They attach to antigens on pathogens and destroy them

• memory cells circulate the blood and tissue fluid, ready to divide if the body is re-infected by the same pathogen

types of immune responses

primary and secondary

Primary immune response

takes place when the body is exposed to a pathogen for the first time. This response is slow and the infected individual experiences symptoms of the disease

secondary immune response

takes place when when the body has been exposed to the same pathogen before. This response is much faster and stronger and pathogens are destroyed before any symptoms appear

why are B cells able to quickly divide into plasma cells in the secondary immune response?

because the memory B cells recognise the pathogen’s antigens

autoimmune disease

occurs when an immune system cannot recognise self antigens and start to attack them

examples of autoimmune disease

• Type 1 diabetes - The immune system attacks the insulin-secreting cells of the pancreas, causing a lack of insulin

• Lupus - The immune system attacks cells in the connective tissues, causing inflammation

antibody structure

• Y-shaped glycoproteins

• made of four polypeptide chains (two heavy chains and two light) which are held together by disulphide bridges

regions on the antibody

• constant - binds to receptors on cells such as B cells

• variable - different for each antibody as its shape is complimentary to a specific antigen

• hinge - allows the antibody to be flexible so it can bind to multiple antigens at once

three roles antibodies carry out to help destroy pathogens

1. Agglutination of pathogens - involves clumping pathogens together to enable easier phagocytosis

2. Neutralisation of toxins - when antibodies bind to toxins to inactivate them

3. Preventing pathogens from binding - when antibodies bind to pathogens to stop them from infecting body cells

types of immunity

active and passive

active immunity

• develops when the immune system makes its own antibodies after exposure to a pathogen’s antigens

• takes a while to become immune to the disease, but provides LT protection because memory cells are produced

passive immunity

• develops when an individual is given antibodies made by a different organism

• provides immediate immunity to the disease, but is ST protection because the antibodies are broken down and memory cells are not produced

vaccination

• involves the introduction of a pathogen’s antigens into the body.

why are vaccines a form of artificial active immunity?

the body is stimulated to produce an immune response to the pathogen

what may vaccines contain?

• dead of inactivated pathogens

• a harmless version of a toxin

• isolated antigens from a pathogen

how vaccinations provide immunity

1. The vaccine, containing antigens, is injected into the blood. 

2. This stimulates the primary immune response to produce antibodies against the pathogen. 

3. Memory cells, capable of recognising these antigens, are produced. 

4. On second exposure to this pathogen, memory cells rapidly divide into plasma cells. 

5. Plasma cells rapidly produce antibodies against the pathogen. 

6. The pathogen is destroyed before any symptoms are experienced.

Criteria for a successful vaccination programme

• Availability - Suitable vaccines must be affordable and available in large amounts for mass immunisation

• Minimal side effects - The fewer the side effects from the vaccine, the better the public acceptance

• Administration - Proper and timely vaccine administration is important, requiring trained healthcare workers

herd immunity

the idea that vaccines provide immunity to those that receive them, but they can also provide some protection to those not vaccinated

• only works when a large portion of a population are vaccinated

Factors that may prevent the elimination of a disease

• individual immunity failures - People with weak immune systems may not be able to withstand vaccines

• Vaccine objections - Personal, religious, ethical, or medical objections to vaccination can hinder disease eradication

• Pathogen mutation and antigenic variability - Rapid antigenic changes due to frequent mutations can make vaccines ineffective

antigenic variability

a process in which pathogens change their antigens, making it difficult to develop vaccines against some pathogens

Antibiotics

drugs that kill or inhibit the growth of bacteria

Examples of how antibiotics affect bacteria

• Preventing the synthesis of bacterial cell walls.

• Disrupting enzyme action.

• Preventing DNA synthesis.

• Preventing protein synthesis.

antibiotic resistance

antibiotics that were once effective against these bacteria no longer work, making it much more difficult to treat bacterial infections

how does antibiotic resistance occur?

• via natural selection

• Genetic mutations occur, making some bacteria resistant to an antibiotic.

• When an infection is treated with antibiotics, resistant bacteria are able to survive.

• Resistant bacteria reproduce, passing on the allele for antibiotic resistance to their offspring.

why is the development of resistant bacteria a problem?

because it means that certain bacterial infections are becoming more difficult to treat

measures that can help reduce the development of antibiotic resistance

1. Choosing appropriate antibiotics for treatment - Antibiotics can be tested against bacterium strains to make sure they are effective in treating the disease.

2. Using antibiotics only when needed - Antibiotics should only be prescribed for bacterial infections, not for viral infections. 

3. Avoiding the use of wide-spectrum antibiotics - The use of narrow-spectrum antibiotics (antibiotics specific for the infection) is less likely to lead to antibiotic resistance.

4. Ensuring that patients complete courses of antibiotic treatment - This ensures all bacteria are killed and so does not give them the chance to develop resistance.

5. Avoiding the use of antibiotics in farming - This reduces the chance of bacteria becoming resistant to antibiotics. 

examples of medicines

• penicillin - an antibiotic extracted from a type of mould

• aspirin - a painkiller based on compounds from willow bark

• prialt - pain-killing drug derived from the venom of a cone snail

personalised medicines

medicines that are tailored to an individual's DNA - a patient's genome is analysed before they are given any treatment and so drugs given are more likely to be effective