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Pathogens can cause communicable diseases

- disease is a condition that impairs the normal functioning of an organism. Both plants and animals can get diseases.

- a pathogen is an organism that causes disease. Pathogens include: bacteria, viruses, fungi and protoctista (type of single-cells eukaryotic organism).

- a communicable disease is a disease that can be spread between organisms.


3 diseases bacteria cause

Tuberculosis (typically humans and cattle)

Bacterial meningitis (humans)

Ring rot (potatoes and tomatoes)


3 diseases viruses cause

HIV/AIDS (humans)

Influenza (animals, including humans)

Tobacco mosaic virus (plants)


3 diseases fungus causes

Black Sigatoka (banana plants)

Ringworm (cattle)

Athletes foot (humans)


2 diseases protoctists causes

Potato/tomato late blight

Malaria (animals, including humans)


Direct transmission of communicable diseases

- direct transmission is when a disease is transmitted directly from one organism to anther.

- could occur through: droplet infection, sexual intercourse or touching an infected organism.

E.g. - HIV between humans via sexual intercourse.
- athletes foot via touch.


Indirect transmission of communicable diseases

Indirect transmission is when the disease is transmitted from one organism to another via an intermediate.

Intermediates include air, water, food, or another organism (a vector)

E.g. - tomato/potato late blight is spread when spores are carried between plants (first in air then water)
- malaria spreads between humans via mosquitos acting as vectors.


3 factors that affect disease transmission

Overcrowded living conditions increase transmission of many communicable diseases.
E.g. Tuberculosis is spread directly via droplet infection and indirectly as bacteria can remain in air for a long time. Risk of TB infection increased when lots of people live crowed together.

Climates affects spread of communicable diseases.
E.g. potato/tomato late blight is especially common during wet summers, since spores need water to spread.
Malaria is most common in tropical countries which are hot/humid. Ideal for mosquitoes to breed.

Social factors can increase transmission of communicable diseases in humans.
E.g. risk of HIV infection is high in places with:
- limited good health care- so less likely to be diagnosed/treated and most effective anti-HIV drugs are less likely to be available.
- limited good health education- to inform people about how HIV is transmitted.


Primary, non-specific defences animals have to prevent pathogens entering an organism

- Skin
- mucous membranes
- blood clotting
- inflammation
- wound repair
- expulsive reflexes


Skin as a primary defence against pathogens

- the skin acts as a physical barrier, blocking pathogens from entering the body.
- it also acts as a chemical barrier by producing chemicals that are antimicrobial and can lower pH, inhibiting the growth of pathogens.


Mucous membranes as a primary defence against pathogens

- these protect body openings that are exposed to the environment (mouth, nostrils, ears, anus...)
- some membranes secretes mucus, a sticky substance that traps pathogens and contains antimicrobial enzymes.


Blood clotting as a primary defence against pathogens

- a blood clot is a mesh of protein fibres.
- blood clots plug wounds to prevent pathogen entry and blood loss.
- they’re formed by a series of chemical reactions that take place when platelets are exposed to damaged blood vessels.


Inflammation as a primary defence against pathogens

- signs of inflammation include swelling, pain, heat and redness.
- can be triggered by tissue damage. Damaged tissue releases molecules which increase the permeability of the blood vessels, so they start to leak fluid tissue into surrounding area.
- causes swelling and isolates any pathogens that may have entered the damaged tissue.
- the molecules also cause vasodilation, increasing the blood flow to the affected area.
- makes area hot and brings white blood cells to fight off any pathogens that may be present.


Wound repair as a primary defence against pathogens

- skin is able to repair itself in the event of an injury and re-form a barrier against pathogen entry.
- surface is repaired by outer layer of skin cells dividing and migrating to the edges of the wound.
- tissue below the wound then contracts to bring the edges of the wounds closer together.
- repaired using collagen fibres.


Expulsive reflexes as a primary defence against pathogens

- e.g. coughing or sneezing.
- a sneeze happens when the mucous membranes in the nostrils are irritated by dust or dirt.
- cough stems from irritation in the respiratory tract.
- both coughing and sneezing are an attempt to expel foreign objects, including pathogens from the body.


Plants physical defences against pathogens

- most plant leaves and stems have a waxy cuticle, which provides a physical barrier against pathogen entry. May also stop water collecting on the leaf, which could reduce risk of infection by pathogens transferred between plants in water.

- plant cells themselves are surrounded by cell walls. Form a physical barrier against pathogens that make it past the waxy cuticle.

- plants produce the polysaccharide callose. Callose gets deposited between plant cell walls and plasma membranes during times of stress (e.g. pathogen invasion). Callose deposition may make it harder for pathogens to enter cells. Callose deposition at the plasmodesmata may limit spread of viruses between cells.


Plants chemical defences against pathogens

- plants produce antimicrobial chemicals (including antibiotics) which kill pathogens or inhibit their growth. For example:

- some plants produce chemicals called saponins. These are though to destroy the cell membranes of fungi and other pathogens.
- plants also produce chemicals called phytoalexins, which inhibit the growth of fungi and pathogens.

- other chemicals secreted by plants are toxic to insects. This reduces amount of insect-feeding on plants, therefore reducing the risk of infection by plant viruses carried by insect vectors.



- antigens are molecules (usually proteins or polysaccharides) found on the surface of cells.
- when a pathogens invades the body, antigens on its cell surface membrane are identified as foreign, which activates the cells of the immune system.
- immune response involves specific and non-specific stages. Non-specific response happens in the same way for all microorganisms, whatever foreign antigens they have. Specific response is antigen-specific, aimed at specific pathogens. It involves white blood cells called
T and B lymphocytes.


Four main stages in the immune response

1- Phagocytes Engulf Pathogens.
2- Phagocytes Activate T lymphocytes
3- T lymphocytes Activate B lymphocytes, which divide into plasma cells.
4- Plasma cells make more antibodies to a specific Antigen


1st stage in the immune response- Phagocytes engulf pathogens

- a phagocyte is a white blood cells that carries out phagocytosis, which is engulfment of pathogens (found in blood and tissues)
Carry out a non-specific immune response:

- phagocyte recognises antigens on a pathogen.

- cytoplasm of phagocyte engulfs pathogen. May be made easier by presence of opsonins, which are molecules that attach to foreign antigens to aid phagocytosis. Some opsonins hide negative charges on the membrane of the pathogen, making it easier for the negatively-charged phagocyte to get closer to the pathogen.

- pathogen now contained inside a phagosome (type of vesicle) in the cytoplasm of the phagocyte.

- lysosome (organelle containing digestive enzymes) fuses with the phagosome. Enzymes break down the pathogen.

- phagocyte then presents the pathogens antigens. Sticks antigens on its surface to activate other immune system cells. Acts as an antigen-presenting cell.

- neutrophils are a type of phagocyte, which move towards a wound in response to signals from cytokines.


2nd stage in the immune response- Phagocytes activate T lymphocytes

- a T lymphocyte is another type of white blood cell.

- their surface is covered with receptors.

- the receptors bind to antigens presented by antigen-presenting cells.

- each T lymphocyte has a different receptor on its surface.

- when the receptor on the surface of a T lymphocyte meets a complimentary antigen, it binds to it. So each T lymphocyte will bind to a different antigen.

- this activates the T lymphocyte. This process is called clonal selection.

- the T lymphocyte then undergoes clonal expansion. Divides to produce clones of itself. Different types of T lymphocytes carry out different functions.


T helper cells

These release substances to activate B lymphocytes and T killer cells.


T killer cells

These attach to and kill cells that are infected with a virus.


T regulatory cells

These suppress the immune response from other white blood cells. Helps to stop immune system cells from mistakingly attacking the host’s body cells.


3rd stage in the immune response- T lymphocytes activate B lymphocytes, which divide into plasma cells

- B lymphocytes are another type of white blood cell.

- covered with proteins called antibodies.

- antibodies bind to antigens to form an antigen-antibody complex.

- Each B lymphocyte has a different shaped antibody on its surface.

- when the antibody on the surface of a B lymphocyte meets a complimentary shaped antigen, it binds to it. So each B lymphocyte will bind to a different antigen.

- this, together with substances released from T helper cells, activates the B lymphocyte. This is clonal expansion.

- activated B lymphocyte divides by mitosis into plasma cells and memory cells. This is clonal expansion.


4th stage in the immune response- plasma cells make more antibodies to a specific antigen

- plasma cells are clones of the B lymphocyte.

- they secrete loads of the antibody, specific to the antigen, onto the blood.

- these antibodies will bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes.


Cell signalling

- cell signalling is how cells communicate.

- cell may release a substance that binds to the receptors on another cell. This causes a response of some kind in the other cell.

- cell signalling is really important in the immune response because it helps to activate all of the different types of white blood cells needed.

- e.g. T helper cells release interleukins, that bind to receptors on B lymphocytes. This activates the B lymphocytes. T helper cells are signalling to the B lymphocytes that there’s a pathogen in the body.


Structure of antibodies

- the variable regions of the antibody form the antigen biding sites.

- shape of the variable region is complementary to a particular antigen. Variable regions differ between antibodies.

- hinge region allows flexibility when the antibody binds to the antigen.

- constant regions allow binding to receptors on immune system cells. Constant region is the same in all antibodies.

- disulphide bridges hold the polypeptide chains of the protein together.


Antibodies help clear an infection by:

- Agglutinating pathogens

- Neutralising toxins

- Preventing the pathogen binding to human cells


Agglutinating pathogens (way antibodies clear an infection)

- each antibody has two binding sites, so an antibody can bind to two pathogens at the same time.
- pathogens becomes clumped together.
- phagocytes then bind to the antibodies and phagocytose a lot of pathogens all at once.
- antibodies that behave in this way are known as agglutinins.


Neutralising toxins (way antibodies clear an infection)

- like antigens, toxins have different shapes.
- antibodies called anti-toxins can bind to the toxins produced by pathogens.
- this prevents the toxins from affecting human cells, so the toxins are neutralised (inactivated). The toxin-antibody complexes are also phagocytosed.


Preventing the pathogen binding to human cells (way antibodies clear an infection)

- when antibodies bind to the antigens on pathogens, they may block the cell surface receptors that the pathogens need to bind to the host cells.
- this means the pathogen can’t attach to or infect the host cells.


Primary response is slow

- when a pathogen enters the body for the first time, antigens on its surface activate the immune system. This is the primary response.

- primary response is slow since there aren’t many B lymphocytes that can make the antibody needed to bind to it.

- person will show symptoms while the body produces enough of the right antibodies.

- after being exposed to an antigen, both T and B lymphocytes produce memory cells, which remain in the body for a long time.

- memory T lymphocytes remember the specific antigen and will recognise it a second time round. Memory B lymphocytes record the specific antibodies needed to bind to the antigen.

- the person is now immune, so immune system has the ability to respond quickly to a second infection.


Secondary response is faster

- if the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response. The secondary response.

- clonal selection happens faster. Memory B lymphocytes are activated and divide into plasma cells that produce the right antibody to the antigen. Memory T lymphocytes are activated and divide into the correct type of T lymphocytes to kill the cell carrying the antigen.

- the secondary response often gets rid of the pathogen before you begin to show any symptoms.


Active immunity

- this is the type of immunity you get when your immune system makes its own antibodies arder being stimulated by an antigen.


Passive immunity

- this is the type of immunity you get from being given antibodies made by a different organism. Your immune system doesn’t produce any antibodies of its own.


Natural active immunity

This is when you become immune after catching a disease. E.g. if you have measles as a child, you shouldn’t be able to catch it again in later life.


Artificial active immunity

This is when you become immune after you’ve been given a vaccination containing a harmless dose of antigen.


Natural passive immunity

This is when a baby becomes immune due to the antibodies it receives from its mother, through the placenta and in breast milk.


Artificial passive immunity

This is when you become immune after being injected with antibodies from someone else. E.g. if you contract tetanus you can be injected with antibodies against the tetanus toxin, collected from blood donations.


Autoimmune diseases

- sometimes an organism’s immune system isn’t able to recognise self-antigens. The antigens present on the organisms own cells.

- when this happens, the immune system treats the self-antigens as foreign antigens and launches an immune response against the organisms own tissues.

- disease resulting from an abnormal immune response is an autoimmune disease.


Examples of autoimmune diseases

Lupus- caused by the immune system attacking cells in the connective tissues. Damages the tissues causing painful inflammation. Lupus can affect skin and joints, as well as organs such as the heart and lungs.

Rheumatoid arthritis- caused by the immune system attacking cells in the joints, causing pain and inflammation.


Vaccines control disease and prevent epidemics

- while B lymphocytes are dividing to deal with a pathogen (primary response), you suffer from the disease. Vaccination helps avoid this.

- vaccines contain antigens, causing the body to produce memory cells against a particular pathogen, without the pathogen causing disease. You become immune without any symptoms.

- if most people in a community are vaccinated, the disease becomes extremely rare. So even people who haven’t been vaccinated are unlikely to get the disease.
This is heard immunity, which helps prevent epidemics.

- vaccinations always contain antigens, which may be free or attached to a dead/attenuated pathogen.

- sometimes booster vaccines are given to make sure memory cells are produced.

- vaccinations cause immunisation.


Vaccination programmes change (e.g. influenza vaccine)

- influenza vaccine changes every year, since then antigens on the surface of the influenza virus change regularly, forming new strains of the virus.

- memory cells produced from vaccination with one strain of the flu won’t recognise other strains with different antigens. Strains are immunologically distinct.

- each year different vaccines need to be made due to different strains of the influenza virus circulating in the population.

- labs collect samples of these different strains, an test effectiveness of different vaccines on them.

- new vaccine developed and most effective one is chosen.

- then government implement a programme of vaccination using the most suitable vaccine.



- antibiotics are chemicals that kill or inhibit the growth of bacteria.

- used by humans as drugs to treat bacterial infections. Useful because they usually target bacterial cells, without damaging human body cells.

- risks of antibiotics include: side effects and antibiotic resistance.


Antibiotic resistance

- there is genetic variation in a population of bacteria. Genetic mutation make some bacteria naturally resistant to an antibiotic.

- for bacteria, ability to resist an antibiotic is an advantage, since it’s better able to survive, so reproduces many more times.

- leads to the allele for antibiotic resistance being passed on to lots of offspring. An example of natural selection.

- problem for people who become infected with this bacteria, since you can’t easily get rid of them with antibiotics.

- increased use of antibiotics means antibiotic resistance is increasing.


Examples of antibiotic-resistant bacteria

MRSA- causes serious wound infections and resistant to several antibiotics.

Clostridium difficile infects the digestive system, usually causing problems in people who have already been treated with antibiotics. Harmless bacteria normally present in digestive system are killed by the antibiotics. Allows C. difficile to flourish and cause diarrhoea, fever and cramp.


How do we reduce the likelihood of antibiotic resistance developing

- doctors are being encouraged to reduce their use of antibiotics. E.g. not prescribe them for minor infections and not to prescribe them to prevent infections.

- patients are advised to take all of the antibiotics they have been prescribed, to make sure the infection is fully clear and all the bacteria have been killed.


Sources of medicines need to be protected

- many medicinal drugs are manufactured using natural compounds found in plants, animals or microorganisms. E.g. penicillin obtained from fungus...

- only small proportion of organisms have been investigated so far, so possible that plants or microorganisms exist that contain compounds that could be used to treat currently incurable disease.

- possible sources of drugs need to be protected by maintaining biodiversity.


Personalised medicines

Personalised medicines are medicines that are tailored to an individual’s DNA. The theory is that if the doctors have your genetic information, they can use it to predict how you will respond to different drugs, and only prescribe ones that will be most effective for you.


Synthetic biology

Synthetic biology involves using technology to design and make things like artificial proteins, cells and even microorganisms.

Has applications in areas like medicine. E.g. scientists are looking at engineering bacteria to destroy cancer cells, while leaving healthy body cells intact.