Unit 2: Section 5

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Infectious Disease

  • Health is a state of physical, mental and social well-being, which includes the absence of disease and infirmity (weakness of body or mind).
  • Disease can be caused by infection with pathogens or parasites.
  • A pathogen is an organism that can cause disease. Bacteria, fungi and viruses are all pathogens.
  • A parasite is an organism that lives on or in another organism (the host) and causes damage to that organism. Tapeworms, roundworms and fleas are all examples of parasites. Some parasites cause disease, so they're also pathogens.
  • Diseases can also be caused by genetic defects, nutritional deficiencies and environmental factors. Infectious diseases are diseases that can be passed between individuals.
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Infectious Disease


  • Malaria is caused by the parasite Plasmodium.
  • Plasmodium is a eukaryotic, single-celled parasite.
  • It's transmitted by mosquitos - insects that feed on the blood of animals, including humans.
  • The moquitos are vectors - they don't cause the disease themselves, but they spread the infection by transfetting the parasite from one host to another.
  • Mosquitos transfer the Plasmodium parasite into an animal's blood when they feed on them.
  • Plasmodium infects the liver and red blood cells, and disrupts the blood supply to vital organs.
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Infectious Disease


  • AIDS is caused by the HIV virus.
  • The human immunodeficiency virus (HIV) infects human white blood cells.
  • HIV can only reproduce inside the cells of the organism it has infected because it doesn't have the equipment (such as enzymes and ribosomes) to replicate on its own.
  • After the virus has reproduced, it kills the white blood cells as it leaves.
  • HIV infection leads to acquired immune deficiency syndrome (AIDS).
  • AIDS is a condition where the immune system deteriorates and eventually fails due to the loss of white blood cells. It makes the sufferer more vulnerable to other infections, like pneumonia.
  • HIV is transmitted in three main ways:
    - Via unprotected sexual intercourse.
    - Through infected bodily fluids e.g. sharing needes, blood transfusions.
    - From mother to fetus (through the placenta, breast milk or during childbirth). 
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Infectious Disease

Tuberculosis (TB)

  • TB is a lung disease caused by the bacterium Mycobacterium tuberculosis.
  • TB spreads by 'droplet infection' - when an infected person coughs or sneezes, tiny droplets of saliva and mucus containing the bacteria are released from their mouth and nose. These droplets are then breathed in by other people.
  • Many people with TB are infected but don't show any symptoms.. If they become weakened (by another disease or malnutrition), then the infection can become active. They'll show the symptoms and be abele to pass on the infection.
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Infectious Disease

Global impact

Malaria, HIV and TB are most common in sub-Saharan Africa and other developing countries, because:

  • There's limited access to good healthcare - drugs are not always available, people are less likely to be diagnosed and treated, blood donations aren't always screened for infectious diseases and surgical equipment isn't always sterile.
  • There's limited health education to inform people how to avoid infectious diseases.
  • There's limited equipment to reduce the spread of infections.
  • There are overcrowded conditions - this increases the risk of TB infection by droplet transmission.

Studying the global distribution of these diseases is important for many reasons:

  • The information can be used to find out where people are most at risk.
  • Any data collected can be used to predict where epidemics are most likely to occur.
  • It's important for research
  • It allows organisations to provide aid where it's needed most.
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The Immune System

Primary defences


  • Acts as a physical barrier, blocking pathogens from entering the body.
  • Also acts as a chemical barrier by producing chemicals that are antimicrobial and can power pH, inhibiting the growth of pathogens.

Mucus membranes

  • Protect body openings that are exposed to the environment (such as the mouth, nostrils, ears, genitals and anus).
  • Some membranes secrete mucus - a sticky substance that traps pathogens and contains antimicrobial enzymes.

If a pathogen gets past the primary defences and enters the body, the immune system will respond. Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells. When a pathogen invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system.

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The Immune System

There are four main stages involved in the immune response

1. Phagocytes engulf pathogens:

A phagocyte (e.g. a macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They're found in the blood and in tissues and are the first cells to respond to a pathogen inside the body:

  • A phagocyte recognises the antigens on a pathogen.
  • The cytoplasm of the phagocyte moves round the pathogen, engulfing it.
  • The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.
  • A lysosome fuses with the phagocytic vacuole. The enzymes break down the pathogen.
  • The phagocyte then presents the pathogen's antigens. It sticks the antigens on its surface to activate other immune system cells.
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The Immune System

2. 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 the phagocytes.
  • Each T lymphocyte has a different receptor on its surface.
  • When the receptor on the surface of a T lymphocyte meets a complementary antigen, it binds to it - so each T lymphocyte will bind to a different antigen.
  • This activates the T lymphocute - the process is called clonal selection.
  • The T lymphocyte then undergoes clonal expansion - it divides to produce clones, which then differentiate into different types of T lyphocytes that carry out different functions:
    - Some activated T lymphocytes (called T helper cells) release substances to activate B lymphocytes.
    - Some attach to antigens on a pathogen and kill the cell (T killer cells).
    - Some become memory cells. 
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The Immune System

3. T lymphocytes activate B lymphocytes, which divide into plasma cells

  • B lymphocytes are another type of white blood cell.
  • They're 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 complementary 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 another example of clonal selection.
  • The activated B lymphocyte divides by mitosis into plasma cells and memory cells. This is another example of clonal expansion.

Cell signalling:

  • Cell signalling is how cells communicate.
  • A cell may release a substance that binds to the receptors on another cell, causing a response.
  • It's important in the immune system as it helps activate different white blood cells.
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The Immune System

4. 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, into the blood.
  • These antibodies will bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes.
  • Structure of antibodies:
    - The variable region of the antibody form the ntigen binding sites. The shape of the variable region is complementary to a particular antigen. The variable regions differ between antibodies.
    - The hinge region allows flexibility when the antibody binds to the antigen.
    - The constant regions allow binding to receptors on immune system cells. The constant region is the same in all antibodies.
    - Disulfide bridges (a type of bond) hold the polypeptide chains together.
  • Antibodies help to clear an infection by:
    - Agglutinating pathogens - antibodies have two binding sites; can bind two pathogens, which become clumped together
    - Neutralising toxins - bind to toxins, preventing them affecting human cells
    - Preventing pathogen binding to human cells - can block surface receptors 
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The Immune System

Primary response

  • When a pathogen enters the body for the first time the antigens on its surface activate the immune system. This is called the primary response.
  • The primary response is slow because there aren't many B lymphocytes that can make the antibody needed to bind to it.
  • Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease.
  • After being exposed to an antigen, both T and B lymphocytes produce memory cells. These memory cells 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 - their immune system has the ability to respons quickly to a second infection.
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The Immune System

Secondary response

  • If same pathogen enters the body again, the immune system will produce a quicker, stronger immune response - the secondary response.
  • Memory B lymphocytes divide into plasma cells that produce the right antibody to the antigen. Memory T lymphocytes 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 any symptoms are shown.
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Immunity and Vaccinations

Active immunity

  • This is the type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen. There are two different types of active immunity:
  • Natural - this is when you become immune after catching a disease.
  • Artificial - this is when you become immune after you've been given a vaccination containing a harmless dose of 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. There are two types:
  • Natural - this is when a baby becomes immune due to the antibodies it receives from its mother, through the placenta and in breast milk.
  • Artificial - this is when you become immune after being injected with antibodies from someone else.
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Immunity and Vaccinations


  • While your B lymphocytes are busy dividing to build up their numbers to deal with a pathogen, you suffer from the disease. Vaccination can help avoid this.
  • Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. THis means you become immune without getting any symptoms.
  • If most people in a community are vaccinated, the disease becomes extremely rare. This means that even people who haven't been vaccinated are unlikely to get the disease, because there's no one to catch it from. This is called herd immunity.
  • Vaccines always contain antigens - these may be free or attached to a dead or attenuated (weakened) pathogen,
  • Vaccines may be injected or taken orally. The disadvantages of taking a vaccine orally are that it could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed into the blood.
  • Sometimes booster vaccines are given later on to make sure that memory cells are produced.
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Immunity and Vaccinations

New influenza vaccines have to developed every year

  • The influenza virus causes influenza.
  • Proteins (neuraminidase and haemagglutin) on the surface of the influenza virus act as antigens, triggering the immune system.
  • These antigens can change regularly, forming new strains of the virus.
  • Memory cells produced from vaccination with one strain of flu will not recognise other strains with different antigens.
  • Every year there are different strains of the influenza virus circulating in the population, so a different vaccine has to be made.
  • Laboratories collect samples of these different strains, and organisations, such as the WHO (World Health Organisation) and CDC (Centre for Disease Control), test the effectiveness of different influenza vaccines against them.
  • New vaccines are developed and one is chosen every year that is the most effective against the recently circulating influenza viruses.
  • Governments and health authorities then implement a programme of vaccination using this most suitable vaccine. This is a good example of how society uses science to inform decision making.
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Smoking and Disease


  • When damage occurs to the lining of an artery, white blood cells move into the area.
  • Over time more white blood cells, lipids and connective tissues build up and harden to form a fibrous plaque at the site of the damage - an atheroma.
  • The atheroma partially blocks the lumen of the artery and restricts blood flow.
  • Atherosclerosis is the hardening of arteries due to the formation of atheromas.
  • Cigarette smoke contains nicotine, which causes an increase in blood pressure. Increased blood pressure can cause damage to the arteries, leading to the formation of more atheromas.
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Smoking and Disease

Coronary Heart Disease (CHD)

  • Coronary heart disease is when the coronary arteries (arteries that supply blood to the heart) have lots of atheromas in them. This restricts blood flow to the heart.
  • A reduction in blood flow reduces the amount of oxygen an area of the heart gets. This can cause pain (angina) or a heart attack.
  • Smoking increases the risk of CHD because carbon monoxide irreversibly combines with haemoglobin, reducing the amount of oxygen transported in the blood, which reduces the amount of oxygen available to tissues, including the heart.
  • Also, nicotine in cigarette smoke makes platelets (cells involved in blood clotting) sticky, increasing the chance of blood clots forming. If clotting happens in the coronary arteries, it could cause a heart attack.
  • The presencce of atheromas also increases the risk of blood clots forming.
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Smoking and Disease


  • A stroke is a rapid loss of brain function due to a disruption in the blood supply to the brain.
  • This can be caused by a blood clot in an artery leading to the brain, which reduces the amount of blood, and therefore oxygen, that can reach the brain.
  • Nicotine increases the risk of stroke because it increases the risk of clots forming.
  • Carbon monoxide also increases the risk of stroke because it reduces the amount of oxygen available to the brain by combining with haemoglobin.
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Smoking and Disease

Lung Cancer

  • Cigarette smoke contains many carcinogens (chemicals that can cause a cell to become cancerous).
  • These carcinogens may cause mutations in the DNA of lung cells, which could lead to uncontrolled cell growth and the formation of a malignant (cancerous) tumour.
  • Malignant tumours grow uncontrollably, blocking air flow to areas of the lung.
  • This decreases gas exchange and leads to a shortness of breath because the body is struggling to take in enough oxygen.
  • The tumour uses lots of nutrients and energy to grow, which causes weight loss.
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Smoking and Disease

Chronic Bronchitis

  • Chronic bronchitis is inflammation of the lungs.
  • The upper respiratory tract is lined with goblet cells that produce mucus to trap microorganisms. The tract is also lined with cilia that 'beat' to move the mucus towards the throat so it can be removed.
  • Cigarette smoke damages the cilia and causes the goblet cells to produce more mucus.
  • The mucus accumalates in the lungs, which causes increased coughing to try and remove the mucus.
  • Microorganisms multiply in the mucus and cause lung infections that lead to inflammation, which decreases gas exchange.
  • Chronic bronchitis is a type of chronic obstructive pulmonary disease (COPD). COPD is a group of diseases that involve permanent airflow reduction.
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Smoking and Disease


  • Emphysema is a lung disease caused by smoking or long-term exposure to air pollution - foreign particles in the smoke (or air) become trapped in the aveoli.
  • This causes inflammation, which encourages phagocytes to the area. The phagocytes produc an enzyme that breaks doen elastin.
  • The alveolar walls are destroyed and the elasticity of the lungs is lost.
  • This reduces the surface area of the alveoli, so the rate of gaseous exchange decreases.
  • Symptoms of emphysema include shortness of breath and wheezing. People with emphysema have an increased breathing rate as they try to increase the amount of air (containing oxygen) reaching their lungs.
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