12. Disease and immunity (12.1, 12.2 & 12.3)

12.1 Disease 

1. Describe a pathogen as a disease-causing organism 

• These include bacteria, viruses, fungi, and protozoa. Pathogenic bacteria produce poisonous waste called toxins. Toxins damage the cells in which the bacteria are growing, upsetting the systems of the body and causing raised temperature, headache, weakness, and diarrhoea. 

2. Describe a transmissible disease in which the pathogen can be passed from one host to another 

3. Understand that a pathogen may be transmitted: 

(a) through direct contact, including through blood or other body fluids 

• HIV can be transmitted through drug users sharing needles or transmitted sexually through bodily fluids if a person has unprotected sex with an infected individual. 

(b) indirectly, including from contaminated surfaces or food, from animals, or the air 

• Raw meat contains bacteria. If fresh food [such as vegetables and fruits] is eaten raw and kept on the surface where raw meat was kept, pathogens from the meat can transfer to the fresh food and infect a person. 

• A person handling the food may infect other people if they do not wash their hands properly after using the toilet, handling rubbish or raw produce 

• When a person sneezes, laughs, talks or breathes, they send out a fine spray of droplets which can settle on food or remain floating in the air. If these droplets contain bacteria or viruses, they may infect a person who breathes in those droplets. When water in these droplets evaporates, most bacteria die out, but viruses remain in the air for a long time. They cause diseases like colds, flu, measles and chickenpox. 

• Houseflies place their mouthparts on the food, injecting saliva and taking up the digested food as liquid. Since flies also lay on human faeces or decaying food, their feet or mouth parts can pick up on the harmful bacteria in these substances. When these flies land on food, harmful bacteria are transferred to the food. Houseflies cause diseases like dysentery and polio 

4. Describe the human body’s barriers to the entry of pathogens, limited to: skin, hairs in the nose, mucus, and stomach acid

• Mechanical barriers – structures that make it difficult for pathogens to get past them and into the body 

• Skin covers almost all parts of your body to prevent infection from pathogens. If it is cut or grazed, it immediately begins to heal itself, often by forming a scab. 

• Hairs in the nose – these make it difficult for pathogens to get past them further up the nose, so they are not inhaled into the lungs

• Chemical barriers – substances produced by the body cells that trap/kill pathogens before they can get further into the body and cause disease 

• Mucus – made in various places in the body [trachea and bronchi], pathogens get trapped in the mucus and can then be removed from the body (by coughing, blowing the nose, swallowing, etc) 

• Stomach acid contains hydrochloric acid, which is strong enough to kill any pathogens that have been consumed in food or water 

• Cells – different types of white blood cells work to prevent pathogens from reaching areas of the body where they can replicate 

• By phagocytosis – engulfing and digesting pathogenic cells 

• By producing antibodies, which clump pathogenic cells together so they can’t move as easily (known as agglutination)and releasing chemicals that signal to other cells that they must be destroyed 

5. Understand the role of the mosquito as a vector of disease 

• The means of transmission of a disease is called a vector. The mosquito is a vector of malaria 

6. Describe the malaria pathogen as an example of a parasite and explain how it is transmitted 

• Malaria is caused by a protozoan parasite called Plasmodium. This disease is transmitted through infected mosquitoes of the genus Anopheles. When a mosquito lands on a person’s skin, it injects its mouthpart into a blood capillary and releases salivary fluid to prevent the blood from clotting. If a mosquito is infected, then it also releases hundreds of malarial parasites into the bloodstream. These malarial parasites reach the liver via the circulation and burrow in the liver where they reproduce. A week or two later, the daughter cells burst out of the liver and start attacking the red blood cells. Here, they reproduce rapidly and escape from the original red blood cells to further invade other cells. This cycle of reproduction in the red blood cell takes about 2-3 days. Every time daughter plasmodia are released from thousands of blood cells, the person experiences violent shivering, fever, and profuse sweating. Since so many blood cells are destroyed, a person becomes anaemic 

7. Describe the control of the mosquito that transmits malaria regarding its lifecycle 

• Elimination of the mosquito 

• Mosquitoes lay their eggs in stagnant water. The pupa and the larvae grow in the water, though they come out of the water to take in oxygen. Spraying any body of fresh or stagnant water with insecticides and oil suffocates the pupae, the larvae, and they die out. 

• Turning swamps and sluggish rivers into fast water streams also destroys the breeding ground of mosquitoes. 

• Spraying homes with DDT [an insecticide]also kills the mosquito; however, many mosquitoes have become resistant to the insecticide 

• Some drugs kill the parasites in the bloodstream but not in the liver. Therefore is possible that the parasites in the liver become active again and the person suffers from malaria. If a healthy person takes the malaria drug before being infected, the parasites are killed as soon as they enter the bloodstream 

• Sterilising male mosquitoes so that the next generation fails 

• Genetically engineering the female mosquito so that it has a flightless phenotype, which reduces the number of mosquitoes 

8. Explain that human immunodeficiency virus(HIV) is a viral pathogen 

9. Describe how HIV is transmitted 

• Direct infection of the blood due to drug users sharing needles. 

• Can be transmitted sexually through bodily fluids when a person has unprotected sex with an infected person 

• It can be passed from a mother to the fetus in the placenta, during birth or breastfeeding. However, if a baby is given drug therapy immediately after birth, the transmission is reduced. 

• Haemophiliacs [inherited bleeding disorder in which the blood does not clot properly] would sometimes receive blood from HIV carriers before the risks were recognised and fell victim to AIDS 

10. Understand that HIV infection may lead to Acquired Immune Deficiency Syndrome(AIDS) 

• If left untreated, HIV infection can lead to AIDS [which is stage 3 of the HIV infection]. However, if it is treated in very early stages through medication, it is possible that the person does not get AIDS and continues to live a normal life 

11. Describe the methods by which HIV may be controlled 

• Avoiding having sexual relations with an infected person 

• Being faithful to one partner 

• Using condoms and femidoms during sex 

12. Describe cholera as a disease caused by a bacterium, which is transmitted through contaminated water 

13. Explain the importance of a clean water supply, hygienic food preparation, good personal hygiene, waste disposal, and sewage treatment in controlling the spread of cholera(details of the stages of sewage treatment are not required) 

Clean water supply:

• Homes and public places should have plumbing and drains to safely remove faeces and waste, which can carry pathogens. 

• Raw sewage should be treated to remove solid waste and kill pathogens before being released into the environment. This is done by filtration and chlorination [chlorine kills all bacteria] 

Hygienic food preparation:

• Keep food cold so bacteria and fungi reproduce more slowly. 

• Prepare food hygienically to avoid contamination from pathogens by washing hands well with soap and cleaning work surfaces with products such as bleach to kill pathogens. 

• Cook food well(long enough at a high temperature)to kill bacteria and fungi 

• Cover food to prevent flies from landing on it before eating 

• Use separate chopping boards/utensils for cutting uncooked meat - wash hands after using the bathroom before handling food 

• Pasteurise milk thoroughly to kill any bacteria 

• Frozen poultry must be thoroughly defrosted 

Good personal hygiene 

• washing with soap removes substances which trap pathogens as well as pathogens themselves, from the skin 

• Use tissues to catch sneezes and coughs 

• Dispose of used tissues as soon as possible, as pathogens can still be alive 

Waste disposal 

• Wastefood is a food source for flies that can act as vectors for transmissible diseases and should be disposed of in a sealed container. 

• Rubbish bins should be covered and removed to the landfill for disposal or burning regularly. 

• All rubbish should be stored before collection, away from human habitation 

Sewage treatment 

• Homes and public places should have plumbing and drains to safely remove faeces and waste, which can carry pathogens. 

• Raw sewage should be treated to remove solid waste and kill pathogens before being released into the environment 

14. Explain that the cholera bacterium produces a toxin that causes the secretion of chloride ions into the small intestine, causing osmotic movement of water into the gut, resulting in diarrhoea, dehydration and loss of ions from the blood 

• Cholera bacteria - Vibrio cholerae - can be taken in if a person eats contaminated food or water. These bacteria reproduce in the small intestine and invade the epithelial lining of the intestine. They also release toxins which can upset the internal lining of the intestine and cause large amounts of secretion of salts and water. This upsets the osmotic potential of the gut contents, attracting more water from the surrounding area[tissues/blood]. This results in the undigested food having more water, leading to serious diarrhoea. The loss of body fluids and salts leads to dehydration and kidney failure 

• Treatment: replacement and rehydration of lost salts [by giving a carefully controlled saline solution], taking an antibiotic to kill the bacteria[tetracycline] 

15. Describe the effects of excessive consumption of alcohol: reduced self-control, depressant effects, effect on reaction times, damage to the liver, and social implications 

• Wines, beersand spirits contain an alcohol called ethanol. It is a depressant drug – it slows down signals in the nerves and brain. Because alcohol increases reaction times, there are legal limits for drinking and driving in many countries. 

• Short-term effects 

• vomiting - alcohol is toxic to the body, and this is the quickest way to get rid of it 

• impaired judgement and potentially violent behaviour - the user has reduced self-control 

• impaired balance and muscle control 

• sleepiness - when consumed in large enough quantities, unconsciousness 

• Causes vasodilation - giving a feeling of warmth, but leading to a greater loss of body heat 

• Long-term effects

• Damages the brain, causing memory loss and confusion 

• Heavy alcohol abuse over a long period damages the liver, causing the liver to produce too many fat droplets, leading to cirrhosis. A diseased liver is less able to stop poisonous blood from the intestinal blood from reaching the general circulation 

16. Describe the effects of tobacco smoke and its major toxic components(nicotine, tar, and carbon monoxide): strong association with bronchitis, emphysema, lung cancer, heart disease, and the association between smoking during pregnancy and reduced birth weight of the baby 

Tar 

• Tar carcinogen is linked to increased chances of cancerous cells developing in the lungs. Carcinogens present in tar can cause tumours to develop. Tumours are balls of abnormal cells which do not allow gaseous exchange like normal lung cells 

• It also contributes to COPD [chronic obstructive pulmonary disease], which occurs when chronic bronchitis and emphysema occur together 

• Chronic bronchitis is caused by tar, which stimulates goblet cells and mucus glands to enlarge, producing more mucus. It destroys cilia, inhibiting the cleaning of the airways, and mucus(containing dirt, bacteria viruses) builds up, blocking the smallest bronchioles. A smoker’s cough is the attempt to move the mucus, but it damages the epithelia, resulting in scar tissue, which narrows the airways and makes breathing difficult. 

• Emphysema is the breakdown of the alveoli and develops as a result of frequent infection, meaning phagocytes are attracted to the lungs where they release elastase, an enzyme that breaks down the elastin in the alveoli walls, to enable them to reach the surface where the bacteria are. Without adequate elastin, the alveoli cannot stretch, so they recoil and many burst. The breakdown of alveoli results in the appearance of large air spaces, reducing the surface area for gas exchange and making sufferers breathe more rapidly. As it progresses, patients become breathless and wheezy – they may need a constant supply of oxygen to stay alive 

Carbon monoxide 

• Carbon monoxide binds irreversibly to haemoglobin, reducing the capacity of blood to carry oxygen. 

• This puts more strain on the breathing system as breathing frequency and depth need to increase to get the same amount of oxygen into the blood. 

• It also puts more strain on the circulatory system to pump the blood faster around the body, and increases the risk of coronary heart disease and strokes 

Nicotine 

• It is the addictive component of tobacco smoke 

• Nicotine narrows blood vessels, so it will put more strain on the circulatory system and increase blood pressure. Narrow blood vessels are more likely to become clogged with fat, including cholesterol. If this happens in the coronary artery, this causes coronary heart disease 

• This means the heart muscle cells do not get sufficient oxygen, and so less aerobic respiration takes place. 

• To compensate, the cells respire anaerobically, producing lactic acid, which cannot be removed (due to lack of blood supply). 

• This creates a low pH environment in the cells, causing enzymes to denature. Eventually, heart muscle cells will die. If enough, this can cause a heart attack 

Other risks 

• Diseases of leg arteries - 

• Strokes [arterial disease in the brain] - 

• Cancer of the bladder, uterus, duodenum, tuberculosis, tooth decay and gum disease - 

• Smaller babies than average - 

• Higher frequency of miscarriage, death rate, and stillbirth rate 

12.2 Antibiotics 

1. Describe a drug as any substance taken into the body that modifies or affects chemical reactions in the body 

2. Describe the use of antibiotics for the treatment of bacterial infection 

3. State that antibiotics kill bacteria but do not affect viruses 

• Antibodies work by disrupting cell membranes, cell walls, protein synthesis the replication of DNA. Viruses have a completely different structure and mechanisms; hence, antibiotics do not affect viruses. 

4. Explain how the development of antibiotic-resistant bacteria, including MRSA, can be minimised by using antibiotics only when essential 

12.3 Immunity 

1. Describe active immunity as a defence against a pathogen by antibody production in the body 

2. State that each pathogen has its antigens, which have specific shapes 

3. Describe antibodies as proteins that bind to antigens, leading to direct destruction of pathogens, or marking of pathogens for destruction by phagocytes 

4. State that specific antibodies have complementary shapes which fit specific antigens 

5. Explain that active immunity is gained after an infection by a pathogen or by vaccination 

• Lymphocytes can ‘read’ the antigens on the surfaces of cells and recognise any that are foreign. They then make antibodies which are a complementary shape to the antigens on the surface of the pathogenic cell. 

• The antibodies attach to the antigens and cause agglutination (clumping together). 

• This means the pathogenic cells cannot move very easily. At the same time, chemicals are released that signal to phagocytes that there are cells present that need to be destroyed. 

• The initial response of a lymphocyte encountering a pathogen for the first time and making specific antibodies for its antigens can take days, during which time an individual may get sick. 

• Lymphocytes that have made antibodies for a specific pathogen for the first time will then make ' memory cells’ that retain the instructions for making those specific antibodies for that type of pathogen. This means that, in the case of reinfection by the same type of pathogen, antibodies can very quickly be made in greater quantities and the pathogens destroyed before they can multiply and cause illness. This is how people can become immune to certain diseases after only having them once. It does not work with all disease-causing microorganisms; some of them mutate fairly quickly and change the antigens on their cell surfaces. 

• Therefore, if they invade the body for a second time, the memory cells made in the first infection will not recall them as they now have slightly different antigens on their surfaces(e.g. the cold virus) 

6. Outline the process of vaccination: 

(a) Weakened pathogens or their antigens are given 

• Vaccines allow a dead, altered form of the disease-causing pathogen, which contains specific antigens, to be introduced into the body. In this weakened state, the pathogen cannot cause illness but can provoke an immune response 

(b)Antigens stimulate an immune response by lymphocytes, which produce antibodies 

• Lymphocytes produce complementary antibodies for the antigens. The antibodies target the antigen and attach themselves to it to create memory cells 

(c) Memory cells are produced that give long-term immunity. 

• The memory cells remain in the blood and will quickly respond to the antigen if it is encountered again in an infection by a ' live’ pathogen. As memory cells have been produced, this immunity is long-lasting 

7. Explain the role of vaccination in controlling the spread of transmissible diseases 

• If a large enough percentage of the population is vaccinated, it protects the entire population because there are very few places for the pathogen to breed – it can only do so if it enters the body of an unvaccinated person. 

• This is known as herd immunity. 

• If the number of people vaccinated against a specific disease drops in a population, it leaves the rest of the population at risk of mass infection, as they are more likely to come across people who are infected and contagious. 

• This increases the number of infections, as well as the number of people who could die from a specific infectious disease 

• This is the reason that many vaccinations are given to children, as they are regularly seen by medical practitioners and can be vaccinated early to ensure the entire vaccinated population remains at a high level. In certain instances, vaccination programmes are run to eradicate certain dangerous diseases, as opposed to controlling them at low levels. 

• An example of a disease which has been eradicated as a result of a successful vaccination programme is smallpox, which was officially eradicated in 1980, a vaccination programme run by the World Health Organisation since the mid-1950s 

8. Explain that passive immunity short-term defence against a pathogen by antibodies acquired from another individual, limited to: across the placenta and in breast milk 

9. Explain the importance of breastfeeding for the development of passive immunity in infants 

• When a mother breastfeeds her baby, the milk contains some of the mother’s white blood cells, which produce antibodies. When a baby is very young, its immune responses are not fully developed, so these antibodies give the baby protection against infection at a time when it is at risk. 

10. State that memory cells are not produced in passive immunity 

11. Outline how HIV affects the immune system, limited to: decreased lymphocyte numbers and reduced ability to produce antibodies, which weakens the immune system 

• HIV primarily targets T lymphocytes, which are crucial for coordinating the immune response against infections. 

• The virus enters T cells, replicates inside them, and eventually destroys them. As a result, the number of CD4 T cells in the bloodstream decreases over time. 

• With fewer T cells available, the ability to mount an effective immune response against pathogens diminishes. 

• If the body cannot respond to infections through the immune system, it becomes vulnerable to pathogens that might not be otherwise life-threatening. As a result, the patient had little or no resistance to a wide range of diseases such as influenza, pneumonia, and blood disorders. 

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School