Disease and Immunity

12.1 Disease

How pathogens are transmitted

• Direct contact, including through blood or other body fluids (for example HIV, passed in blood or sexual fluids).
• Indirectly:
  • from contaminated surfaces or food (for example cholera from contaminated water or food),
  • from animals (for example a mosquito, which is a vector),
  • through the air (for example in droplets from a cough or sneeze).

The body's barriers to pathogens

Malaria

• Malaria is caused by a parasite (a Plasmodium protoctist) that lives in red blood cells and the liver.
• It is transmitted by the female Anopheles mosquito, which acts as a vector: the mosquito picks up the parasite when it feeds on an infected person's blood, and passes it into the blood of the next person that it bites.

Controlling mosquitoes. The mosquito's life cycle has eggs, larvae and pupae in still water, and adults that fly. Control measures include:

• draining or covering standing water (removes breeding sites);
• putting oil on the surface of ponds (the larvae and pupae cannot breathe);
• introducing fish that eat the larvae;
• using insecticides on adults and in homes;
• using mosquito nets and repellent to prevent bites.

HIV and AIDS

• HIV (human immunodeficiency virus) is a viral pathogen.
• It is transmitted by sexual contact, by blood (sharing needles, infected blood transfusions) and from mother to baby (across the placenta, during birth or in breast milk).
• HIV infection may lead to AIDS (Acquired Immune Deficiency Syndrome), when the immune system is so weak that the person gets many infections.
• Control: using condoms, not sharing needles, screening blood donations, and giving antiviral drugs to infected people, including pregnant women to protect the baby.

Cholera

• Cholera is caused by a bacterium, transmitted in contaminated water (and food).
• The bacterium makes a toxin that causes cells in the small intestine to secrete chloride ions into the gut. The water potential in the gut falls, so water moves by osmosis from the blood into the gut. The result is severe diarrhoea, dehydration and loss of ions from the blood.
• Controlling the spread: a clean water supply (treated with chlorine), hygienic food preparation, good personal hygiene (washing hands), proper waste disposal and sewage treatment. Treatment is by oral rehydration (water with salts and glucose).

Alcohol

Effects of excessive consumption of alcohol: reduced self-control, it is a depressant (slows the nervous system), it slows reaction times, and it can cause damage to the liver (cirrhosis). There are also social implications, such as accidents, violence and effects on families and work.

Tobacco smoke

Smoking is strongly associated with bronchitis, emphysema, lung cancer and heart disease. Smoking during pregnancy is associated with a reduced birth weight of the baby (because the blood carries less oxygen to the fetus).

12.2 Antibiotics

A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

• Antibiotics (for example penicillin) are used to treat bacterial infections. They kill bacteria (or stop them growing) but do not affect viruses, so they do not work against colds, flu or HIV.
• Antibiotic resistance. Some bacteria are resistant because of a gene mutation. When antibiotics are used, the resistant bacteria survive and reproduce, and so the resistant strain spreads (this is natural selection). MRSA is a bacterium that is resistant to many antibiotics.
• To reduce the development of resistance: use antibiotics only when essential, and always complete the course so that all the bacteria are killed.

12.3 Immunity

• Each pathogen has its own antigens: molecules (usually proteins) on its surface that have specific shapes.
• Antibodies are proteins made by lymphocytes. Each antibody has a complementary shape that fits one specific antigen. The antibody binds to the antigen, which leads to direct destruction of the pathogen, or marks it for destruction by phagocytes.

Active immunity

After the first infection, memory cells are produced, which remember the antigen. If the same pathogen enters again, the memory cells make the right antibodies very quickly in large amounts, so the person does not become ill: they are immune. This gives long-term immunity.

Vaccination

1. A weakened or dead pathogen, or its antigens, is given (injected or swallowed).
2. The antigens stimulate an immune response: the lymphocytes make antibodies.
3. Memory cells are produced that give long-term immunity.

Vaccination protects the individual without them having the disease. If a large proportion of the population is vaccinated, the pathogen spreads less, so the spread of transmissible disease is controlled, and even people who are not vaccinated are protected.

Passive immunity

The person's own lymphocytes are not stimulated, so no memory cells are produced and the protection does not last. Breast-feeding is important because the baby gets antibodies from the mother's milk, which protects the infant before its own immune system is fully developed.

HIV and the immune system

HIV infects and destroys lymphocytes, so their numbers decrease and the body is less able to make antibodies. The immune system is weakened, and the person is more likely to get infections that they would normally fight off.

Influenza is caused by a virus, and antibiotics do not affect viruses (they kill bacteria only).