Diseases and Immunity

Pathogens and Transmission

Some diseases are caused by organisms that invade the body. These organisms are pathogens, and the diseases they cause can often be passed from person to person.

How pathogens are transmitted

A pathogen that is transmitted indirectly is carried by a surface, food, an animal or the air before it reaches a new host. A pathogen transmitted by direct contact, including through blood and other body fluids, needs no such carrier.

Worked example (classifying transmission): A pathogen is passed on when a person eats food that was prepared on a dirty surface. Is this direct or indirect? Step 1. The pathogen reaches the new host through the surface and the food. Step 2. Something in between carries it, so it is not direct contact. Answer: Indirect transmission.

Body Defences

The body has several defences that stop pathogens getting in, and others that destroy those that do.

White blood cells

If a pathogen gets past the barriers, white blood cells deal with it in two ways: they engulf it by phagocytosis, and they produce antibodies.

Worked example (naming the defence): A pathogen is swallowed in food. Name the defence in the stomach that kills it. Step 1. The pathogen passes into the stomach. Step 2. Stomach acid kills pathogens that are swallowed. Answer: Stomach acid.

Controlling the Spread of Disease

Good public health measures control the spread of disease by cutting off the routes by which pathogens are passed on.

The stages of sewage treatment are not required. You only need to explain why it is important.

Linking the measures to transmission

Each measure breaks one of the indirect routes shown on page 3: contaminated water, food and surfaces are made safe, and waste that could contaminate food, water or surfaces is removed.

Worked example (explaining a measure): Explain why hands should be washed before preparing food. Step 1. Hands can carry pathogens. Step 2. Washing removes them, so they are not passed on to the food. Answer: Prevents food contamination.

Antigens and Antibodies

ExtendedAntigens

Each pathogen has its own antigens, which have specific shapes. The body recognises them as foreign.

ExtendedAntibodies

Antibodies are proteins that bind to antigens. Specific antibodies have complementary shapes, which fit specific antigens.

ExtendedWhat antibodies do

Once bound to an antigen, an antibody leads to the direct destruction of the pathogen, or marks the pathogen for destruction by phagocytes. Lymphocytes make the antibodies.

ExtendedActive immunity

Active immunity is defence against a pathogen by antibody production in the body.

Active Immunity and Vaccination

ExtendedGaining active immunity

Active immunity is gained after an infection by a pathogen, or by vaccination.

ExtendedThe process of vaccination

Step 1: weakened pathogens, or their antigens, are put into the body.

Step 2: the antigens stimulate an immune response by lymphocytes, which produce antibodies.

Step 3: memory cells are produced, giving long-term immunity.

ExtendedVaccination and the spread of disease

If the pathogen enters the body again, memory cells allow antibodies to be made quickly, so the person does not become ill. When many people are vaccinated, fewer people can pass the pathogen on, so the spread of the disease is controlled.

Worked example (explaining vaccination): Explain why a vaccinated person is protected if the pathogen later enters the body. Step 1. Vaccination caused lymphocytes to produce antibodies and memory cells. Step 2. The memory cells give long-term immunity. Step 3. The antibodies are made quickly, so the pathogen is destroyed. Answer: Memory cells give long-term immunity.

Passive Immunity

ExtendedPassive immunity

In passive immunity the antibodies are acquired from another individual, including across the placenta and in breast milk. It is a short-term defence, and memory cells are not produced.

ExtendedImportance of breast-feeding

Breast-feeding is important for the development of passive immunity in infants, because breast milk contains antibodies that protect the baby while its own immune system develops.

ExtendedActive and passive immunity compared

Worked example (active or passive?): A baby is protected against a pathogen by antibodies from its mother’s milk. Is this active or passive immunity? Step 1. The antibodies come from another individual. Step 2. The baby’s own lymphocytes did not make them, so no memory cells are produced. Answer: Passive immunity.

Cholera

ExtendedThe disease

Cholera is caused by a bacterium that is transmitted in contaminated water.

ExtendedHow the toxin causes symptoms

The cholera bacterium produces a toxin. The toxin causes the secretion of chloride ions into the small intestine. Water then moves into the gut by osmosis, causing diarrhoea, dehydration and loss of ions from the blood.

ExtendedWhy water moves

The chloride ions in the gut lower the water potential there. Water therefore moves from the blood into the gut, from the blood (higher water potential) into the gut (lower water potential), down a water potential gradient, by osmosis.

Worked example (explaining dehydration): Explain why a person with cholera becomes dehydrated. Step 1. The toxin causes chloride ions to be secreted into the small intestine. Step 2. Water moves into the gut by osmosis. Step 3. The water is lost in diarrhoea, so the body is dehydrated. Answer: Water lost in diarrhoea.