Topic 3: Infection and response

Communicable (infectious) diseases are caused by pathogens — microorganisms that can spread between organisms. Pathogens include bacteria, viruses, protists and fungi, and they infect both animals and plants. This topic looks at named examples of diseases, how the human body keeps pathogens out and destroys those that get in, and how medicine prevents and treats infection through vaccination, drugs and monoclonal antibodies. You also need to know how plants are affected by disease and how they defend themselves.

Pathogens and how disease spreads

A pathogen is a microorganism that causes communicable (infectious) disease. The four types are bacteria, viruses, protists and fungi. Bacteria and viruses can reproduce very rapidly once inside the body.

• Bacteria are small living cells that may produce toxins (poisons) which damage tissues and make us feel ill.
• Viruses are not cells. They live and reproduce inside body cells, which damages or destroys the cell.

Pathogens are spread in three main ways: through the air (droplets when you cough or sneeze), through water (drinking dirty water) and by direct contact (touching infected surfaces, skin or sexual contact). Spread can be reduced by hygiene, destroying vectors, isolating infected individuals and vaccination.

Viral diseases

You need three named viral diseases.

• Measles — spread by droplets from sneezes and coughs. Symptoms are fever and a red skin rash. It can be serious or even fatal, so most young children are vaccinated against it.
• HIV — spread by sexual contact or exchange of body fluids such as blood (e.g. sharing needles). At first it causes flu-like symptoms. It is controlled with antiretroviral drugs. If untreated, the virus attacks the body's immune cells and the immune system becomes so badly damaged it cannot deal with other infections — this late stage is called AIDS.
• Tobacco mosaic virus (TMV) — a plant pathogen affecting many species including tomatoes. It gives leaves a distinctive mosaic pattern of discolouration. This reduces photosynthesis, so growth is affected.

Bacterial, fungal and protist diseases

Bacterial:

• Salmonella food poisoning — bacteria spread by food prepared in unhygienic conditions or from infected poultry. Toxins cause fever, abdominal cramps, vomiting and diarrhoea. In the UK poultry are vaccinated to control it.
• Gonorrhoea — a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge and pain on urinating. Once treated easily with the antibiotic penicillin, but resistant strains have appeared. Spread is controlled with treatment and barrier methods such as condoms.

Fungal:

• Rose black spot — a fungus that causes purple or black spots on leaves, which then turn yellow and drop early. This reduces photosynthesis and growth. Spread in water and by wind, it is treated with fungicides and by removing affected leaves.

Protist:

• Malaria — caused by a protist with part of its life cycle in the mosquito vector. Mosquitoes carry the pathogen to people when they feed on blood, causing repeating episodes of fever; it can be fatal. Spread is reduced by preventing mosquitoes breeding and using mosquito nets to avoid bites.

Human defence systems

The body has non-specific defences that try to stop pathogens entering in the first place:

• Skin — a physical barrier; it also produces antimicrobial secretions and scabs over cuts.
• Nose — hairs and mucus trap particles and pathogens.
• Trachea and bronchi — secrete mucus to trap pathogens, and have cilia (tiny hairs) that waft the mucus up to the throat to be swallowed.
• Stomach — produces hydrochloric acid that kills most swallowed pathogens.

If pathogens get past these barriers, white blood cells of the immune system defend the body by:

1. Phagocytosis — engulfing and digesting pathogens.
2. Producing antibodies — proteins that lock onto specific antigens, marking pathogens for destruction.
3. Producing antitoxins — which neutralise the toxins released by bacteria.

Vaccination

A vaccine contains small quantities of dead or inactive forms of a pathogen. When injected, the white blood cells respond by producing antibodies against the pathogen's antigens. If the same live pathogen later enters the body, the white blood cells can respond quickly to produce the correct antibodies, preventing illness.

If a large proportion of the population is vaccinated, the spread of a pathogen is greatly reduced because there are far fewer people for it to infect — this also protects unvaccinated people (sometimes called herd immunity). For higher tier, you should be able to evaluate the benefits and risks of vaccines using given data, but you do not need to remember specific vaccination schedules.

Antibiotics and painkillers

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. Important points:

• Specific bacteria should be treated by specific antibiotics.
• Antibiotics cannot kill viruses (such as colds and flu), because viruses live inside body cells, making it hard to develop drugs that kill them without harming the cell.

The emergence of strains of bacteria resistant to antibiotics (e.g. MRSA) is of great concern. Resistance develops by natural selection, so doctors should not over-prescribe antibiotics and patients should always complete the full course.

Painkillers and other medicines treat the symptoms of disease (such as pain or fever) but do not kill the pathogen.

Discovery and development of drugs

Traditionally drugs were extracted from plants and microorganisms.

• Digitalis, a heart drug, originates from foxgloves.
• Aspirin, a painkiller, originates from willow.
• Penicillin, an antibiotic, was discovered by Alexander Fleming from the Penicillium mould.

Most new drugs are now synthesised by chemists in the pharmaceutical industry, but the starting point may still be a chemical from a plant. New medical drugs must be tested and trialled before use to check they are safe and effective. Preclinical testing is done in a laboratory using cells, tissues and live animals. Clinical trials use healthy volunteers and patients: very low doses are given at first; if safe, the optimum dose is found.

In a double blind trial, some patients are given a placebo (a treatment containing no drug). Neither the doctors nor the patients know who has received the placebo until the trial is complete, removing bias. Tests for toxicity, efficacy and dose are carried out, and results are peer reviewed before publication.

Monoclonal antibodies (higher tier)

Monoclonal antibodies are produced from a single clone of cells, so they are all identical and specific to one binding site on one protein antigen. This means they can target a specific chemical or specific cells in the body.

They are made by stimulating mouse lymphocytes to make a particular antibody. These lymphocytes are combined with a type of fast-dividing tumour cell to make a hybridoma cell. The hybridoma can both divide and make the antibody. Single hybridoma cells are cloned to produce many identical cells that all make the same antibody, which is then collected and purified.

Uses include:

• Pregnancy tests — detecting the hormone in urine.
• Research and diagnosis — measuring or locating a substance, or detecting pathogens.
• Treatment — e.g. binding to a cancer cell so a radioactive substance, toxic drug or chemical that stops cells growing is delivered directly to the cancer cells without harming other cells.

Monoclonal antibodies created more side effects than expected, so they are not yet as widely used as hoped when they were first developed.

Plant diseases and defences

Plants can be infected by viral, bacterial and fungal pathogens, and by insect pests such as aphids. Signs of disease include stunted growth, spots on leaves, areas of decay (rot), growths (galls), malformed stems or leaves, discolouration and the presence of pests. A plant disease can be identified by reference to a gardening manual or website, by taking the plant to a laboratory, or using testing kits containing monoclonal antibodies.

Plants may be short of mineral ions taken from the soil: lack of nitrate ions causes stunted growth (needed for protein synthesis), and lack of magnesium ions causes chlorosis (yellow leaves) because magnesium is needed to make chlorophyll.

Plant defences include:

• Physical: a waxy cuticle and cellulose cell walls; layers of dead cells around stems (e.g. bark) that fall off.
• Chemical: antibacterial chemicals, and poisons to deter herbivores.
• Mechanical: thorns and hairs to deter animals; leaves that droop or curl when touched; mimicry to trick animals.