Module One 11.4 What causes infectious diseases and how can our bodies defend themselves against the

Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens. They can produce antibodies to destroy pathogens, and antitoxins to neutralise toxins.

In vaccination pathogens are introduced into the body in a weakened form. The process causes the body to produce enough white blood cells to protect itself against the pathogens, while not getting diseased.

Antibiotics are effective against bacteria, but not against viruses. Some strains of bacteria are resistant to antibiotics.

Diseases

Diseases can be caused when micro-organisms such as certain bacteria and viruses enter the body.

Micro-organism	Description
Bacteria cell	A bacterial cell consists of cytoplasm and a membrane surrounded by a white cell wall. The genes are NOT in a nucleus. Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include: food poisoning cholera typhoid whooping cough gonorrhoea - a sexually transmitted disease
Virus	Viruses are smaller than bacteria. They consist only of a protein coat surrounding a few genes. Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes. Diseases caused by viruses include: influenza - flu colds measles mumps rubella chicken pox

Causes of disease

Diseases are more likely to occur if large numbers of micro-organisms enter the body. The chance of this occurring increases if:

Organisms live in unhygienic conditions.

Organisms come into contact with infected organisms.

Defence against micro-organisms

The body defends against micro-organisms in two ways:

It tries to prevent their entry into the body.

White blood cells actively counteract microbes inside the body.

White blood cells can:

• ingest pathogens and destroy them
• produce antibodies to destroy pathogens
• produce antitoxins that neutralise the toxins released by pathogens

Preventing microbe entry.

Counteracting microbes inside the body

There are many types of white blood cell in the body. They help defend against infective micro-organism in 3 ways.

Defence	Function
Ingestion	Some white blood cells engulf and ingest microbes, thus destroying them. Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging the body.
Antibodies	Some white blood cells produce antibodies that destroy a particular microbe. Pathogens contain certain chemicals that are foreign to the body and are called antigens. Each lymphocyte carries a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly, and makes many copies of the antibody that neutralises the pathogen. Antibodies neutralise pathogens in a number of ways: they bind to pathogens and damage or destroy them they coat pathogens, clumping them together so that they are easily ingested by phagocytes they bind to the pathogens and release chemical signals to attract more phagocytes
Antitoxins	Some white blood cells produce antitoxins which counteract the toxins (poisons) produced by the microbe.

It is the toxins produced by the microbes that make us feel ill.

Vaccination

When antibodies are produced they only work against one specific microbe. Once they have killed that microbe, some of the antibody-producing white blood cells remain in the bloodstream. If the same microbe returns, the white blood cells quickly replicate the particular antibody and the microbe is killed before any ill effects are felt.

This is called immunity.

Scientists have taken advantage of this natural defence in a process called vaccination. When people are vaccinated they are immunised against a disease by having a mild or dead form of the disease introduced into their body. This causes white blood cells to create antibodies without the person suffering any symptoms. If the infective organism enters the body, antibodies are rapidly produced to destroy it.

Vaccines and boosters

Vaccines in early childhood can give protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.

Sometimes vaccine boosters are needed, because the immune response 'memory' weakens over time. Anti-tetanus injections may need to be repeated every ten years

Antibiotics

Antibiotics are substances that kill bacteriaor stop their growth. They do not work against viruses: it is difficult to develop drugs that kill viruses without also damaging the body's tissues.

A bacterium is damaged and distorted by penicillin

The first antibiotic - penicillin - was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould.

Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most antibiotics used in medicine have been altered chemically to make them more effective and safer for humans.

Resistance

Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic due to variation or a mutation of the bacterial DNA. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.

MRSA is methicillin-resistant staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. It is important to avoid over-use of antibiotics, so we can slow down, or stop, the development of other strains of resistant bacteria.

Cleanliness

One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

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