natural defence

In biology, the defence against disease provided by natural barriers. In animals this is provided by natural barriers to infection such as the skin, and defence systems of the blood circulation such as the production of antibodies. If the natural defences work, they provide immunity (see immunization) to infection. Plants also have natural defences – for example, spines or poisonous chemicals.

Organisms may also need to defend themselves against being eaten by another organism. Animals and plants use a variety of mechanisms. Animals need to be able to detect a predator and take appropriate action. This may involve escaping (for example, rabbits), defending itself using a frightening display (for example, a butterfly opening its wings to reveal eye spots on its wings), or using teeth or claws (for example, a cat). However, many animals and plants rely on camouflage to make them ‘invisible’ – for example, deer or a bulb growing in grassland with leaves looking like grass. Some plants may have defences such as spines (holly), chemicals with unpleasant effects (the stings of nettles), or poisonous chemicals (bracken).

Skin as a defence The barrier of the skin helps prevent infection by being tough. It repels water and so tends to be dry – these are conditions in which disease-causing micro-organisms (pathogens) do not thrive. The skin also produces anti-bacterial peptides.

Tears, sweat, and saliva There are other important barriers. The tears produced by the eyes, sweat, and saliva are also protective. One way they protect is by containing an enzyme – lysozyme – that digests and weakens the cell walls of bacteria, causing them to die.

Internal mechanisms Once a disease-causing organism (pathogen) gains entry to the body, a variety of mechanisms in the blood help protect the body. White blood cells help in several ways. Some engulf micro-organisms and then kill them; others can kill infected cells; and yet others make antibodies. Antibodies are special chemicals of many different types, each of which can bind to one particular chemical specific to a micro-organism. Chemicals like this, called antigens, act as a label marking out the micro-organism as being foreign or ‘non-self’. Micro-organisms that have antibodies attached to them can become sticky and join in clumps. This prevents them from spreading so efficiently. The antibody also encourages white cells to engulf the clumps. The white cells are able to retain a memory of the antibody that works against the micro-organism. When that organism next contacts the body, large amounts of antibody can be made quickly and the organism killed before it can cause disease. This is an important way of becoming immune to disease.

Defences in babies When a baby has just been born it cannot make its own antibodies. However, its mother's antibodies are found in breast milk and these enter the baby and protect it until it makes its own.

Vaccination Vaccination provides immunity using antigens. A vaccine contains antigen from a specific pathogen, which could be a bacterium or a virus. It does not, however, contain any active pathogen and so cannot cause disease. The vaccination causes the body's natural defences to react just enough to create a memory of the antigen and provide immunity for some years.