If a pathogen evades the barriers to infection a second line of defence is activated. This is the white blood cells of the immune system. As this is a response to a particular pathogen, it is called active or acquired immunity.

But how does your body recognise when a foreign pathogen has entered?

The surface of every cell is covered with molecules that give it a unique set of characteristics. These molecules are called antigens. Antigens are generally fragments of protein or carbohydrate molecules. There are millions of different antigens and each one has a unique shape that can be recognised by the white blood cells of your immune system. The white blood cells then produce antibodies to match the shape of the antigens. The antibodies latch onto the antigens, smother the pathogenic invader and kill it.

The antigens on the surface of pathogenic cells are different from those on the surface of your own cells. This enables your immune system to distinguish pathogens from cells that are part of your body.

Antigens are found on the surface of foreign materials like pollen, pet hairs and house dust where they can be responsible for triggering hay-fever or asthma attacks.

There are several different types of white blood cells. In general there are two groups:

• lymphocytes make antibodies to destroy foreign pathogens
• macrophages eat them.

Both of these cells are found all over the body but especially in the lymph glands. These glands often become swollen when your body is mounting an immune response and so an infected person often says they have swollen glands.

When a pathogen enters the body it stimulates specific lymphocytes to make antibodies. These are proteins of a very specific shape and they will attach to the antigens on the surface of the pathogen. This kills the pathogen which is then completely destroyed when it is eaten and digested by macrophages.

If a pathogen enters your body, white blood cells of your immune system quickly recognise its foreign antigens. This stimulates specific lymphocytes to grow, multiply and finally produce antibodies that will stick to the antigens on the invading pathogens and destroy them.

This initial response takes a few days before it is large enough to fight off the infection. During this time, damage to body tissues will happen and you will feel the symptoms of the infection. This can cause serious damage and some infections can be fatal. However, with most infections, your immune system is able to produce enough antibodies to kill the pathogens that are causing the infection and the symptoms disappear. Once this happens, your immune system switches off its response to this infection but remains active, waiting for other pathogens.