Foreign Invaders
Every day our bodies are under threat from a huge variety of foreign invaders that can cause disease, even death. Colds, upset stomachs, chickenpox, malaria, tuberculosis and hundreds of other illnesses are caused by foreign invaders.

Luckily our bodies have developed defences to protect us from these invaders. Grouped together they make up our IMMUNE SYSTEM. It is very important: without it none of us would survive.

Invaders come in all shapes and sizes: the smallest can only be seen with an electron microscope that magnifies them 100 000 times. The largest can reach 4 metres in length.

They also come in many forms: bacteria, viruses, fungi, parasites and poisons are all foreign invaders. They can be passed to us in many different ways: in water droplets in the air, through insect bites, domestic pets, dirt, infected food.

The Natural Immune System
Foreign invaders get into our bodies through our skin, airways and digestive system.

We have barriers to try and stop these attacks:

• the skin is a tough, practically waterproof layer,
• the lining of the airways traps the invaders, sweeps them upwards out of the body or destroys them.
• the intestines also have a lining which stops the attachment of harmful microbes and enzymes that attack invaders.

If invaders manage to get through these surface barriers a second line of defence comes into play: special cells called phagocytes eat up and destroy invaders. There are two types of phagocytic cell: macrophages and neutrophils. Macrophages live in body tissues, neutrophils circulate in the blood.

The Specific Immune System
Some invaders overcome the natural immune system. The Specific Immune System then comes into action. It deals with each foreign invader separately, using cells and molecules that work together to protect us. It operates through lymphoid tissues and the lymphatic and blood circulations.

The most important cells in this system are lymphocytes (T cells - made in the thymus - and B cells - made in the bone marrow). They carry receptors on their surface that can recognise the specific surface structures of foreign invaders, known as antigens. It is this recognition that causes the lymphocytes to set off on an immune response specific to each foreign substance.

The response has many features: cells seek out and kill infected cells in the body tissue; some stimulate bacteria eating cells to engulf and digest the invading bacteria. Other cells stimulate the production of antibodies to combat infection at distant sites and others circulate in the blood carrying the memory of an infection, ready to respond should you become infected again.

Antibodies
During the immune response B cells are stimulated to produce cells that in turn produce antibodies. They have a number of very important roles in your body's response to foreign invaders:

• they latch on to invading bacteria and viruses in your blood and tissues to get them ready to be destroyed by phagocytic cells,
• they bind to poisons, bacteria and viruses in your gut and airways to stop them attaching to your cells,
• with complement, they kill microbes or infected cells directly by a process called lysis,
• they help in the production of further, more specialised antibodies by your immune system.

Memory of past encounters provides long-term protection
Your immune system can 'remember' past infections and store products from that immune response to give you better protection the next time that same infection tries to attack you. It is the memory T cells, memory B cells and antibodies that do this. This is the reason why vaccination provides long-term immunity from infection.

Adaptable Antibodies
Antibodies are very useful tools for use in medical research and care. This is because of the ability of each antibody to recognise only one type of antigen molecule.