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The defences that our bodies have developed to fight against foreign invaders are collectively known as the immune system. The immune system is made up of the natural and specific barrier defences.

Foreign invaders enter our bodies through the skin, respiratory and digestive systems. The natural barriers developed by the body include:

• intact skin
• sweat glands
• tears
• saliva
• normal flora
• lining of the respiratory tract including cilia and cough reflex
• digestive tract changes in pH, peyers patches in the small intestine
• urinary system, flushing of the bladder and acidic nature of urine.

There are two types of phagocytic cells that ingest and destroy invaders, these are macrophages that live in the body tissue and neutrophils that live in the blood.

The specific system uses cells and molecules working together through the lymphoid tissues and the lymphatic and circulatory systems. The most important cells are lymphocytes, T cells made in the thymus and B cells made in the bone marrow. Receptors are carried on the surface of the cells that allow them to recognise the specific surface structures of foreign invaders known as antigens. These help lymphocytes switch on and off the immune response to each foreign substance.

The immune response has features that include:

• seeking out and killing infected cells
• stimulating bacteria eating cells to engulf and digest invading bacteria
• producing antibodies to fight infection at distant sites in the body
• activating memory cells that circulate in the blood carrying memory of an infection to respond when the infection is repeated.

The immune response stimulates B cells to produce cells that make antibodies. These are able to:

• attach to bacteria and viruses and make them ready for destroying by phagocytic cells
• stop toxins, bacteria and viruses attaching to body cells by binding on to them
• use complement to directly kill microbes and infected cells by a process called lysis
• help to produce other more specialised antibodies.
  

The immune system can remember previous infections to give us better protection should we come into contact with it again. This is responsibility of the T and B memory cells and antibodies. These are mimicked by the process of vaccination to provide long-term immunity to specific infections.

This is a condition where the body treats its own cells and tissues as if they were foreign and results in diseases such as diabetes and rheumatoid arthritis. The exact cause is not yet known and genetic factors are involved in some cases as autoimmunity can run in families.

Autoimmune diseases can be grouped together as those that are organ specific or non-organ specific. Organ specific autoimmune disease occurs when antibodies no longer recognise a molecule in the body as ‘self’ found in a particular organ or body tissue. Examples include Graves Disease affecting the thyroid gland and insulin dependent diabetes mellitus affecting the production of insulin. Non-organ specific autoimmune disease occurs when autoantibodies cause damage to a wide range of cells and tissues throughout the body. This includes diseases such as Systemic Lupus Erythmatosus (SLE) and Rheumatoid Arthritis.

About 15% of the population in the UK suffers from an allergy at one time or another. The immune system overreacts to foreign invaders that in others would be harmless. Substances that cause an allergic response are called allergens. Common allergens include:

• house dust mite from bedding, carpets and furniture in the home
• rye grass and ragweed pollen
• fungi and fungal spores
• insect stings and bites (bees, wasps, ants, mosquitoes)
• drugs (e.g. penicillin)
• food (e.g. peanuts, cheese, chocolate, milk, shellfish, wheat).

When in contact with something that causes an allergy the body causes an over-production of IgE antibody, which results in an immediate hypersensitivity. Tissue mast cells are also stimulated and produce mucus, swelling and other symptoms. Common forms of allergy are:

• eczema
• asthma
• urticaria (itching)
• hay fever.

Food and drug allergies can lead to anaphylaxis when the whole body reacts and emergency medical treatment is needed.