Vaccination, allergy and autoimmunity

The essentials

Vaccination and immunisation are based on the memory of the immune system. A preparation of antigenic proteins of a pathogen, or an inactivated or weakened pathogen (the vaccine), is introduced into the body. The antibodies produced neutralise the pathogenic agents during actual infection, and the vaccine also generates memory B and T cells that recognise the pathogen quickly on later exposure. This gives active immunity. If a quick response is needed, as for tetanus, preformed antibodies or antitoxin are injected (passive immunisation). Recombinant DNA technology has allowed antigenic polypeptides of pathogens to be produced in bacteria or yeast; the hepatitis B vaccine is produced from yeast.

Allergy is an exaggerated response of the immune system to certain antigens present in the environment. The substances that cause it are allergens, such as mites in dust, pollen and animal dander. Antibodies of the IgE type are produced. Allergy is due to the release of chemicals like histamine and serotonin from mast cells. Symptoms include sneezing, watery eyes, running nose and difficulty in breathing. Drugs such as anti-histamines, adrenaline and steroids reduce the symptoms. Modern lifestyles may lower immunity and increase sensitivity to allergens.

Autoimmunity: higher vertebrates can normally distinguish "self" from "non-self". Sometimes, due to genetic and other unknown reasons, the body attacks self-cells, resulting in damage. This is an autoimmune disease; rheumatoid arthritis is an example.

Key points

• Vaccines give active immunity; antitoxins give passive immunity.
• IgE and mast cells are central to allergic reactions.

Common mistakes

• Allergy is an over-reaction to harmless substances, not a failure of immunity.