Antibodies and antigens Types of immunity Active immunity Vaccinations Passive immunity I Immunity

Antibodies and antigens Invading pathogens can be recognised by your white blood cells as foreign because of their surface protein molecules. These protein molecules are called antigens. Your white blood cells construct Y-shaped antibodies that lock onto the antigens, making it easy for other white blood cells to destroy the pathogen.

Characteristic surface molecules (called antigens) on the pathogen are identified, and the antibodies exactly fit these molecules. With the antibody attached, the pathogen is quickly destroyed by white blood cells. Some pathogens (such as the flu virus) change their antigens frequently, so that last year’s antibodies no longer fit. Antibodies When a pathogen starts to multiply within the body, special blood cells trigger the production of purpose- built antibodies to fight it. 6H 5 Friendly fungi 6H 6 Resisting resistance

Types of immunity Once your body has made antibodies for a particular pathogen, the cells that made it stay available in case they are needed again. A few antibodies remain in the blood ‘on patrol’. If particles carrying those specific antigens return, the antibodies lock on and a large number of new antibodies are produced, preventing illness. We say you are immune to that disease. Most of the time, once you’ve recovered from an infectious disease, you’ll be immune to it. However, some pathogens, such as colds, influenza and noroviruses, keep on changing their surface antigens so they can reinfect the same people many times.

Active immunity When the body makes its own antibodies we say it has active immunity to that particular pathogen. Anyone who has got sick and then recovered from the disease will have active immunity. Vaccines A vaccine is an injection of dead or weakened pathogens. These develop active immunity by triggering the formation of antibodies but are unable to cause the illness. It often takes more than one dose of the vaccine to build up full immunity to the disease.

A kitten develops the skills needed to catch and kill mice by training on harmless dead mice or toys. The dead or weakened pathogens in a vaccine act like this kitten’s toy mouse. Your body can study the pathogen and develop antibodies to it without any risk of illness. Some vaccines today simply contain a shell covered in the key surface proteins, without any viral genetic material – very much like this toy!

Passive immunity Young babies are very vulnerable to diseases because they are still making their own antibodies. To help them survive those early months, the mother's antibodies pass through the placenta before birth and her breast milk contains additional antibodies. The maternal antibodies remain ‘on patrol’ for a few months, but are eventually destroyed by the baby’s own immune system. Immunity provided by maternal antibodies is passive immunity because the antibodies were made by the mother, not the baby.

Sometimes patients infected with a particularly serious disease will be given injections of antibodies (also called immunoglobulin) to that disease. These antibodies may have come from a person who has recovered from the disease, or sometimes from the blood of an animal such as a horse that has been stimulated to produce the required antibodies. Injections of this type provide artificial passive immunity for a short period.

Immunity Immunity from disease is created by antibodies, which are specific to each pathogen. Some pathogens change frequently, so last year’s antibodies no longer work. Natural active immunity Body makes own antibodies after getting sick and recovering from an infection. Natural passive immunity Babies get short-term protection from mother’s antibodies via placenta and breast milk. Artificial active immunity Body makes own antibodies after injection of dead or weakened pathogens (vaccination). Artificial passive immunity Injections of antibodies (immunoglobulin) to fight some serious illnesses. 6H 5 Friendly fungi 6H 6 Resisting resistance

Proceed to next chapter End of chapter