Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Infection and Disease
The human body is constantly under attack by diseasecausingorganisms called pathogens. Many of these pathogens are tinymicrobes, such as bacteria and viruses that try to enter our bodies and live within. The invasion of the body by pathogens is called infection.
How the body prevents infection
The human body has several ways of defending itself. The first line of defence is the presence of physical and chemical barriers that prevent pathogens getting into the body tissues. This is also known as the general defence system; it includes the skin, mucus in the breathing passages, blood clotting at a wound and acid in the stomach – all of which inhibit the entry of harmful microbes.
Once the tissues have been invaded by a pathogen, a second, more specific defence system known as the immune response takes over. The immune system involves a number of different types of white blood cells, such as the phagocytes andlymphocytes as well as specialised organs of the body including the spleen, thymus and the lymph glands. The phagocytes, also known as macrophages, are large white blood cells that engulf and destroy harmful bacteria and viruses.
The lymphocytes provide a more specialised protection. Because the membranes of all cells are covered with special ‘marker’ chemicals called antigens, it is important for the body to be able to distinguish between its own cells and ‘foreign’ cells. When apathogen enters the body, the immune system responds by making a specific antibody to the ‘foreign’ antigen. Antibodies generally work by causing the pathogens to clump together and this enables the phagocytes to engulf and destroy them.
The type of antibody made depends on the type of antigen invading. For example an invading cold virus antigen will stimulate the production of a cold virus antibody that will only destroy the cold virus.