Immunity is the body’s defence system against germs and cell changes that could make you ill. It is made up of many organs, cells and proteins that work together. Whenever the immune system recognises something that it doesn’t like or can’t deal with, it triggers an immune response that fights the invader.
The first line of defence is the innate immune system, which is active from birth. It creates physical barriers to infection, such as the skin and mucous membranes that line the respiratory, gastrointestinal and genitourinary tracts. It also includes soluble factors, such as complement proteins and cytokines, that help kill invading germs or stimulate the immune cells to engulf and destroy them.
As the immune system recognises a germ, it makes a record of it so that if it comes back, the body will know what to do. This is called immunological memory and it allows the body to respond much faster and more effectively. The innate immune system is triggered by proteins on the surface of bacteria, viruses and fungi, which are called antigens. These bind to special receptors on the immune cells (called lymphocytes) and trigger the process of a specific immune response.
Some of these responses are very general and others are highly specific to a particular pathogen or abnormal cell, for example cancer cells. Some of these responses are inherited, while others are acquired through immunisation. This is when a vaccine (a small amount of a specially treated virus, bacterium or toxin) is injected into the body, and it mimics the natural immune response to a specific disease so that the body learns to recognise and attack the antigen.