The immune system

The function of the immune system is to protect an individual against becoming sick from an infection. Infections are usually caused when a pathogen such as a disease-causing bacterium, fungus or virus gets somewhere it should not be. The immune systems of all mammals function in a similar way.

Keeping foreign substances out; deactivating those that get in

The mammalian body is designed to keep pathogens out. Skin is a natural barrier which protects most of the body and fluids such as ear wax, tears and snot help protect the natural openings in this barrier.

If a pathogen gets through this first line of defence, specialised cells inside the body take action. The part of a pathogen that stimulates such an immune response is called an antigen.

Cells of the immune system

A large number of cells called neutrophils float through blood and lymph vessels ingesting and breaking up anything they don’t recognise as part of the body. This recognition is based on matches (and mis-matches) between proteins and other molecules on the surfaces of the neutrophils and other cells they come across. Neutrophils lead a short, busy life and often self-destruct in the process of destroying foreign invaders.

Dendritic cells have long tendrils which allow each dendritic cell to be in contact with 200 or more other cells. They make up the gossip network of the immune system. If a particular antigen is discovered by a dendritic cell, the dendritic cell releases chemical messages which quickly activate T-cells circulating in the body's blood and lymph systems.

The T-cells now begin to take action. Over a period of about a week, the body develops specialised T-cells to target the particular infection that has been identified. Some are killer T-cells. Their job is to seek and destroy infected cells. Others, the helper T-cells, are in charge of deploying the killer T-cells, making sure there are enough and that they go to the right place. Memory T-cells are long-lived and allow your body to respond more quickly to a subsequent invasion.

Meanwhile, in the bone marrow, B-cells are also being produced. Like the T-cells, B-cells are specific to the one particular antigen identified by the dendritic cell. B-cells produce antibodies which attach to antigens on foreign objects like bacteria, and label them for destruction by other parts of the immune system.

Macrophages clean up the mess. They roam the body absorbing anything from clumps of antigens grouped together by antibodies, to strands of DNA exposed when the cells they have been in have been destroyed. The body then gets rid of these long-serving macrophage cells either in the lymph system, or as pus.

By acting together, neutrophils, dendritic cells, T cells, B cells, and macrophages form an efficient team which protects your body against disease.