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 past this first line of defence, for example, through a cut in your skin, and an infection develops, the second line of defence becomes active. This ‘immune response’ is where specialised cells inside the body take action.
The immune response is triggered by antigens – molecules from pathogens or foreign substances such as bacteria or viruses. Antigens appear on the surface of cells that have ingested or been invaded by a pathogen.
The immune system
This interactive explains the different cells, microorganisms and molecules involved in the human immune system.
To use this, move your mouse or finger over any of the labelled boxes and select to obtain more information.
Select here for the full transcript and copyright information.
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. Neutrophils find pathogens by detecting common antigen patterns, sensing ‘tags’ on the invaders from other immune system cells or by following chemical trails to the infection site. Neutrophils lead a short, busy life and often self-destruct in the process of destroying foreign invaders. Pus is made up of dead neutrophils.
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 one particular antigen identified by the dendritic cell.
B cells produce antibodies that 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 through the lymphatic system or as pus.
By acting together, neutrophils, dendritic cells, T cells, B cells and macrophages form an efficient team that protects your body against disease.
The immune system in action
This interactive is a simple version of the human immune response to two different pathogens.
Related content
To plan teaching and learning around the immune system, take a look at Fighting infection – introduction, Fighting infection: the immune system – unit plan or the Fighting infection: reducing infection – unit plan.
Related concepts include Microorganisms – introduction, Antimicrobial resistance explained, Immunisation in New Zealand and Vaccines and therapies.
Activity ideas
These activities below introduce some of the big science ideas related to our immune system.
Useful Links
Health NZ has a number of resources on the Immune system.
Healthify He Puna Waiora looks at autoimmune diseases – conditions where the immune system mistakenly attacks the human body.
