Summary: How the immune system works

• The human immune system protects the body against foreign substances, such as micro-organisms. It is made of many different cells that are spread throughout the body, each playing different roles and moving about the body as needed.
• There are two major types of cells in the blood. The most common are red blood cells or erythrocytes, which carry oxygen to the body tissues, and carry away carbon dioxide. The other group is white blood cells, or leukocytes. The leukocytes are the immune cells.
• Some white blood cells recognise specific foreign organisms, and can remember which foreign organisms the body has been exposed to in the past. These specific immune cells are called lymphocytes. Other white blood cells are non-specific and can attack a range of different foreign organisms, including neutrophils, eosinophils and natural killer cells.
• There are two different types of lymphocytes. B-lymphocytes (sometimes just called B-cells) produce antibodies. An antibody is a protein that can lock onto a distinct part of a specific foreign organism. When this happens, the antibody acts as a signal to other immune cells to attack the organism.
• T-lymphocytes (sometimes just called T-cells) have different names depending on the molecules on their surface.
• CD4 T-cells (also known as CD4 T-lymphocytes, or T-helper cells) play a co-ordinating role in the immune system. They help B-cells identify foreign organisms. They also secrete substances that enable CD8 T-cells to reproduce themselves. CD4 T-cells also activate macrophages to kill certain organisms, including many causes of AIDS diseases. When HIV destroys CD4 T-cells, all these parts of the immune system are disrupted.
• CD8 T-cells (also known as CD8 T-lymphocytes or cytotoxic T-cells) attach themselves to body cells identified as abnormal, notably tumour cells or cells that have been infected by viruses, and kill them.
• Natural killer cells (or NK cells) attack tumour cells and virus-infected cells in a similar way to lymphocytes. But while each lymphocyte can only recognise and attack cells infected by one specific virus, natural killer cells can attack a wider range.
• Eosinophils attack large organisms, like worms.
• The phagocytes attack and destroy foreign cells by engulfing them. There are two main different types of phagocytes: macrophages and neutrophils.
• Macrophages roam the blood and the body tissues, killing organisms, some of which can cause AIDS-related diseases and cells infected by viruses.
• Neutrophils leave the blood to go to tissues where infection or inflammation is developing. They mainly attack bacteria and fungi.