• The human immune system protects the body against foreign objects, 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 are white blood cells, or leukocytes. The leukocytes are the immune cells.
• Some white blood cells recognise specific foreign organisms to which the body has been exposed 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 signals to other immune cells to attack the organism.
• T-lymphocytes (sometimes just called T-cells) are called different names depending on the molecules on their surface.
• CD4 cells (also known as CD4 T-lymphocytes, or T-helper cells) play a coordinating role in the immune system. They help B-lymphocytes identify foreign organisms (which they produce antibodies against). They also secrete substances that enable CD8 cells to reproduce. CD4 cells also activate macrophages (see below) to kill certain organisms, including many causes of AIDS diseases. When HIV destroys CD4 cells, all these parts of the immune system are disrupted.
• CD8 cells (also known as CD8 T-lymphocytes or cytotoxic T-cells) attach themselves to body cells identified as abnormal, notably 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 organisms that are too big to be eaten by a single phagocyte (see below), like worms.
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