The human immune system protects the body against foreign objects such as microorganisms. It is made up of various cells that are scattered throughout the body, each carrying different roles and moving through the body as needed.
There are two main types of cells in the blood. The most common are red blood cells or erythrocytes, which carry oxygen to body tissues and remove carbon dioxide. The other group is white blood cells, or leukocytes. These are the immune cells.
Some white blood cells recognize specific foreign bodies for the body to have been exposed in the past. These immune cells are called lymphocytes. Other white blood cells are not specific and can attack a range of different foreign organisms: these include neutrophils, eosinophils and natural killer cells.
There are two different types of lymphocytes. B lymphocytes (sometimes called B cells only) produce antibodies. An antibody is a protein that can block in a particular role of a particular foreign body. When this happens, the antibody signals to other cells immune to the body's attack.
T lymphocytes (sometimes called T-cells) are called different names depending on the molecules on their surface. CD4 cells (Also known as CD4 T lymphocytes and T-helper cells) play a coordinating role for the immune system. They help B lymphocytes identify foreign organisms (which they produce antibodies against). They also secrete substances that activate CD8 cells to reproduce. CD4 cells also activate macrophages (see below) to kill certain organisms, including many of the diseases subsequent to AIDS, opportunistic diseases. When CD4 cells are destroyed by HIV, all these parts of the immune system are disrupted. CD8 cells (also known as CD8 T-lymphocytes or cytotoxic T cells) connect in the body to abnormal cells, especially cells that have been infected by viruses, and kill them.
Other immune cells
Natural killer (NK) cells attack tumor cells and virus-infected cells in a manner similar to lymphocytes. But at the same time, each lymphocyte can only recognize and attack cells infected by a specific virus, natural killer cells can attack a wider range.
Eosinophils attack organisms that are too large to be eaten by single phagocytes, such as worms.
Phagocytes are cells that attack and destroy foreign cells, choking them. There are two main types of phagocytes.
- Macrophages move in the blood and tissues of the body, killing organisms that can cause AIDS-related diseases and virus-infected cells.
- Neutrophils leave the bloodstream and go to the tissues where infection or inflammation is developing. These are mainly bacteria and fungi.