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The immune system is comprised of millions of cells that help to protect us against a wide variety of pathogens such as bacteria and viruses. This is quite a task, so to be able to do it effectively we require many types of cells.

All the cells of the immune system start life in the bone marrow, located in the hollow part of the long bones in our body. This hollow cavity is full of cells that look after the most remarkable cells in our entire body, the stem cell. These stem cells are able to produce nearly every other cell in the human body and it is for this reason that they are used frequently in the treatment of patients who have had cancer. Every time they divide, they are able to produce another stem cell alongside any other cell type that is needed. In the case of the immune system, this might be a lymphocyte.

Lymphocytes are specialised cells within the blood that are able to recognise specific bacteria or viruses and target them to be killed. There are 2 main types, B and T. B lymphocytes or B cells as they are otherwise known are responsible for producing antibodies which bind to viruses and bacteria, allowing them to be recognised by other components of the immune system. The second type, known as T lymphocytes or cells, are able to find these bacteria and viruses within cells and destroy the cells that have been infected.