Our immune system protects us from infection and disease. It is a complex system made up of the bone marrow, the thymus gland (which lies behind the breast bone), the spleen and the lymph nodes (or lymph glands).
One of the most important cells in our immune system is a type of white blood cell called a lymphocyte. Lymphocytes are made in the bone marrow, and they circulate all around the body in the blood and lymph vessels. Lymphocytes recognise unwanted or abnormal cells and act quickly to destroy them.
There are two types of lymphocytes: B-cells and T-cells. B-cells develop into cells (called plasma cells) that make specialised proteins called antibodies. Antibodies circulate in the blood and react with toxins, bacteria and some cancer cells. The body can then identify and remove these unwanted cells.
However, some foreign substances in the body can hide from the B-cells by growing within the body's own cells. T-cells can sense when the body's own cells have become abnormal and can destroy them. The whole process is known as an immune response.
After the abnormal cells or bacteria have been destroyed, the surviving B-cells and T-cells develop into specialised memory cells that remain on watch in the lymph nodes, and are reactivated if that particular abnormal cell or substance appears in the body again.