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The body’s defence system normally provides a formidable barrier against attack by ‘foreign’ invaders such as viruses, bacteria and parasites. Although the way it works is still not fully understood, what is known is that many actions depend on two kinds of lymphocytes (white blood cells) – T cells and B cells – responsible for fending off attackers from outside.

When the body is under attack by invaders, the immune system sends T cells to the affected site to find out what is happening. There are two types of T cells: helper cells and killer cells. Helper cells help the immune system by identifying antigens, a chemical substance that marks the invaders as ‘foreign’. Once the helper cells have recognized a foreign antigen, killer cells are despatched to attack and destroy them. To protect the body against future attack by the same foreign invaders, killer T cells retain a ‘memory’ of their antigen. If the body is threatened again by the same invader, these killer T cells are quickly activated and sent in for the kill. This entire process is known as cellular immunity.

B cells work in a similar way except that they fight off an attack by producing protein antibodies known as immunoglobulins; these are produced specific to the invader. When the immune system identifies a particular invader, B cells are stimulated to produce a large quantity of a specific immunoglobulin that will attach itself to the invading antigens and immobilize them. Once this has happened, the antigens are devoured by other white cells called phagocytes. This process is known as humoral immunity.