The immune system - how the body reacts
The immune system is the combination of tissues and cells (and their products) that provides the body with its ability to acquire immunity to invading agents, such as pathogenic micro-organisms, viruses, toxins and foreign tissues. This ability is known as the immune response. There are two types of immune response: cell-mediated immune response and antibody-mediated immune response.
The immune response is specific to the invading cells or viruses and can recognise self from non-self. It also confers upon the body a memory of most previously encountered invaders, enabling the body to respond more quickly and effectively against repeat invasions or infections. This is known as specific resistance or immunity. Foreign agents that provoke the immune response are known as antigens.
The tissues and cells which provide this ability include: red bone marrow; the thymus; the spleen; circulating lymphatic system; white blood cells.