Overview of the human immune system A general overview of the immune system is provided so that an appreciation can be gained of how biological molecules from certain mushrooms may modulate the immune response and tackle cancer cells. Immunology is the study of the methods by which the body defends itself from infectious agents and other foreign substances in its environment. The immune system is a remarkably adaptive defence system that has evolved in humans to protect against invading pathogenic microorganisms and evidence is accumulating that the immune system can provide protection against some tumours (Wood, 2001). An infectious organism that causes a disease is called a pathogen and the individual (person or animal) that is infected by a pathogen is called the host. There are thousands of components to the immune system and it would appear that the immune system is far more complicated than necessary for achieving what is, on the surface, a simple task of eliminating a pathogenic organism or abnormal ‘self’ cells. However there are a number of reasons for this complexity, including the desirability of eliminating pathogens without causing damage to the host. Getting rid of a pathogen or dead host cells is theoretically easy, but eliminating these without damaging the host is much more complicated. The immune system must be able to distinguish between pathogens or abnormal cells and healthy host cells so that it can direct its destructive powers towards their elimination. As a consequence of this dynamic complexity, the immune system is able to generate a tremendous variety of cells and molecules capable of specifically recognising and eliminating an apparently limitless variety of foreign invaders, in addition to the recognition and destruction of abnormal cells. Furthermore, these host cells and molecules act together in an exquisitely adaptable dynamic manner