ABSTRACT
The function of the immune system is to protect the host against foreign organisms (viruses, bacteria, fungi), tumor cells, and other foreign substances such as xenobiotics. The importance of the immune system is evidenced by the consequences of immunodeficiency: patients with this disorder are prone to be susceptible to infections and develop tumors. Immunodeficiency can be congenital or acquired; the latter is also known as acquired immunodeficiency syndrome (AIDS).
