In immunodeficiency disorders, part of the body's immune system is missing or defective, thus impairing the body's ability to fight infections. As a result, the person with an immunodeficiency disorder will have frequent infections that are generally more severe and last longer than usual.

The immune system is the body's main system to fight infections. The normal immune system involves a complex interaction of certain types of cells that can recognize and attack "foreign" invaders, such as bacteria, viruses, and fungi. It also plays a role in fighting cancer. People are born with certain parts of the immune system; such parts are called innate immunity. Other parts of the immune system people develop over time, which is called adaptive immunity.

The innate immune system is made up of the skin (which acts as a barrier to prevent organisms from entering the body), white blood cells called phagocytes, a...