The immune system is composed of white blood cells. These cells are made in the bone marrow and travel through the bloodstream and lymph nodes. They protect and defend against attacks by "foreign" invaders such as germs, bacteria and fungi. Antibodies are proteins that are made in response to infection or immunization and help fight infections. Complement is another blood protein that plays a protective role in the immune system. In the most common PIDDs, different forms of these cells or proteins are missing or do not funtion. This creates a pattern of repeated infections, severe infections and/or infections that are unusually hard to cure. These infections may attack the skin, respiratory system, the ears, the brain or spinal cord, or in the urinary or gastrointestinal tracts.