This protocol includes studies of genetic defects of the immune system that cause failure of host defenses against infections, immune dysregulation and autoimmune diseases. Numerous rare disorders result from inherited or newly arising mutations in genes involved in the development and function of innate and adaptive immune systems or both. As specific disease syndromes are defined and the responsible genes identified, mutations in individual families can be sought. Correlation of mutation sites with clinical information helps to determine how specific gene segments encode important functional domains of the proteins of the immune system within the same genetic defect. Rare, single gene disorders identify immunologic pathways that might contribute to more common conditions, such as failure to respond to vaccines, susceptibility to allergies, or autoimmune diseases like arthritis or lupus.