CD55-deficiency is an autosomal recessive genetic disorder. This means that a patient will have two mutations inherited from both parents (bi-allelic mutations), while carriers of only one mutation are generally healthy. The disease is caused by mutations in a gene called CD55, which encodes a protein that regulates the complement system, which is part of the immune system. The role of CD55 is to prevent tissue self-injury by complement system components. Mutations causing CD55-deficiency are those predicted to cause loss of the protein function; therefore, loss of CD55 causes injury and inflammation in the intestine, which leads to the clinical manifestations of the disease.
CD55