A group of non-neoplastic and neoplastic disorders resulting from the deregulation and/or deficiency of immune system functions. It includes autoimmune disorders (e.g., lupus erythematosus, dermatomyositis, rheumatoid arthritis), congenital and acquired immunodeficiency syndromes including the acquired immune deficiency syndrome (AIDS), and neoplasms (e.g., lymphomas and malignancies secondary to transplantation.)