the oesophagus. very low numbers of circulating t cells are seen. the condition is diagnosed by fluorescent in situ hybridization and treated with thymus transplantation. severe combined immunodeficiency (scid) are group of rare congenital genetic diseases that can result in combined t, b, and nk cell deficiencies. these syndromes are caused by mutations that affect the maturation of the hematopoietic progenitor cells, which are the precursors of both b and t cells. a number of genetic defects can cause scid, including il-2 receptor gene loss of function, and mutation resulting in deficiency of the enzyme adenine deaminase. autoimmune polyendocrine syndrome autoimmune polyendocrine syndrome type 1, is a rare genetic autoimmune syndrome that results from a genetic defect of the thymus tissues. specifically, the disease results from defects in the autoimmune regulator (aire) gene, which stimulates expression of self antigens in the epithelial cells within the medulla of the thymus. because of defects in this condition, self antigens are not expressed, resulting in t cells that are not conditioned to tolerate tissues of the body, and may treat them as foreign, stimulating an immune response and resulting in autoimmunity. people with apeced develop an autoimmune disease that affects multiple endocrine tissues, with the commonly affected organs being hypothyroidism of the thyroid gland, addison's disease of the adrenal glands, and candida infection of body surfaces including the inner lining of the mouth and of the nails due to dysfunction of th17 cells, and symptoms often beginning in childhood. many other autoimmune diseases may also occur. treatment is directed at the affected organs. thymoma-associated multiorgan autoimmunity thymoma-associated multiorgan autoimmunity can occur in people with thymoma. in this condition, the t cells developed in the thymus are directed against the tissues of the body. this is because the malignant thymus is incapable of appropriately educating developing thymocytes to eliminate self-reactive t cells. the condition is virtually indistinguishable from graft versus host disease. myasthenia gravis myasthenia gravis is an autoimmune disease most often due to antibodies that block acetylcholine receptors, involved in signalling between nerves and muscles. it is often associated with thymic hyperplasia or thymoma, with antibodies produced probably because of t cells that develop abnormally. myasthenia gravis most often develops between young and middle age, causing easy fatiguing of muscle movements. investigations include demonstrating antibodies (such as against acetylcholine receptors or muscle-specific kinase), and ct scan to detect thymoma or thymectomy. with regard to the thymus, removal of the thymus, called thymectomy may be considered as a treatment, particularly if a thymoma is found. other treatments include increasing the duration of acetylcholine action at nerve synapses by decreasing the rate of breakdown. this is done by acetylcholinesterase inhibitors such as pyridostigmine. cancer