Leukopenia, which occurs when autoimmune diseases occur, is called autoimmune leukopenia. This is a condition in which the number of white blood cells in the blood is significantly reduced. This usually occurs as a result of a malfunction of the immune system. Autoimmune diseases can lead to leukopenia, as immune cells attack their own organs and tissues, mistaking them for foreign ones. Among such diseases are diabetes mellitus or Sjögren's syndrome, in which the level of white blood cells decreases. Leukopenia can also be caused by rheumatoid arthritis, Hashimoto's thyroiditis, Ghislaine Loeffler's syndrome and many other autoimmune diseases.