An antibody is an immunoglobulin, a specialized immune protein, produced as a response to the introduction of an antigen into the body. An antibody possesses the ability to combine with the antigen that triggered its production. The production of antibodies is a major function of the immune system and is carried out by a type of white blood cell called a B cell (B lymphocyte). Antibodies can be triggered by and directed at foreign proteins, microorganisms, or toxins which serve as antigens. Some antibodies are auto-antibodies and attack the host’s own tissues producing autoimmune disease, for example lupus erythematosis.