Systemic lupus erythematosus, often abbreviated as SLE or lupus, is a systemic autoimmune disease (or autoimmune connective tissue disease) in which the body’s immune system mistakenly attacks healthy tissue.ystemic lupus erythematosus, often abbreviated as SLE or lupus, is a systemic autoimmune disease (or autoimmune connective tissue disease) in which the body’s immune system mistakenly attacks healthy tissue.