Lupus is an autoimmune disease where the body’s immune system becomes hyperactive and attacks normal, healthy tissue. This results in symptoms such as inflammation, swelling, and damage to joints, skin, kidneys, blood, the heart, and lungs.

Under normal function, the immune system makes proteins called antibodies in order to protect and fight against antigens such as viruses and bacteria. Lupus makes the immune system unable to differentiate between antigens and healthy tissue. This leads the immune system to direct antibodies against the healthy tissue – not just antigens – causing swelling, pain, and tissue damage.
(* An antigen is a substance capable of inducing a specific immune response.)

(information from MedicalNewsToday.com)