Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves. When someone has lupus, the immune system can’t tell the difference between the body’s healthy cells and bacteria and viruses, so the antibodies attack the body’s healthy cells.