According to the Mayo Clinic's website, lupus is defined as a "chronic inflammatory disease" where one's own immune system attacks his or her organs and tissues. Given that lupus weakens the immune system, people who have lupus are more likely to get infections. Given the weakened state of the immune system, people with lupus are more likely to be susceptible to kidney damage and failure, anemia, pleurisy, cardiovascular disease, heart attacks, cancer, and bone tissue death.
The immune system of a person with lupus does not attack "invading organisms" (like bacteria). Instead, lupus turns the immune system against the "host," and the immune system invades and attacks the tissues and organs.