In autoimmune diseases, what is the primary reason the immune system attacks the body's own cells?

The immune system normally learns to ignore the body’s own cells, a process called immune tolerance. In autoimmune diseases this tolerance breaks down, so the immune system mistakenly labels healthy cells as threats. This can happen because of genetic factors, infections that mimic body proteins, or errors in cell signaling. When the immune system starts attacking, it releases antibodies or T‑cells that attack those self‑targets, like in type 1 diabetes where insulin‑producing cells are destroyed. Thus, the main reason is a failure of self‑tolerance that lets the immune system attack its own tissues.