When does immunity harm the body?

Immunity acts to the detriment of our body when the pathogen has a very strong resemblance to the cells of the body. During the disease, not only the cells of the pathogen are destroyed, but also its own cells and tissues. Example: streptococci, which are the causative agents of sore throat, are very similar in structure of membrane proteins to the muscle cells of our heart, therefore, after sore throat, there may be a complication in the heart.
In other words: a malfunction of the immune system can lead to autoimmune diseases, during which the body accepts its own cells as foreign and damages them.