The immune system protects the body against infection and disease. It is a complex and integrated system of cells, tissues, and organs that have specialized roles in defending against foreign substances and pathogenic micro-organisms, including bacteria, viruses, and fungi. The immune system also functions to guard against the development of cancer. For these actions, the immune system must recognize foreign invaders as well as abnormal cells and distinguish them from self. However, the immune system is a double-edged sword in that host tissues can be damaged in the process of combating and destroying invading pathogens.
Autoimmune diseases, of which HIV and AIDS are part, are related to chronically suppressed immune systems.