The immune system can be activated by many “non-self” substances or antigens.3 For example, the proteins on the surfaces of bacteria, fungi and viruses are all antigens, and when the antigens bind to special receptors on the defense cells, a series of cell processes is started. Then the immune system can recall stored “memories” in order to more quickly be ready to defend against known pathogens.3
The body’s own cells also have surface proteins. However, the immune system does not attack them, because it has already learned at an earlier stage to identify specifically these cell proteins as “self.”3