My 5 year old had a canker sore. however it disappeared on its own without any treatment at all in a week . did his immune system play a part in healing it ?

Very likely yes but … since it is unknown what causes canker sores, its also unknown what part of the immune system makes it disappear. Canker sores are probably caused by a viral infection, but a specific virus has never been identified.

The human body does have a remarkable ability to heal itself, and the immune system often plays a role. Some believe that canker sores are caused by an over active, or auto-immune response, although admittedly our knowledge of the exact cause is somewhat limited. Canker sores can sometimes be confused with cold sores which are due to a form of herpes virus, but they are different.

Most dental lesions, and there are textbooks full of different varieties, heal in 2 weeks or less. Rule of thumb is that any lesion that lasts longer than 2 weeks should be examined by a Dentist. As your sons lesion only lasted a week, don't worry.