The immune system protects against infectious disease, but it may also at times cause disease.

Disorders of the immune system fall into two broad categories: 

(1) those that arise when some aspect of the host's immune mechanism fails to prevent infection (immune deficiencies) 

(2) those that occur when the immune response is directed at an inappropriate antigen, such as a noninfectious agent in an allergic reaction, the body's own antigens in an autoimmune response, or the cells of a transplanted organ in graft rejection.

Medical and Alternative Medical Terminology, Explanations -  by Courtesy of Encyclopedia Britannica® 2001

My question is here : 

We are protected by our own immune system.

Are we always protected by our own immune system alone? 

No, not always!

Then
What, Which, Who,
protects us...?