The Real Definition of Hypersensitivity

I share this in the spirit of making your life easier for the rest of your medical/scientific education/career, because sometimes teachers complicate life by giving out wrong definitions here and there.

Wrong Defintion:

Our teachers' definitions of hypersensitivity lie along the lines of:

"Hypersensitivity is the aberrant reaction of the immune system to an innocuous antigen." 

This is the definition of allergies. Surprisingly (not), this is not the definition of hypersensitivities. Allergies and hypersensitivities are NOT synonymous. Allergies are only one type of hypersensitivity. 

Building on this, the entire 3 years of my medical education I have been struggling with hypersensitivities because they do not match up with this definition. 

Until yesterday. I searched up the definition in a book I recently acquired (and wished to God I'd found it 2 years ago).

Definition of hypersensitivity:

"Hypersensitivity is a process of reactions of antigen with antibodies or sensitized lymphocytes that are harmful to the host. Hypersensitivity refers to processes in which the immune response itself is primarily responsible for the induction and/or exacerbation of disease." (Dunn and Hawley, 2002)

In the newer edition of Kaplan's Immunology and Microbiology Notes:
"Hypersensitivity diseases are conditions in which tissue damage is caused by immune responses. They may result from uncontrolled or excessive responses against foreign antigens or from a failure of self-tolerance, in which case they are called autoimmune diseases." (Hawley and Ruebush, 2009)

Reference:

Dunn, D., Hawley, L. (2002). Failures of the ImmuneSystem: Hypersensitivity. USMLE Step 1: Immunology and Microbiology Notes. (pp. 401-418). Kaplan: Medical

Hawley, L., Ruebush, M. (2009). Diseases Caused by Immune Responses: Hypersensitivity and Autoimmunity. USMLE Step 1: Immunology and Microbiology Notes. (pp. 149-168). Kaplan: Medical