Allergies:

Hapten: small molecule which can not generate an immunological reaction by itself, but which acquires this property when it is bound to a so-called carrier protein.

Hemagglutination: A method for detection of antigen-antibodies. The antigens are bound to red blood cells and demonstration of antigens with bivalent antibodies induces agglutination.

Herbaceous Plants: group of plant families not belonging either to the Graminaceae nor to trees and whose stem is soft, generally green and little resistant.  

Heredity: transmission of genetic characteristics by the parents to their offspring.

Histamine: the compound which comes from the conversion of the amino acid (a component of proteins) hisdidine. It plays the role of a chemical messenger in several mechanisms, including the triggering of an allergic reaction. It is the first mediator of allergy to be demonstrated. Research on histamine led to the discovery of antihistaminic drugs

HLA (system - Human Leukocyte   Antigens): synonym for Human Major Histocompatibility Complex (MHC).

Hymenoptera: order of insects characterized by four transparent membranous wings, for example bees, wasps, ants.

Hyperreactivity (nasal or bronchial): increased and excessive nasal or bronchial reaction against inhaled non specific provocating agents (e.g. tobacco, temperature variation, etc.). 

Hyposensitization: synonym for "de-sensitization".

Hyposmia : partial loss of smell