haplotype
A set of genetic determinants located on a single chromosome.

helper T lymphocyte (Th)
A subclass of T-cells which provide help (in the form of cytokines and/or cognate interactions) necessary for the expression of effector function by other cells in the immune system

hematology
The study of the diagnosis, pathology, and treatment of diseases of the blood, blood-forming tissues and organs.

hemolysis
Lysis of erythrocytes with the release of hemoglobin.

heparin
A substance that interferes with blood clotting.

histamine
Vasoactive amine present in basophil and mast cell granules which, following degranulation, causes increased vascular permeability and smooth muscle contraction.

hydralazine
A drug that dilates blood vessels.