Hormone
One of many types of circulating chemical signals in all multicellular organisms that are
formed in specialized cells, travel in body fluids, and coordinate the various parts of the
organism by interacting with target cells.
Host
An organism on or in which a parasite lives.
Human
The modern species of humans, the only extant species of the primate family Hominidae.
Human Genome Project
An international collaborative effort to map and sequence the DNA of the entire human
genome.
Humerus
The long bone of the arm or forelimb, extending from the shoulder to the elbow.
Humoral immunity
The type of immunity that fights bacteria and viruses in body fluids with antibodies that
circulate in blood plasma and lymph, fluids formerly called humors.
Hummingbird
Any of numerous New World birds of the family Trochilidae, usually very small in size and
having brilliant iridescent plumage, a long slender bill, and wings capable of beating very
rapidly, thereby enabling the bird to hover.
Hybrid
Offspring of two different varieties or of two different species.
Hybrid zone
A region where two related populations that diverged after becoming geographically
isolated make secondary contact and interbreed where their geographical ranges overlap.
Hydrocarbon
An organic molecule consisting only of carbon and hydrogen.
Hydrogen bond
A type of weak chemical bond formed when the slightly positive hydrogen atom of a polar
covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent
bond in another molecule.
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