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• Immunology Terms

- H -

• H-2 complex

The major histocompatibility complex situated on chromosome 17 of the mouse; contains subregions K, I and D.



• Haplotype

A particular combination of closely linked genes on a chromosome inherited from one patient.



• Hapten

A compound, usually of low molecular weight, that is not itself immunogenic but that, after conjugation to a carrier protein or cells, becomes immunogenic and induces antibody, which can bind the hapten alone in the absence of carrier.



• Heavy chain (H chain)

The larger of the two types of chains that comprise a normal immunoglobulin or antibody molecule.



• Helper T cells

A class of T cells which help trigger B cells to make antibody against thymus-dependent antigens. Helper T cells also help generate cytotoxic T cells.



• Heterophile antigen

A cross-reacting antigen that appears in widely ranging species such as humans and bacteria.



• Hinge region

A flexible, open segment of an antibody molecule that allows bending of the molecule. The hinge region is located between Fab and Fc and is susceptible to enzymatic cleavage.



• Histocompatibility

Literally, the ability of tissues to get along; in immunology, it means identity in all transplantation antigens. These antigens, in turn, are collectively referred to as histocompatibility antigens.



• HLA complex

See 'Major histocompatibility complex'.



• Humoral immunity

Any immune reaction that can be transferred with immune serum is termed humoral immunity (as opposed to cell-mediated immunity). In general, this term refers to resistance that results from the presence of specific antibody.



• Hybridoma

A hybrid cell that results from the fusion of an antibody-secreting cell with a malignant cell; the progeny secrete antibody without stimulation and proliferate continuously both in vivo and in vitro.



• Hypersensitivity

State of reactivity to antigen that is greater than normal for the antigenic challenge; hypersensitivity is the same as allergy and denotes a deleterious outcome rather than a protective one.



• Hypervariable regions

Portions of the light and heavy immunoglobulin chains that are highly variable in amino acid sequence from one immunoglobulin molecule to another, and that, together, constitute the antigen-binding site of an antibody molecule. Also, portions of the T-cell receptor which constitute the antigen-binding site.