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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.
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