Affinity: the binding strength between the binding site on an anitbody and an antigen.

Antibody: a protein, also known as an immunoglobulin, consisting of two heavy chains and two identical light chains, that binds to an antigen.  They are produced by white blood cells in response to foreign antigens within the body.

Antigen: any foreign substance that has entered the body and that binds specifically to an antibody or to a T-cell receptor.

Complementarity determining region:  one of three regions within the variable domain of each chain in antibodies that exhibits the most sequence variability and contributes the most to the antigen-binding site.

Epitope: The site on an antigen that is recognized and bound by a particular antibody or T-cell receptor.

Fab Fragments: an antigen-binding fragment of an immunoglobulin that consists of both light chains and part of one heavy chain.

Germ Line: the unmodified genetic material that is transmitted from one generation to the next through gametes.

Heavy chain:  the larger polypeptide of an antibody molecule composed of one variable domain (VH) and three or four constant domains (CH1, CH2, etc.).

Hapten: small organic compounds that are coupled to larger protiens called carriers.  The resulting hapten-carrier conjugate functions as an antigen.

Immunoglobulin: see antibody.

Light chain: the smaller polypeptide of an antibody molecule composed of one variable domain (VL) and one constant domain (CL).

Pathogen: any disease causing organism.

Somatic mutation: a change in genetic information occuring in one of the cells in the body that is not destined to become a gamete.  These mutations are not passed from one generation of organisms to the next.