Terminology

1. Immunogenicity (antigenicity): the ability of an antigen to elicit an immune response. Some antigens (such as D and K) are much more immunogenic than others. An important factor affecting immunogenicity is antigen molecular weight. The lower limit for an antigen is a molecular weight of approximately 5,000 daltons. To be a good antigen, the molecular weight should be at least 40,000 daltons. (One dalton = 1.66 x 10^-24 g.)

2. Antigenic determinant or site (epitope): a 3-dimensional chemical configuration on an antigen to which antibody will react. Each antigen may contain several antigenic sites. Note: the part of the antigen to which antibody does not react is called the carrier region. Its purpose is to help the antigen keep its 3-D shape.

3. Hapten: a small molecule which, by itself, cannot stimulate the production of antibody but which can combine with antibody.

Terminology