d. The determination of the amino acid sequence, of both light and heavy
immunglobulin polypeptide chains, show that these chains can be divided into two
domains. A light chain has two such domains, one variable (VL) and one constant (CL).
An lgG heavy chain has one variable (VH), and three constant (CH1, CH2, CH3)
domains. The variable domains confer specificity on the antibody molecule, and two of
them; one from the light and one from the heavy chain, form the binding site for antigen
(see figure 1-3). The constant region of both heavy and light chains imparts the class
and subclass characteristics to the immunoglobulin molecule.
e. Papain cleavage of the molecule yields three fragments. Two, composed of
part H chain and part L chain (Fab fragments), retain the antigen-binding properties of
the whole antibody molecule, whereas, a third, the Fc fragment, is concerned with a
variety of general biologic functions such as complement fixation, placental transfer, and
skin sensitization (see figure. 1-2).
f. On the basis of antigenic and functional differences, each immununoglobulin
class can be shown to consist of subclasses. In the case of lgG, four such subclasses
are recognized: lgG1, lgG2, lgG3, and lgG4. The various immunoglobulin classes,
although primarily identified by differences In the amino acid composition of the
constant region of their heavy chains, demonstrate differences in a matter of other
properties such as: physico-chemical properties (electrophoresis, sedimentation,
carbohydrate composition); antigenic properties; biologic properties (skin-sensitizing
properties, ability to cross the placenta, complement fixation, and so forth); and
serologic behavior. The types of immununoglobulins and their properties are
summarized in Table 1-1.