Which region of the Ig molecule interacts with the antigen? What makes the region suited for this purpose?

Answer and Explanation:

Immunoglobulins (Ig) are proteins that are composed of 4 polypeptidic chains: 2 heavy chains (H) and two light chains (L). They assemble in a structure with a 'Y' form. This structure has two regions: the Fab region and the Fc region.

The region of Ig which interacts with the antigen is the Fab region. That is possible because the Fab region presents variability in the amino acid sequence which determines the specific complementary shape with the antigen structure (like a lock and the key).