Final answer:
Antibodies have unique molecular structures with variable and constant domains, which allow them to recognize and bind to specific antigens. This variability in antibody structure makes it challenging to determine the complete structure of all possible antibody molecules.
Step-by-step explanation:
The reason there are so few full-length antibody structures is because antibodies have unique molecular structures designed to match with only one type of antigen. The regions on the antibody that recognize and bind to antigens are variable domains, while the base of the antibody is composed of constant domains. These variable domains allow antibodies to recognize a wide range of antigens, resulting in a large diversity of antibody molecules. However, this variability makes it difficult to determine the complete structure of all possible antibody molecules.