The structural resemblance between molecules of an antigen and molecules of the corresponding antibody may be visualized as analogous to the relationship between a mould and its replica or casting. Each is specific for the other. If the antigen molecule is small and contains only one combining site, then only one antibody molecule can combine with each such antigen molecule (though 2 antigen molecules may combine with each antibody molecule since antibody molecules are typically bivalent).
Such antigen-antibody combinations are small and soluble. The more complex the form of the antigen molecule, the more recognition sites there are for specific combination with antibody,and the larger, more stable and insoluble the combination is.