Final answer:
IgM is the most efficient immunoglobulin isotype at initiating a complement fixation cascade due to its pentameric structure, enabling it to bind multiple antigens and robustly activate complement.
Step-by-step explanation:
The immunoglobulin isotype most efficient at initiating a complement fixation cascade is IgM. IgM is produced initially as a monomeric form on the surface of B cells and upon secretion transforms into a pentamer linked by a J chain. This pentameric structure of IgM allows it to bind ten antigens simultaneously, providing a robust mechanism for complement activation. Although IgG can activate complement, it does so less effectively than IgM. IgG is mainly involved in late primary and secondary immune responses, is monomeric, and has the ability to cross the placenta to provide immunity to the developing fetus.