Final answer:
The immunoglobulin class most effective at causing agglutination and binding complement is IgM, due to its pentameric structure and ability to bind multiple antigens effectively.
Step-by-step explanation:
The class of immunoglobulin that is most effective at causing agglutination of cells and binding complement is IgM. IgM is produced as a pentamer, meaning it consists of five immunoglobulin molecules joined together, which allows it to bind to multiple antigens effectively. This structure largely accounts for its high avidity for antigens and its efficiency in activating the complement system, which enhances opsonization and cell lysis. IgM is often the first antibody to appear in response to initial exposure to an antigen.