Final answer:
The statement that does not apply to IgG is that it a) appears early in the primary immune response; this role is filled by IgM. IgG comes into play later in the immune response, particularly during secondary responses, and possesses many crucial functions, including the ability to cross the placenta.
Step-by-step explanation:
The correct answer to the question is A: Appears early in the primary immune response. Immunoglobulin G (IgG) does not appear early in the primary immune response; rather, IgM is the antibody that appears first. IgG is the major antibody of late primary responses and is the main antibody of secondary responses. The process known as class switching enables IgM to change to IgG, IgA, or IgE during a primary immune response, but IgG itself is not the initial antibody formed.
As for the other statements, IgG does neutralize bacterial toxins, can fix complement (though not as efficiently as IgM), crosses the human placenta to protect the fetus, and it can opsonize bacteria, making other immune cells more capable of attacking them. These functions make IgG a versatile and critical component of the adaptive immune system, particularly in subsequent encounters with familiar pathogens. The answer is option a.