Final answer:
IgG is the antibody isotype of the highest concentration in the blood that activates complement, antibody-dependent cell cytotoxicity, and opsonization.
Step-by-step explanation:
The antibody isotype of the highest concentration in the blood that activates complement, antibody-dependent cell cytotoxicity, and opsonization is IgG. IgG is a major antibody of late primary responses and the main antibody of secondary responses in the blood. It can bind to cellular antigens, leading to complement activation and cell lysis. IgG can also mediate ADCC with cytotoxic T cells, natural killer cells, macrophages, and neutrophils.