IgG is the main isotype in the extracellular fluid and blood. It opsonizes pathogens for consumption by phagocytes and activates the complement system. IgG antibodies that are high-affinity are able to neutralize bacterial toxins (they bind to the receptor-binding site on the toxin molecule that stops the toxin from binding to the cell. Since these antibodies are able to scatter into tissue and entangle the toxins quickly, they are considered to be the principal neutralizing antibodies.