Many mechanisms can explain the mode of action of IVIg in immune disorders. Macrophage blockade and interference in the idiotypic network are supported by some experimental data. Among the other mechanisms, two are considered in greater detail. Firstly, in some disorders, the patients could improve simply because the infused Ig contains antibodies directed against the infectious antigen causing the disease. Secondly, one can expect that IVIg increases the IgG catabolism and therefore the elimination of the autoantibodies. When the concentration of IgG in the plasma reaches 200% of the normal value, for example, the fractional catabolic rate increases up to 180% of its normal value. In other words, the half-life of IgG is decreased from 21 days to 12 days.