Metabolism of Immunoglobulins
Half-life (T 1/2) of immunoglobulin is one of the most com-monly
used parameters to assess the catabolic rate of immunoglobulins. The half-life
corresponds to the time elapsed for a reduction to half of a circulating
immunoglobulin concentration after equilibrium has been reached. This is
usu-ally determined by injecting an immunoglobulin labeled with a radioisotope
(131I).
The IgG is the immunoglobulin class with the longest half-life
(average of 21 days), with the exception of IgG3. The IgG3 has a considerably
shorter half-life (average of 7 days) that is nearer to that of IgA (5–6 days)
and IgM (5 days).
The synthesis rate of IgA1 (24 mg/kg/day) is not very different
from that of IgG1 (25 mg/kg/day), but the serum concentra-tion of IgA1 is about
one-third of the IgG1 concentration. This is explained by a fractional turnover
rate three-times greater for IgA1 (24%/day). The highest fractional turnover
rate and shorter half-life are those of IgE (74%/day and 2.4 days,
respec-tively). The lowest synthesis rate is that of IgE (0.002 mg/kg/day,
compared to 20–60 mg/kg/day for IgG).
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