正常血清水平 — IgA是循环中数量第二多的抗体亚型，仅次于IgG。出生时体内一般不存在IgA，出生后第1年水平逐渐增加，1岁时大约达到成人水平的30%，到青春期时可达到成人水平。
Although IgG is the clearly predominant isotype in the blood, dimeric IgA is the dominant immunoglobulin isotype in the mucosal secretions as well as in breast milk and colostrum.
In the blood, 10% to 15% of the Ig is IgA (vs 65%-75% IgG). Moreover, IgA has a shorter half-life than IgG in serum.
[1]IgA1 is the predominant (>80%) IgA subclass in the serum;
IgA2 is the major form in some human mucosal secretions
Shorter hinge region: IgA2--increased resistance to bacterial proteases
extended hinge region: IgA1--permit molecules of this isotype to accommodate variable epitope spacings on multivalent antigens. 允许这种同种型的分子适应多价抗原上可变的表位间距
IgA does not activate the classical complement pathway. Deposition of polymeric IgA1 in IgA nephropathy can, however, trigger the alternative and lectin complement pathways. Only the latter appears to be dependent on mannose-binding lectin.
The two α H chain C region genes correspond to two IgA subclasses, IgA1 and IgA2. IgA1 is the predominant (>80%) IgA subclass in the serum. Although IgA2 is the major form in some human mucosal secretions, such as those in the large intestine and the female genital tract, there is variation in the relative proportion of IgA1 and IgA2 in different secretions. The shorter hinge region of IgA2 confers increased resistance to bacterial proteases that might be encountered in the mucosal environment. The extended hinge region of IgA1 is believed to permit molecules of this isotype to accommodate variable epitope spacings on multivalent antigens. Although the long hinge region of IgA1 molecules might be expected to confer relatively high susceptibility to proteolysis (Fig. 21.14), relative protection against the activity of bacterial proteases is provided by heavy O-linked glycosylation in the hinge. Nevertheless, the IgA1 hinge is uniquely susceptible to IgA proteases produced by certain pathogenic bacteria.↩︎