IgG is said to be bivalent because it has 2 Fab fragments. IgA occurs as a monomer, dimer, and trimer. In secretion, secretory IgA has a J-chain which holds the two IgA molecules together. It also has a secretory piece which protects the IgA molecule from degradation. IgM is a cyclic pentamer which is multivalent, and has 5 J-chains to hold it together. (Figure 2-9).
A comparison of the three immunoglobulin classes involved in blood banking is shown in Table 2-2. In terms of the relative concentrations in human serum, from most to least, the order is IgG > IgA > IgM > IgD > IgE or "GAMDE".
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