[Isotypic characterization of IgA paraproteins: association with other clinical and analytic data]

A study was conducted to investigate the relationships between the characteristics of IgA paraprotein and the clinical findings in a group of 196 patients. In 19 cases IgA paraprotein was associated with another monoclonal immunoglobulin (IgG or IgM); the other 177 patients had a single IgA paraprotein; 145 of them corresponded to multiple myeloma (MM) and the other 32 to other diagnostics. Class and type of paraproteins were identified by immunoelectrophoresis and subclass by an enzyme-immunoassay specifically developed for this study. The degree of polymerization of the protein was determined by gel filtration; quantitation of monoclonal IgA and polyclonal IgG and IgM was obtained by kinetic nephelometry. Out of 196 paraproteins, 96.4% were classified in IgA1 subclass and only 3.6% in IgG2. In 14 cases, all of them diagnosed with MM, monoclonal IgA in serum was associated with Bence-Jones protein; in more than 78% of them light chains corresponded to type lambda, whereas type kappa predominated (over 60%) in cases without Bence-Jones protein in serum. Significantly higher serum levels of monoclonal IgA were associated with the diagnosis of myeloma, with type kappa paraproteins, and with the presence of Bence-Jones protein in serum. The cases with two paraproteins (IgA and IgG or IgM) had significantly lower serum levels of IgA, with comparable levels of total paraprotein (the addition of both monoclonal immunoglobulins). Serum levels of polyclonal IgG and IgM, which appeared decreased in cases of MM, were normal in cases with other conditions. In all these cases, monoclonal IgA showed a monomeric character, whereas relevant amounts of polymerized IgA paraprotein was found in almost a third part of myeloma cases, particularly in those with higher serum levels of paraprotein, or when paraprotein belonged to type kappa. The 5 IgA2 paraproteins analyzed had a polymeric character. In conclusion, a detailed, both qualitative and quantitative, analysis of IgA paraproteins can lead to a better knowledge of conditions associated with their presence and at the same time provides useful data for a clinical evaluation of patients.