Jardin et al.  characterised serum IgM, IgG and IgA HLC in DLBCL. An abnormal IgMκ/IgMλ, IgGκ/IgGλ or IgAκ/IgAλ HLC ratio was observed in the sera of 19.1%, 6.4% and 2.9% of patients, respectively . This is consistent with the relative frequencies of B-cell receptor isotype expression by DLBCL cells . The most frequent HLC abnormalities were reduced serum concentrations, observed in 11.2% (IgMκ) to 21% (IgGκ) of patients. There was little overlap between patients with sFLC and IgM HLC abnormalities (Figure 31.4 ).
Patients with an abnormal IgMκ/IgMλ HLC ratio had an increase in Cμ mRNA expression and a higher rate of IgM positive cases by immunohistochemistry compared with patients with normal HLC ratios . Therefore, in the absence of any coexistent monoclonal gammopathies (e.g. monoclonal gammopathy of undetermined significance, Chapter 13), the source of a monoclonal protein in DLBCL is likely to be the tumour.