Because the plasma cells are clonal, each one secretes the same, identical immunoglobulin, called an M-component when detectable amounts accumulate in the serum. In some cases the kidneys are so ravaged that the intact immunoglobulin is no longer retained by the glomerulus, and an M-protein spills into the urine. Each M-protein has exclusively either kappa or lambda light chains and is most frequently IgG, less often IgA, and rarely another isotype. Only rare MMs fail to secrete detectable amounts of immunoprotein. The latter 2 modalities are more sensitive, difficult and expensive and may be necessary to detect a small M-component.
Some MMs secrete only light chains.
Unlike intact immunoglobulin, these are freely filtered by the kidney and not resorbed. Thus clonal light chains do not usually accumulate in the serum but may be detected in the urine. Some MMs secrete light chains in excess of heavy chains, so that both a serum M-component and urinary light chains are present. It deposits in various locations, stiffening, displacing, and harming the normal tissue. It may be present in other, non-neoplastic diseases too.