The malignancy, in this case, is of differentiated B lymphocytes, known as plasma cells. Because it involves an overgrowth of plasma cells specific to one antigen, only one antibody (immunoglobulin) type is secreted. The overabundance of this immunoglobulin is detected as the "M Protein" or monoclonal protein. It is diagnostic of plasma cell dyscrasias such as Multiple Myeloma and occasionally other malignancies such as lymphomas and lymphocytic leukemias.
Immunoglobulin proteins, including just the kappa or lambda light chains, can also be excreted in the urine. The light chains in the urine are sometimes called "Bence Jones Protein." The light chains can often be deposited in distal tubules of the kidney, leading to kidney disease. Hypercalcemia, which can commonly occur in this disease (due to increased bone resorption), can also contribute to kidney disease.
Other pathological findings include bone lesions caused by expanding plasma cell tumors, known as plasmacytomas. These plasmacytomas can cause diffuse osteoporosis or discrete lesions. Characteristic "punched-out lesions" can be seen on x-rays.
Anemia is found in many patients and is usually after renal disease suppressing erythropoietin secretion. Sometimes the anemia is secondary to other related problems.
Bacterial infections are common due to a lack of normal immune function; viral infections are often seen, although they are commonly a result of the treatment.
