Immunoglobulins are protein-like substances with antibody activity that are found mainly in human plasma and to a lesser extent in tissue fluids as well as body fluids. Immunoglobulins have two identical heavy chains and two identical light chains, so the light chains are part of the immunoglobulins. Immunoglobulin light chains, also known as periplasmic proteins, are not present in normal urine because of their small molecular weight and can pass through the glomerular filtration membrane but are reabsorbed in the renal tubules. Positive urine periplasmic protein is most often seen in diseases such as multiple myeloma, lymphoma, and chronic nephritis.