Measurement in a specified sample of lymph node tissue of the amount of corpuscular or electromagnetic radiations emitted consequent to nuclear disintegration of a radioactive isotope which is replacing a stable chemical element in a compound that has been introduced into lymphocytes (small agranulocytic leukocytes that originate from fetal stem cells and develop in the bone marrow) rendering them able to be followed or tracked through the body. Lymph nodes are any of the accumulations of lymphoid tissue organized as definite lymphoid organs along the course of lymphatic vessels, consisting of an outer cortical and an inner medullary part; they are the main source of lymphocytes of the peripheral blood and, as part of the reticuloendothelial system, serve as a defense mechanism by removing noxious agents, e.g., bacteria and toxins, and probably play a role in antibody formation.