The bone marrow is a highly radiosensitive tissue and is of particular important when total body irradiation is delivered.
Mechanism of Immune Suppression
All blood cells arise from a population of immature stem cells within the bone marrow. These cells may divide to produce an ongoing stem cell and a progenitor cell. The progenitor cell undergoes many divisions as it progresses along a determined path (eg. erythrocytes, lymphocytes etc).
Radiation prevents the activation of or kills the immature stem cells. The progenitor cells continue to divide into their various descendents. Without replacement of the progenitor cells by the stem cells, the number of circulating blood cells will drop after a period related to their half life.
Myelocytes (neutrophils, eosinophils, basophils) are one line of immune cells affected by radiation. Neutrophils have a half life of approximately 8 days. Following radiation, there is a surge in neutrophil numbers before they fall to low levels within 2 – 4 days. The lag in neutropenia is due to minimal effect of radiation on the already dividing progenitor cells.
Lymphocytes are a special case. Unlike other terminally differentiated cells, they are exquisitely sensitive to radiation and may undergo apoptotic death after exposure to small amounts of radiation.