Radiation Effects on Lymphocytes 139 



atypical lymphocyte, similar to that seen in infectious mononucleosis, was 

 observed in survivors of the atomic bomb attacks. 41 



Stage of Recovery 



Lymphoid Tissue. The fourth stage in the radiation reaction is the 

 stage of recovery. This stage is associated with a gradual increasing number 

 of mitoses in reticular cells and in medium-sized lymphocytes. 11 The mitotic 

 cells were observed not only in the germinal centers of persisting follicles 

 but also diffusely throughout the node. The lymph node appeared histo- 

 logically normal five days after irradiation of rabbits with 400 r. 



The recovery was slower in lymph nodes of rabbits exposed with 600 or 

 (S00 r. In these nodes the lymph follicles had been largely destroyed. Early 

 diffuse regeneration was first observed 5 days after irradiation with gradual 

 repopulation of lymphocytes by mitoses of medium-sized lymphocytes. 

 Nodular regeneration started 21 days after irradiation with production of 

 small, "bare" germinal centers consisting of medium-sized lymphocytes, 

 some in mitosis. The lymph node was reconstituted in 59 days in rabbits ex- 

 posed to 600 r. This may be contrasted to the rapid regeneration in 5 days 

 of lymph nodes of rabbits exposed to 400 r. 



Abdominal irradiation with 400 r in rabbits resulted in lymphoid hyper- 

 plasia in the omentum with the production of many new lymph follicles. 33 

 Overcompensation of mitotic activity was observed in the lymphoid tissues 

 of mice 23 and of chick embryos. 03 In spite of its radiosensitivity, lymphoid 

 tissue recovers rapidly after moderate doses of x-rays and shows a tendency 

 to hyperplasia and overcompensation of mitotic activity. 



In Japanese individuals dying three or more weeks after the atomic- 

 bomb attacks, the spleen had small lymphocytes and occasional mitotic 

 figures in syncytial, spindle-shaped reticulum cells near the central arteri- 

 oles. 31 Early follicles developed with a peripheral "germinal center" similar 

 to the "hematopoietic peri-follicular envelop" seen in the spleen of rats 

 by Krumbhaar.- In lymph nodes, formation of germinal centers was ob- 

 served in only one patient after the sixth week. In seven patients irradiated 

 for cancer of the cervix, the lymph nodes showed a reduced number of 

 follicles three to six months after irradiation. 57 The findings indicate that 

 regeneration of follicles is slow after heavy irradiation. 



Blood Lymphocytes. Recovery of the number of lymphocytes in the 

 lymph of the thoracic duct started two weeks after irradiation of rats with 

 750 r. 45 During the regenerating phase as many as 70 per cent of the white 

 cells were large lymphocytes as compared to 2 to 5 per cent in non- 

 irradiated animals. Many of the large lymphocytes were abnormal and had 

 lobed, eccentric, or multilobed nuclei. The number and the morphology of 



