892 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
000 
~T 1 1 1 1 1 r 
SMALL LYMPHOCYTES 
-oTD LYMPHOCYTES / KILO/ HR 
-• PB LYMPHOCYTES / KILO 
AVERAGE OF NINE CALVES 
0 
8 
10 12 14 
DAYS OF ECIL 
24 
Figure 5. — The effect of continuous ECIL on small lymphocytes in the peripheral blood (PB) (• •) and tho- 
racic duct (TD) lymph (O O). The decrease in thoracic duct output is represented by two components: the 
first with a halving time of 1.2 days; the second with a halving time of 29.5 days. (Reprinted from Cronkite 
et al., 1968.) 
nodes as a function of time after onset of con- 
tinuous ECIB is shown in Figure 6. As with 
blood and lymph, depletion of lymphoreticular 
organs follows an exponential with two compo- 
nents. Best estimates of the relative size of the 
easily mobilizable pool are : lymph node medulla 
< 10%, lymph node cortex 18%, splenic red 
pulp 37%, splenic dense white pulp 55%, and 
splenic loose white pulp 60%. Germinal centers 
remain intact. 
Recovery from lymphocyte depletion induced 
by ECIB or ECIL is prolonged.^ Depending 
upon the degree of depletion, blood lymphocyte 
counts may remain below pre-irradiation levels 
for at least 6 months. This is mainly due to a 
lag in recovery of small lymphocytes as large 
lymphocyte levels return to normal in about 3 
weeks. 
Collectively, these studies indicate that ECIB 
and ECIL are effective in depleting the body of 
an easily mobilizable pool of lymphocytes which 
circulate from blood to lymphoreticular organs 
and back again. This pool is composed prin- 
cipally of small, non-dividing, long-lived lympho- 
cytes which following depletion are replaced 
very slowly. Thymectomy and lymphocyte label- 
ing experiments in rodents suggest that the 
recirculating pool of lymphocytes is principally 
