104 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURESFigure 5.6. Comparison of effects of small versus large amounts of whole-body irradiation
administered to rabbits at various times before or after secondary
immunization with SRBC. Adapted from Taliaferro and Taliaferro.6
results are expressed both in absolute terms (dashed line) and as a percent of
the control response (shaded areas). An augmentation of the response is
clearly evident with low-dose exposures (5-50 rad). This radiation-induced
augmentation is associated with reduced numbers of viable cells (Figure
5.10), suggesting the inhibition or death of a cell responsible for modulating
the response.
In order to determine the cell type involved in low-dose augmentation,
the immune response was divided into its T and B cell components, and
each was irradiated individually and then recombined with the correspond
ing (nonirradiated) cell type. Results from this approach are shown in Fig
ures 5.11 and 5.12.16 Figure 5.11 shows the effect of irradiation of the B cell
component on the in vitro response to SRBCs. The resultant dose-response
curve shows three apparent components:
- equivocal augmentation associated with low (0-20 rad) doses
- marked loss of activity associated with moderate (25-75 rad) doses
- minimal additional loss of activity associated with large (>100 rad) doses
A complementary experiment utilizing irradiated T cells is shown in Figure
5.12. This dose-response curve similarly shows three components but dif
fers from Figure 5.11 as follows: