EFFECTS ON IMMUNE RESPONSE 101Figure 5.2. Cumulative number of viable T cells mobilizable by thoracic duct cannulation
as a function of radiation dose and time after exposure. Data derived from
Table 5.3. Adapted from Anderson and Williams.1746 hr after exposure. After the initial 4 hr, a dose-dependent decrement in
the number of thoracic duct lymphocytes (TDLs) is apparent. As shown in
Figure 5.2, this decrease is primarily related to a loss of recirculating T
cells.
Recovery of lymphoreticular tissues after irradiation is much more com
plex than might be anticipated. In part, this complexity relates to the obser
vation that the restoration of some cell types requires the expansion of a
precursor pool anatomically removed from the tissue of interest. In addi
tion, the lymphoreticular tissues themselves have a hierarchy of recovery,
and regeneration of so-called “peripheral” organs, such as the spleen and
lymph nodes, is in part dependent upon recovery of the “central” thymus.
Some of these interrelationships are illustrated in Figures 5.3 and 5.4.18
Although the immunosuppressive effects of ionizing radiation have been
recognized for some time, more recently it has been shown that such expo
sures may also occasion an augmented response.19 This immunostimulatory
effect is dependent particularly on the dose of radiation and the time
between exposure and the introduction of antigen. Much of the early experi
mental work concerned with this phenomenon was performed by Dixon and
Taliaferro and their respective co-workers.5’620-22 Figures 5.5 and 5.6, from
Taliaferro et al.,5*6 show the effects of small versus large amounts of whole