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thymocytes and other lymphoid tissues does, however, raise a problem in
that similar studies with goldfish membrane Ig's have revealed presumed
size differences in H chains derived from thymocytes and splenocytes
(116). Another somewhat disconcerting aspect of the bluegill results
reported here was the observation that large amounts of extensively
reduced immunoprecipitated radioactivity failed to penetrate the SDS
gels. This raises the question of whether the high molecular weight
material actually had antigenic determinants in common with serum immu
noglobulin or alternatively was a ntag-along" portion of incompletely
solubilized membrane. Certainly the finding that gel filtration (in
the absence of SDS) yielded immunoprecipitable material only in >
180,000 molecular weight fractions suggests difficulty in totally
solubilizing the bream membrane Ig with nonionic detergents. It would
therefore seem appropriate here to take a relatively conservative view
point in the sense that these studies do not, in fact, prove which of the
components demonstrable on SDS gels actually contained the Ig antigenic
determinants. Future approaches aimed at assessing the antigenicity
of SDS solubilized membrane components may resolve this issue.
Finally, in light of the as yet unresolved issue of membrane recep
tors on mammalian T-cells, it would seem appropriate to comment further
on the results with fish thymocytes. As discussed in Chapter II, in
vivo studies indicate that fish have cellular immune functions which,
by analogy, can be called T-like and B-like. Furthermore, In vitro
studies with trout (46) and bluegill (see Chapter II) seem to leave
little doubt that fish have a heterogeneity of lymphocytes much akin to
that seen in mammals. Thus since bluegill thymuses (as well as other
lymphoid tissues) appear to contain T-like cells and since at least 90%