Development of the Immune System 
sity), and J (joining) genes. During development, 
the avian V|Sl genes are rearranged before the 
V/32 genes, and T cells using the V/31 gene family 
preferentially migrate to the intestines. Notably 
also, the relative frequencies of alloreactive 
TCR2 and TCR3 cells var\^ according to the MHC 
class II gene combination in a graft-versus-host 
assay. 
Studies of T cell development in the South Mri- 
can frog Xenopus laevis are also beginning to re- 
veal interesting information. Monoclonal antibod- 
ies have been made that identify 76 and a/3 TCR 
candidates expressed as dimeric units by separate 
lymphocyte subpopulations in the frog thymus, 
spleen, and intestine. The frog T cells that bear 
the putative 76 and a/3 TCR homologues exhibit 
specialized tissue localization patterns virtually 
identical to those defined for their avian and 
mammalian counterparts. Present eflfons are di- 
rected toward identification of the frog TCR 
genes and exploitation of the advantages of this 
amphibian developmental model. 
B Cell Development 
B cell precursors are identifiable in the bone 
marrov^ as lymphocytes that contain hea\y 
chains in their cytoplasm. Lacking conventional 
light chains that are needed for surface expres- 
sion of immunoglobulins, pre-B cells have been 
considered "blind" to environmental antigens. 
This \iew has been challenged by the recent dis- 
covery of genes called X5 and V pre-B that encode 
a surrogate light-chain complex apparently ex- 
pressed together with /Lt-chains on leukemic pre- 
B cells. We have identified a subpopulation of 
normal pre-B cells that express the surrogate 
light-chain/ju-chain receptor complex on their 
surface. The data imply that pre-B cells are vul- 
nerable to clonal selection. Current experiments 
are directed toward identification of the natural 
ligands for these unusual receptors and character- 
ization of the cellular response. 
More than half of the antibodies that we pro- 
duce daily are of IgA isotype, and many of these 
are transported across mucosal surfaces of the 
body for protective immunity. We have identified 
a receptor for IgA antibodies on phagocytic white 
blood cells and have characterized its biochemi- 
cal nature. Monoclonal antibodies made against 
these IgA receptors are being used to define their 
precise tissue distribution and for functional stud- 
ies. These studies focus attention on IgA anti- 
body-mediated phagocytosis as a potentially im- 
portant defense mechanism. 
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