CLYDE J. STORMONT 
511 
cattle also has its homologue in those species. 
But the gene or genes determining the A factor 
in such species as domestic sheep appears to be 
at or near fixation, as is true in most instances 
for the gene or genes controlling Forssman 
blood factors. Undoubtedly, many additional 
homologies will be brought to light on further 
comparisons, and all will be useful in construct- 
ing phylogenetic trees based on biochemical and 
antigenic markers. 
SUMMARY 
Cross matching blood before initial transfu- 
sions in cattle, dogs, pigs and sheep is of little 
practical importance because of the rarity of 
transfusion reactions. Where multiple transfu- 
ions are given, severe and sometimes fatal re- 
actions may be expected as a result of isoim- 
mune antibodies engendered by the recipients. 
Transfusion reactions are also expected when 
recipients have been immunized with homolo- 
gous tissue vaccines. 
None of the blood group loci in the species 
under consideration functions as a major histo- 
compatibility locus. Consequently, pairing of 
donors and recipients for allografts on the basis 
of their blood groups serves no worthwhile pur- 
pose. On the other hand, chimeric cattle twins 
should be ideal for such experiments. Not only 
do they share each other's hematopoietic tissues 
and therefore can be cross-transfused at will, 
but they are also tolerant to homograf ts of each 
other's tissues. It is recommended that such 
twins, which are easily diagnosed by blood typ- 
ing or by exchanging skin grafts, be used for 
organ transplantation experiments. 
There are numerous firsts, such as the discov- 
ery of blood group chimerism in dizygotic twins 
and higher zygotic multiples, which have been 
brought to light as a consequence of studies on 
blood groups of such species as cattle and sheep. 
Some of those firsts are outlined here. 
All of these break-throughs have come about 
by the use of hemolytic tests. The importance of 
utilizing hemolytic typing procedures cannot be 
over-emphasized. 
The antigenic factors on red cells which de- 
fine individual differences fall naturally into 
one or another phenotypic system of blood 
groups. The number of recognized systems var- 
ies between species, ranging from 7 in sheep to 
15 in pigs. Cattle, with 12 systems, exhibit the 
most complex systems of all and thereby the 
most phenotypic variability in blood types of 
any species so far studied. 
There are numerous blood group homologies 
between cattle and sheep, as would be expected 
of species that belong to the same family. There 
are also broad-range or heterogenetic homolo- 
gies which include the ABO system of man, the 
A-0 system of pigs, the J-Oc system of cattle, 
the R-0 system of sheep, and the Tr system of 
dogs. 
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2a. Owen, R. D. Immunogenetic consequences of vas- 
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