In their early development and before their antibodyforming\ud mechanism is fully established the young of mammals\ud depend entirely on the maternal supply of antibodies.\ud The passive transfer of immunity acquired by the young\ud from its mother can occur before birth, after birth, or\ud both. In man, rabbit and guinea-pig, for example, the\ud transfer of immunity from mother to young occurs entirely\ud prenatally. In the ungulate, horse and pig the passive\ud transfer of immunity occurs postnatally, whilst in dog,\ud rat and mouse it occurs both pre- and postnatally.\ud \ud The difference between these groups in the mechanism\ud of antibody transfer from mother to young was earlier\ud thought to be due to the difference in the number of\ud layers intervening between the maternal and the foetal\ud circulations (Kuttner and Ratner, 1923). In other\ud words the structure of the placenta was the decisive'\ud factor in the mechanism of the transfer of passive\ud impunity to the young.\ud \ud According to Grosser's (1909; 1927) classification,\ud there are four types of mammalian placental structure.\ud (1) the epitheliochorial with six intervening layers,\ud (2) the syndesmochorial with five, (3) the\ud endotheliochorial with four and (4) the haemochorial\ud placenta with three. These were later modified by Mossnan (1926), who added a fifth type, the haemoendothelial with one.\ud \ud Since animals with epitheliochorial and syndesmochorial placentae transfer antibody entirely postnatally to their young, whereas in those with haemochorial and haemoendothelial placentae transfer is mainly prenatal (with the exception of rats and mice where transfer is mainly postnatal), and in those with endotheliochorial placentae both prenatal and postnatal transfer is considerable, it was perhaps not surprising that a'purely physical hypothesis of antibody transfer should be generally accepted.\ud \ud But in 1946 Hartley showed that the human placenta\ud was selective in transferring antibody, so, that refined\ud horse antibody was not transferred to the foetus at all,\ud while human. antibody passed freely. Shortly after, in\ud a series of papers, Brambell and his colleagues (1949,\ud 1950,1951,1952) showed that there was considerable\ud doubt whether in rabbits antibody is transferred via the\ud placenta at all. They suggested that antibody\ud circulating in pregnant rabbits is secreted into the\ud uterine cavity, whence it passes via the yolk-sac\ud splanchnopleur into the foetal circulation. Moreover\ud they showed that the yolk-sac splanchnopleur is a highly\ud selective membrane, differentiating between -globulins\ud not by molecular size but by*species of origin. In\ud particular Brambell, Hemmings and Oakley (1959) showed\ud that rabbit antibody digested with pepsin was less\ud readily transferred to the foetus, notwithstanding its\ud lower molecular weight, than the unmodified antibody.\ud When rabbit -globulin wap digested with papain (Brambell,\ud Hemmings, Oakley and Porter, 1960) to yield Porter's\ud fractions 1, II and III and these fractions were\ud injected into the rabbit uterine cavity, fraction III\ud was transferred to the foetus almost as well as the\ud unmodified -globulin, fraction I and II much less well.\ud \ud In the Guinea-pig the route of antibody transfer from mother to young was also shown to be via the yolksac\ud splanchnopleur and the vitelline circulation of the\ud foetus (Barnes, 1957).\ud \ud The present work was undertaken to investigate by\ud the technique of intra-uterine injections of antitoxic\ud sera used by Brambell and colleagues the selectivity of\ud the yolk-sac splanchnopleur of the guinea-pig foetus for\ud homologous and various heterologous antitoxins and for\ud the antibody fragments obtained by the peptic digestion\ud of the antitoxin molecule
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