Interesting as the wider questions may be, we are here more immediately concerned with the distribution of the blood groups amongst our own population. The percentages in which the four groups occur have been estimated by various observers, and, as will be readily understood from the foregoing remarks, the numbers show some variation. The approximate figures as worked out by three observers in America are as follows:

BernheimMossCulpepper
(1,600 tests)(5,000 tests)
I 2103per cent.
II404038 ” ”
III15 718 ” ”
IV434341 ” ”

The percentages found among the first hundred men whom I tested in the British Army in 1917 conformed almost exactly to the first of these series of figures, and they may be taken as an average result for Western peoples. It will now be seen upon what grounds it was stated in the last chapter that the chances were in favour of the blood of a donor chosen at random being compatible with that of the recipient. If the patient belong to Group II, then 83 per cent. of other bloods will be compatible. If he belong to Group III, 58 per cent. will be compatible. Only if he belong to Group IV will the chance in favour of compatibility fall below 50 per cent.

This statement of the facts concerning distribution of the blood groups will serve to emphasize the absolute necessity for the careful testing of a donor before his blood is used for transfusion. But, further than this, it is necessary to clear away several widely spread misapprehensions as to the group relations between an infant and its mother and between the various members of a family. It has several times been stated in print that a mother’s blood must be compatible with that of her child, or sometimes that a baby has no blood group, so that it may be safely transfused with blood taken from its mother or its father without preliminary testing. On other occasions the statement has been made that the brother or sister of a patient is more likely than other people to belong to the same or a compatible blood group, so that untested blood may be transfused from one member of a family to another with little risk. Knowledge of the existence of blood groups has become somehow mixed up with vague popular beliefs concerning “affinities” and “blood relations.” Such confusions must, however, be dissipated, for none of these statements are more than partially true, and they may lead to a false sense of security and to disaster.

The assertion that an infant has no blood group was tested by the writer some time ago and shown to be false. On several occasions a newly born infant was tested and found to show well-marked agglutination reactions indicating Groups II or III as the case might be. Even in 1905 it had been shown by Martin that reactions could often be demonstrated between an infant’s corpuscles and the maternal serum, and sometimes between the infant’s serum and the maternal corpuscles. More recently (March 1920) the results of a full investigation into the reactions found in infants and children have been published by W. M. Happ in America. These researches began with the testing of blood from the umbilical cord, and this was seldom found to show the blood reactions as given by the adult. So far the statement quoted above was justified. It is even true that the serum of an infant’s blood will usually not give any reaction at birth or during the first month. The percentage in which it does give a reaction increases with the age of the child; after one year it is usually, and after two years always, established. On the other hand, the agglutination reaction in the corpuscles appears before that in the serum, so that the grouping tested in this way may be present immediately after birth, as I found to be the case. It is possible that the grouping which first appears may afterwards be modified, but any change which occurs is always by the addition of factors and not by their subtraction; thus an apparent Group IV may become a Group II or III, or an apparent Group II or III may become a Group I. It is found that when a reaction is present in both the corpuscles and the serum, the group does not afterwards change. Happ’s conclusion, based on his investigations, was that it is unsafe to transfuse an infant with its mother’s blood without first making the usual tests, and the reasons for this will now be evident. In the first place an infant may be possessed of its final blood reactions very shortly after birth, and should therefore be treated in the same way as if it were an adult. In the second place, although its serum may be without agglutinating powers, so that transfused corpuscles will not be attacked, yet its corpuscles may be possessed of pronounced agglutinophilic properties, so that they may be seriously affected by the serum of transfused blood from an incompatible group. In the third place, as will presently be seen, it is by no means the rule that an infant should belong to the same group as its mother, whatever its blood reactions may be.

Another set of observations, leading to precisely the same conclusions, have been made by F. B. Chavasse of Liverpool. He terms the potential agglutination of the fœtal corpuscles by the mother’s serum, and of the maternal corpuscles by the serum of the fœtus, the “maternal threat” and the “fœtal threat” respectively, and states that there is no obvious relationship between the “fœtal threat” and eclampsia or the toxæmias of pregnancy. The inference is therefore justified that there is no transference of the agglutinating substances in either direction across the placental membranes. No chemical “immunity” is acquired, therefore, on either side, since the protection is mechanical. This agrees with the fact observed by Happ that the mother’s milk contains the same agglutinins as the serum of her blood; but these do not have any deleterious effect upon the infant, and are therefore either not absorbed at all or are destroyed in the process of digestion.

The statement that the blood group of an infant is not necessarily the same as that of its mother can be amplified, for it has been found that blood groups are inherited on a definite plan, so that if the groups of the parents be known, certain predictions can be made as to the possible groups that may be found among their offspring. Many characters in animals and plants have been shown during the last twenty years to be transmitted according to the Mendelian plan of inheritance, but up to the present time very few normal characters in man have been isolated, and their manner of inheritance demonstrated, though a number of pathological conditions have been shown to conform to the theory. It is therefore of much interest to find that the inheritance of blood groups in man can be quite satisfactorily and consistently explained in Mendelian terms.

According to this theory, each quality in an organism which can be isolated and investigated independently of other qualities, is termed a “unit character,” and the appearance of each such unit character is determined by the presence of something called a “factor” in the sexual cells or “gametes,” male and female, by the union of which the individual is formed. Further, these unit characters are believed to occur in alternative pairs, and at first it was supposed that each alternative pair consisted of “dominant” and “recessive” characters, the second of which could only make its presence apparent in the individual if the dominant character were absent. Subsequently it was seen that the dominant and recessive characters need not necessarily consist of two positive, though opposite, qualities, but might better be regarded as consisting of the presence of a character and its absence. To use a classical illustration of this view, sweet peas may be classified into tall peas and dwarf peas. At first the unit characters were taken to be tallness (dominant) and dwarfness (recessive). Later this idea was modified, and it was said that potentially all peas are dwarf, but to some is added a factor producing tallness, this factor being absent in those that are dwarf. To represent this idea more simply a conventional notation has been used, according to which the large letters of the alphabet indicate the presence, and the small letters the absence, of each factor.

In order to apply this theory to the case under consideration, it has been suggested that two pairs of factors are concerned:

A the presence of the character producing Group II.
a the absence of the character producing Group II.
B the presence of the character producing Group III.
b the absence of the character producing Group III.