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 the passive immunity of man. According to the nature of the substances injected into the former, its serum may be antitoxic, if it has been immunized against any particular toxin, or antibacterial, if against an organism. Familiar examples of these are, of the former diphtheria antitoxin, of the latter anti-plague and anti-typhoid sera. An antitoxin exerts its effects by actual combination with the respective toxin, the combination being inert. It is probable that the ultimate source of the antitoxin is to be found in the living cells of the tissues and that it passes from them into the blood. The action of an antibacterial serum depends on the presence in it of a substance known as “immune-body,” which has a special affinity and power of combining with the bacterium used. In order that it may exert this power it requires the presence of a substance normally present in the serum known as “complement.” The development of these “anti-bodies,” though it has been studied mainly in connexion with bacteria and their toxins, is not confined to their action, but can be demonstrated in regard to many other substances, such as ferments, tissue cells, red corpuscles, &c. In some animals, for example, the blood serum has the power of dissolving the red corpuscles of an animal of different species; e.g. the guinea-pig’s serum is “haemolytic” to the red corpuscles of the ox. This haemolytic power (haemolysis) can be increased by repeated injections of red corpuscles from the other animal, in this case also, as in the bacterial case, by the production and action of immune-body and complement. The antiserum produced in the case of the red corpuscles may sometimes, if injected into the first animal, whose red corpuscles were used, cause extensive destruction of its red corpuscles, with haemoglobinuria, and sometimes a fatal result.

Opsonic action depends on the presence of a substance, the “opsonin,” in the serum of an immunized animal, which makes the organism in question more easily taken up by the phagocytes (leucocytes) of the blood. The opsonin becomes fixed to the organisms. It is present to a certain extent in normal serum, but can be greatly increased by the process of immunization; and the “opsonic index,” or relation between the number of organisms taken up by leucocytes when treated with the serum of a healthy person or “control,” and with the serum of a person affected with any bacterial disease and under treatment by immunization, is regarded by some as representing the degree of immunity produced.

Agglutinative action is evidence of the presence in a serum of a somewhat similar set of substances, known as “agglutinins.” When a portion of an antiserum is added to an emulsion of the corresponding organism, the organisms, if they are motile, cease to move, and in any case become gathered together into clumps. In all probability several different bodies are concerned in this process. This reaction, in its practical applications at least, may be regarded as a reaction of infection rather than of immunization as ordinarily understood, for it is found that the blood serum of patients suffering from typhoid, Malta fever, cholera, and many other bacterial diseases, agglutinates the corresponding organisms. This fact has come to be of great importance in diagnosis.

The precipitin test depends on a somewhat analogous reaction. If the serum of an animal be injected repeatedly into another animal of different species, a “precipitin” appears in the serum of the animal treated, which causes a precipitate when added to the serum of the first animal. The special importance of this fact is that it can be utilized as a method of distinguishing between human blood and that of animals, which is often of importance in medical jurisprudence.

In this summary the facts adduced are practically all biological, and are due to the extraordinary activity with which the study of (q.v.) has been pursued in recent years. The chemistry of the blood has not hitherto been found to give information of clinical or diagnostic importance, and nothing need here be added to what is said above on the physiology of the blood. Enough has been said, however, to show the extraordinary complexity of the apparently simple blood serum.

The methods at present employed in examining the blood clinically are: the enumeration of the red and white corpuscles per cubic millimetre; the estimation of the percentage of haemoglobin and of the specific gravity of the blood; the microscopic examination of freshly-drawn blood and of blood films made upon cover-glasses, fixed and stained. In special cases the alkalinity and the rapidity of coagulation may be ascertained, or the blood may be examined bacteriologically. We have no universally accepted means of estimating, during life, the total amount of blood in the body, though the method of J. S. Haldane and J. Lorrain Smith, in which the total oxygen capacity of the blood is estimated, and its total volume worked out from that datum, has seemed to promise important results (Journ. of Physiol. vol. xxv. p. 331, 1900). After death the amount of blood sometimes seems to be increased, and sometimes, as in “pernicious anaemia,” it is certainly diminished. But the high counts of red corpuscles which are occasionally reported as evidence of plethora or increase of the total blood are really only indications of concentration of the fluid except in certain rare cases. It is necessary, therefore, in examining blood diseases, to confine ourselves to the study of the blood-unit, which is always taken as the cubic millimetre, without reference to the number of units in the body.

Anaemia is often used as a generic term for all blood diseases, for in almost all of them the haemoglobin is diminished, either as a result of diminution in the number of the red corpuscles in which it is contained, or because the individual red corpuscles contain a smaller amount of haemoglobin

than the normal. As haemoglobin is the medium of respiratory interchange, its diminution causes obvious symptoms, which are much more easily appreciated by the patient than those caused by alterations in the plasma or the leucocytes. It is customary to divide anaemias into “primary” and “secondary”: the primary are those for which no adequate cause has as yet been discovered; the secondary, those whose cause is known. Among the former are usually included chlorosis, pernicious anaemia, and sometimes the leucocythaemias; among the latter, the anaemias due to such agencies as malignant disease, malaria, chronic metallic poisoning, chronic haemorrhage, tubercle, Bright’s disease, infective processes, intestinal parasites, &c. As our knowledge advances, however, this distinction will probably be given up, for the causes of several of the primary anaemias have been discovered. For example, the anaemia due to bothriocephalus, an intestinal parasite, is clinically indistinguishable from the other forms of pernicious anaemia with which it used to be included, and leucocythaemia has been declared by Löwit, though probably erroneously, to be due to a blood parasite closely related to that of malaria. In all these conditions there is a considerable similarity in the symptoms produced and in the pathological anatomy. The general symptoms are pallor of the skin and mucous membranes, weakness and lassitude, shortness of breath, palpitation, a tendency to fainting, and usually also gastro-intestinal disturbance, headache and neuralgia. The heart is often dilated, and on auscultation the systolic murmurs associated with that condition are heard. In fatal cases the internal organs are found to be pale, and very often their cells contain an excessive amount of fat. In many anaemias there is a special tendency to haemorrhage. Most of the above symptoms and organic changes are directly due to diminished respiratory interchange from the loss of haemoglobin, and to its effect on the various organs involved. The diagnosis depends ultimately in all cases upon the examination of the blood.

Though the relative proportions of the leucocytes are probably continually undergoing change even in health, especially as the result of taking food, the number of red corpuscles remains much more constant. Through the agency of some unknown mechanism, the supply of fresh red corpuscles from the bone-marrow keeps pace with the destruction of effete corpuscles, and in health each corpuscle contains a definite and constant amount of haemoglobin. The disturbance of this arrangement in anaemia may be due to loss or to increased destruction of corpuscles, to the supply of a smaller number of new ones, to a