THE DEFENCE MECHANISM OF THE HUMAN BODY*

By Iv. V. KRISHNAN, m.b., m.r.c.p.e. d.b. (Lond.), d.sc.

Kala-azar Enquiry, Indian Research Fund Association (From the Calcutta School oj Tropical Medicine)

Before I proceed to tell you something about the defence mechanism of the human body, I think it is

essential that I should explain to you the reasons that led to the choice of the subject. Some of you who were present on the inauguration day of this academy will remember what Sir C. V. Raman, the President, said to us while explaining the objects of the academy. He said that the primary object of the academy was to make every member tell the others about the problems he is most interested in. The subject of immunology being the one that is engaging my atten- tion at present I could not think of a better theme for my lecture than the defence mechanism of the human body. The second reason for my choice of this subject is because I consider that for a mixed gathering of scientists such as the one before me, there is perhaps no other medical subject of greater interest than the one I have chosen. For, although originally the science of immunology was studied only by medical men for the purpose of solving urgent problems concerning diagnosis, prevention and cure of disease, it is now

being increasingly recognised that immunology is but an important branch of general biology and that its problems are of interest not only to the medical man and the biologist but also to the chemist, the physicist, and the mathematician. Furthermore it is the one

branch of science, more than any other, that has bene- fited most by the recent advances in the sister sciences and thereby shown that the line of demarcation between the sciences is very faint indeed. The third and last reason for my choice of the subject is because immu- nology is a young science of recent development and our knowledge of it consists chiefly of a great volume of insufficiently correlated information, much of which is pure theory. There are more things unknown than known about immunology and this lack of definite and systematised knowledge on several questions connected with it has made the study of the subject all the more alluring. For after all you will admit that, human nature being what it is, has always a special attraction for things new, for things imperfectly understood, and for things hidden. I, therefore, feel confident that the subject -of immunity will interest you all alike. That a protective mechanism exists for guarding the

human body from the ravages of disease was recognised centuries ago. In ancient China and India the people had made the simple observation that a person who has recovered from an infectious disease, such as small- pox, is thereafter resistant to reinfection by the virus of the same disease. Thucydides in Greece had also made somewhat similar observations with regard to the plague. But none of these people tried to explain or

understand the nature of the protective mechanism involved in this process. A few centuries later Jenner made the remarkable discovery that inoculation with

cowpox protected against smallpox and introduced vaccination with calf lymph as a method for preventing the disease. Then came Pasteur and his monumental work on rabies. He found that the spinal cords of rabbits dying of experimental rabies gradually lost their virulence when dried, and that emulsions of these dried cords when inoculated into human beings, bitten by mad dogs, protected them from hydrophobia and death. These two discoveries in fact laid the foundations of the science of immunology and indicated the far-

reaching importance of a study of its problems. Then came Metchnikoff with his phagocytic theory of

immunity and Ehrlich with his humoral theory of

immunity. While the one claimed that certain cells of the body were responsible for the overcoming of

disease, the other maintained that certain constituents of the blood serum and body fluids were the chief

agents of protection. The story of the ardent contest between these two champions is quite familiar to most of you. All that I need to tell you now is that to the modern immunologist this classical controversy has lost all its pristine interest and significance. To him it is but an important landmark in the history of immu-

nology?a landmark that served as a very effective stimulus to both experiment and criticism. One direct outcome of this heated controversy was the discovery of a series of new serological reactions such as the

* Being a lecture delivered on the 27th August, 1932, at the Calcutta Academy of Arts and Sciences.

638 THE INDIAN MEDICAL GAZETTE [Nov.', 1932

agglutination reaction by Gruber and Durham, the

precipitation reaction by Ivraus, the complement- fixation reaction by Bordet and Gengou and several such others. Side by side with these discoveries the value of immune sera in the treatment of certain infec- tious diseases, like diphtheria and tetanus, were being demonstrated and explanations for the; various serological reactions, and the beneficial effects of antisera in treat- ment, were being sought. Very soon Ehrlich came

forward with his ingenious chemical theory of immunity, with its fascinating side chains and receptors and it was

accepted practically by everyone with awe and reverence. Then came Bordet. He recognised the close similarity between the immunological reactions and the reactions of colloidal chemistry and put forward his physical theory to explain the various serological phenomena. His experiments showed that immunological reactions followed the laws governing the phenomena associated with adsorption at surfaces and interfaces, and as such were subject to physical rather than chemical laws. This theory has only one great shortcoming and that

is, it fails to explain adequately the specificity of the

immunological reactions. More recent studies, however, show that a correct explanation for all the serological phenomena noted is possible only by a combination of the physical and chemical theories of immunity and

assuming a primary colloidal adsorption and a secondary chemical union. Last of all comes the work- of Loeb on the behaviour of protein solutions, and the investiga- tions of Landsteiner and Pick 011 the chemical structure of protein molecules and their relation to immunological specificity. These important researches that are being conducted at the present time bid fair to throw more light not only 011 our knowledge of immunity reactions but also 011 the chemistry of proteins which is at present very little understood.

Having outlined the historical development of our

knowledge regarding the defence mechanism of the human body, let me tell you something about it from the evolutionary point of view. In the long course of evolution of living things, it seems probable that the lower forms of life arose far in advance of the more

highly differentiated forms. So much so that the latter from the very moment of their first appearance on this earth had to compete for their place in nature with a vast number of microbial forms. In the course of the

adjustments necessitated by this complex communal existence various forms of parasitism were established. As a defence against parasitism and the abnormal condi- tions arising from it, mechanisms of protection of different degrees of efficiency were developed. Even in the most primitive forms of life one or more simple means of self-defence are discernible. Higher up the scale as the needs for self-preservation grow greater, the mechanism of defence also becomes more and more complex, and when we reach man we find that this mechanism is very intricate and very difficult to understand. By way of illustration let me outline the course of development ot the defence mechanism from the lowest to the highest animal.

Take a unicellular animal like the amu'ba, and give it to eat a red blood corpuscle and a carbon particle. It. takes up both, but it very soon rejects the carbon particle and digests the red blood corpuscle. By repeating this experiment several times it has been shown that the amda can be trained not to take the carbon particle at all. This then is the simplest form of defence mechanism and is just an adaptation of the ordinary feeding mechanism. Xext let us take a simple multicellular animal such as

the hydra or the sea anemones, here we find that certain cells of the mesenterial filaments do exactly the same thing that the amoeba does. The onlv difference is that being a multicellular animal, there is division of labour and that to certain special cells have been rele- gated the power to deal with animate and inanimate particles that are swept into the interior of the animal and to dispose of them iu the way best suited to the interest of the animal.

Next passing to the daphnia or the water flea which is .still higher up in the evolutionary scale, the defence mechanism is found to be principally a phagocytic one. When you place this minute aquatic animal in water containing Monospora or yeast c^lls and observe it under the microscope you can see the spores penetrating the wall of the intestinal canal and making their way into the body cavity of the animal. Here they are

attacked by certain wandering phagocytic cells which first engulf them and later digest them completely. This prevents the proliferation of the parasites within the body cavity of the daphnia and the infection is soon overcome. If, on the other hand, the arrival of the

phagocytic cells to the focus of infection is delayed or their phagocytic and digestive powers are interfered with, then the spores rapidly proliferate and a generalised infection results leading to the speedy death of the

daphnia. In this aquatic animal again we find that

phagocytosis is the principal factor in the overcoming of its infections. When next we take the lower cold-blooded vertebrates,

we find that their mechanism is somewhat similar to

that in the daphnia. If for instance we inject into these animals a culture of Bacillus <mlhraci#, they respond to it by a local accumulation of wandering phagocytic cells which rapidly ingest and digest the bacilli.

Finally when we take up the study of the mechanism of defence in the warm-blooded vertebrates, we find that it is essentially the same as in the less complex forms, but that in addition several other mechanisms are also involved. Details of these mechanisms have been worked out in animals such as guinea-pigs, rabbits and dogs by injection of various substances into them, and our knowledge of the mechanism in man is to a

large extent derived from these experiments. As our

chief interest lies in the understanding of the defence mechanism in man let me take if up next for detailed consideration. The defence mechanism of the human body is

something like a motor engine. It is composed of several parts and in order to understand how the mechanism as a whole works one has to know first what the parts are and then how each part functions. Of the component parts, there are three, that are rela- tively more important than the rest, namely, the phagocytic mechanism, the antibody mechanism, and the anaphylactic mechanism, and these three alone need be considered here.

In the blood and tissues of man there are certain phagocytic cells usually spoken of as polymorphonuclear cells, and the wandering cells of the reticulo-endotlielial system which have retained the primitive capacity for intracellular digestion. Roth these cells have a very wide distribution throughout the body and their chief function is to pick up all foreign particles animate and inanimate that gain access into the body and to

destroy or dispose of them in a suitable manner. They are both provided within their cytoplasm with proteo- lytic ferments that are capable of digesting proteins to the amino-acid stage. Depending upon the nature of the invading organism one or the other of these two cells assumes the role of phagocytosis and destruction. For example when the infecting agent is a pyogenic coccus like the staphylococcus, then the polymorphonuclear leucocytes play an important part in their destruction: but if the agent is a protozoon like the malarial parasite then the cells of the reticulo-endotlielial tissue play the predominant part. The latter cells appear to be con- cerned chiefly with the removal of those foreign particles that are somewhat difficult to destroy or digest. This nartlv explains why we fret different types of cell reactions in different infectious diseases and why the medical man studies the number and distribution of blood cells to find out the probable nature of the disease.

It is not to be supposed that in every instance the phagocytes are successful in destroying the parasites they pick up. At times even growth and multiplication of the ingested organisms take place within the phago- cytic cells. When such cells die, the living organisms

Nov., J932J DEFENCE MECHANISM OF BODY: KRISHNAN 639

contained in them are liberated and cause an infection of the tissue in which they chance to be deposited. This shows how infection is carried from one tissue to another and how the phagocytes prove at times harmful instead of helpful. It can be shown by simple test tube experiments that one of the factors that prevent such occurrences and help 1he phagocytes to perform their destructive function better and more efficiently, is the

antibody mechanism. If for example phagocj^tes are

washed free of serum, mixed with certain bacteria, and examined under the microscope, very few phagocytes will be seen to have taken up the bacteria. If next we add to this mixture a small quantity of fresh normal serum then we can see a relatively larger number of

phagocytes taking up the bacteria. But if instead of normal serum we add immune serum, that is serum

containing specific antibodies for the bacteria then not only can an enormous number of phagocytes be seen with bacteria in their cytoplasm but several of the bacteria contained within the phagocytes are found to be in various stages of disintegration. This shows that in the presence of antibodies large numbers of bacteria arc taken up and successfully destroyed by phagocytes and that phagocytosis represents only one part of the storjr and to get a complete picture of immunity we will have to consider other mechanisms as well. One of the interesting problems in connection with

the phagocytes is the mechanism of phagocytosis. The question is how do the phagocytes take up particulate matter from their environment? Is it a process analogous to ingestion by the relatively highly developed organisms involving a. definite expenditure of energy or is it a process involving decrease of free energy? That is to say does the phagocyte ingest substances because of surface tension or in spite of it. The researches conducted so far show that surface tension is only one of the factors concerned in phagocytosis and that possibly other factors also play a part. Next we come to a consideration of the antibody

mechanism. This mechanism owes its existence to the fact that certain tissue cells of our body have the power to respond in a specific way when substances of a given chemical and physical structure gain an entrance into the blood or tissues. That is to say when certain protein substances called antigens are injected into our blood or tissues there appear after a time in the circu- lating blood certain substances called antibodies which specifically react with the injected antigen when the two are brought together. Remembering that the etio- logical agents of infectious diseases are composed of

protein substances which possess this antigenic property, it can easily be understood how readily they can

stimulate antibody production. Wheji pathogenic organisms gain access to our bodies

antibodies are formed. Each antibody produced is

specific for the organism that stimulates its production. If in one and the same organism there are more than one antigenic substance then antibodies will be formed against each one of these substances and if the organism is a toxin producer then antibodies will be produced against tin; toxin as well. Thus for one organism there may be produced more than one antibody and each

antibody will react only with the antigenic substance that stimulated its production. It is this extreme

specificity of the antigens and antibodies that has made immunology a very interesting subject for stud}'. Recent investigations show that this immunological specificity is a function of the chemical structure of the particular protein antigen.

Efforts have been made 1o isolate antibodies in a

pure state, but so far no success has resulted. Therefore, we do not possess any definite knowledge regarding their chemical nature or structure. All that we know about them i< that they are associated with the globulins of the blood plasma, and that their presence can be

demonstrated by certain serological tests. For example if we mix serum containing antibodies with the organ- isms that stimulated its production in the presence of

electrolytes in a test lube, then the organisms can be seen to clump together and settle down to the bottom

of the lube. What happens is that the organism adsorbs the antibody and becomes very sensitive to the presence of electrolytes. The latter bring about a reduction in the electrical charge on the surface of the organism with the result that the cohesive force between the individual

organisms is increased, the organisms clump together and finally settle down to the bottom of the test tube. The above phenomenon that occurs in the test tube has experimentally been shown to take place inside the animal body and it is this phenomenon that is respon- sible for the increased success of the phagocytes in

destroying organisms in the presence of antibodies.

Attempts have been made to find out where and how the antibodies are formed. Although there is no definite information on the subject it is supposed that antigenic substances when introduced into the body are picked up by phagocytic cells and taken to the spleen and liver and deposited there. Here certain cells of the reticulo- endothelial system are stimulated by these substances in such a way that specific antibodies are produced. When the amount of antibodies produced reaches a

certain concentration, they begin to appear in the

circulating blood.

The role of circulating antibodies in immunity has also been studied very extensively. Taking the evidence as a whole it appears that whenever a disease is due to the toxins of an organism rather than to the organism itself then an increase of antibody in the circulating blood means an increase of protection. In diseases like

diphtheria and tetanus where the damage is caused

chiefly by the toxin of the organism, marvellous results can be achieved by the use of serum containing anti- toxic antibodies. This is so because all that is required to overcome the disease is to neutralise the toxin. The antitoxin does this perfectly and in proportion to the amount available. The greater this amount, the better the neutralisation and the better the immunity. The relation between antitoxin and immunity is similar to the relation between the quantity of petrol in a motor engine and the distance travelled; just as you can go a longer distance with more petrol than with less, you can cure cases better with more antitoxin than with less.

On the other hand in diseases that are directly due to the multiplication of the invading organisms, it is found that increase of antibody in the circulating blood does not necessarily mean increase of immunity. This is so because the function of antibodies in these diseases is

only to prepare the organisms for phagocytosis and other destructive processes, and unless the latter

processes are efficient, antibodies by themselves will not be of much use. For preparing the organism for

phagocytosis only a limited amount of antibody is

required and when more than this is present it will not be of any additional benefit. This explains why in diseases like pneumonia administration of serum con-

taining antibodies to the patient is of very limited value. The best way to understand the true role of these antibacterial antibodies is by taking the analogy of the engine again. Antibodies as I said before cor-

respond to the petrol; more petrol in the tank does not mean more efficiency or speed; some petrol in the tank is certainly better than no petrol, but a full tank is surely not better than a half tank. The same is true of this type of antibody.

Finally we come to a consideration of anaphylactic immunity. In some cases the antigenic substances liberated from parasitic organisms within the body act by increasing the reacting capacity of the body cells in such a way that even when a very small amount of the same antigen is introduced at a later date, the body responds by a violent reaction. This form of immunity is best illustrated in connection with tuberculosis. If. when a person has a mild tuberculous infection, we inject into his skin a small amount of an extract of tubercle bacilli, he will react with a violent inflammation; but the same dose of extract when injected into a healthy person produces no such reaction

640 THE INDIAN MEDICAL GAZETTE [Nov., 1932

whatsoever. This difference in response in the two

persons is because in the first individual the infection with tuberculosis has brought his body cells to a state of great sensitiveness or irritability, and is thereby preventing any further entry of the same organism. This peculiar protective mechanism functions only so

long as the primary infection that causes it lasts._ It

is, therefore, sometimes called ' infection immunity' because the immunity is present only when the infection is present- In those diseases where this is the chief

type of immunity concerned, it is better to have a mild infection of the disease well under check than to be

completely free from it, for in the first case one will be immune to fresh doses of infection and in the second one will not be.

Having explained to you the three chief component parts of the defence mechanism, I wish to point out that every case of cure and immunity cannot be

explained away purely on the basis of these mechanisms. There are occasions when one is at a loss to find an

explanation for the immunity that has occurred. There are as many cases on record of protection in the absence of demonstrable antibodies as there are of absence of protection in the presence of antibodies. These have

naturally led immunologists to recognise that possibly there are other mechanisms concerned in protection as well. Recently Besredka has put forward his new and interesting theory of local immunity in an attempt to explain some of the observed discrepancies. His theory supposes that the cells of the tissues attacked are the cells primarily concerned in protection and not anti- bodies or phagocytes. Besredka says

' the antibodies should be without hesitation stripped of their importance as their function in immunity is in reality entirely secondary or negative in certain cases'. He supports this assumption by the following line of arguments. Many pathogenic organisms have a tendency to get localised in some one particular tissue of the body, irrespective of the route of entry of the organisms. Tor

example the typhoid and dysentery bacilli attack the intestines and anthrax bacilli the skin. If these tissues are rendered insusceptible to attack, then the whole animal will be immune. Therefore what is to be aimed at is not the production of antibodies or phagocytes but effective local tissue immunity. He thinks that this can be done by introducing the virus of disease in a

particular form directly into the tissue concerned, and by stimulating the production of ' anti-virus'. This anti-virus saturates the tissue cells and renders them insensitive to the toxic action of the organisms. Taking the available evidence as a whole, there appears to be some support for the view that local tissue immunity as opposed to a general cellulo-humoral immunity plays some part in resistance to bacterial infection; but there is little or no reason to believe that the mechanism involved in this immunity is in any way different from what we have already considered. For, when local immunisation is pushed to a point at which general resistance is markedly increased it is always possible to demonstrate the presence of protective antibodies. Therefore, while admitting that the method of vaccina- tion advocated by Besredka is good, most immunologists do not accept his theory of local immunity, as sound.

By way of conclusion I may add that in presenting the subject of immunity in the manner I have done, it has been my endeavour to show how the problems of immunity are of interest not only to the medical man, but to workers in other branches of science as well. As I told you in the beginning the unsolved problems in immunology are_ numerous and the subject-matter of these problems lies on the border line between different branches of science. No solution of these problems is possible unless it be through team work or the willing co-operation of specialists in the various branches of science. Therefore there is no better way of concluding this lecture than by an appeal for such a co-operation. In doing that I am sure I will also be indirectly appeal- ing for the success of this academy under whose auspices we have met here to-day.