Body Defense Against Diseases

the social and economic tragedies resulting from infectious disease are appalling not only does the care of the sick cost millions but time lost from employment adds tremendously to this waste of even more importance is the unhappiness and suffering of those afflicted war banners which may be observed on every hand are constant reminders of the serious efforts being made in the fight against disease ultimate victory in this fight requires continued study of both of the organisms causing disease and of the complicated interplay of defensive mechanisms that operate within the body the skin an armor-like coating is an important part of the first line of defense here the structure of the skin is indicated at the surface is a hard layer of compact cells constantly replaced by cells growing outward from this region as long as this protective coat remains uninjured bacteria cannot easily break through this primary line of defense continues as a sticky mucous membrane which lines the walls of breathing channels and digestive passages a greatly enlarged cross-section of this membrane in the nose shows the blanket of sticky mucus propelled along by the cilia here we see a small object actually being carried along by this ciliary action in the mucous membrane of a frog in a similar way as long as this blanket remains intact most bacteria which invade the nose strike it and stick to its surface this mucous coat bearing bacteria travels from end to end of the nasal passage about every 10 minutes and is usually swallowed in the stomach the mucous membrane still acts as a barrier against the bacteria furthermore the stomach lining secretes gastric juice which tends to destroy them by the time the food mixture passes from the stomach few living bacteria remain the mucous lining continues through the intestines injury to this delicate membrane may result in local inflammation and in the region of the appendix may cause appendicitis this appendix is normal this cross-section shows the normal condition of the mucous membrane but in this appendix part of the membrane is destroyed and bacteria are growing into its wall and here is shown the extremely serious condition of a ruptured appendix this occurred when inflammation failed to overcome the bacteria and they spread into the peritoneal cavity causing peritonitis whenever and wherever the body's first line of defense is broken bacteria may enter inflammation is the protective and defensive mechanism that immediately starts to operate poisons from nearby bacteria affect capillary walls so that white blood cells called leukocytes normally circulating in the blood tend to stick to them and then begin to squeeze through the walls and move toward the bacteria the microscope shows us how one of these cells engulfs foreign particles it is called a phagocytic or devouring cell within a few hours a wall of these phagocytic cells aided by similar cells already in the tissue are enclosing the bacteria meanwhile blood fluids have leaked through the capillary wall and they support the walling off action by forming thread-like nets of fibrin eventually the whole interior softens resulting in a pus-filled abscess here are shown the beginnings of an elaborate second line of defense the lymphatic system bacteria from this wound enter these lymphatic channels and are carried in the lymph through larger channels to filtering bodies called lymph nodes within the nodes are maze-like passages lined with devouring or phagocytic cells past which the bacteria must go these cells engulf the bacteria so that they thus tend to become concentrated within the node and are eventually destroyed this secondary line of defense is well distributed throughout the body here are groups of lymph nodes in the neck and here in the thorax and here in the abdomen closely associated with this secondary line of defense is the third line the blood circulatory system including the heart the liver and spleen if bacteria penetrate the first and second lines of defense they enter the bloodstream the walls of this normal vessel prevent the bacteria from passing readily from the blood to the body tissues they are thus kept in the course of the stream since the cells of vessel walls are only slightly phagocytic little happens to these bacteria immediately in this diagram let us follow a small group of bacteria coursing through the bloodstream they pass along the vein to and through the heart and then through the arterial blood to the body as a whole eventually most of them reach the liver and spleen in these organs the blood flows more slowly here we see bacteria running the gauntlet of devouring cells which line these passages because of this ability of the liver and spleen to remove bacteria from the blood these two organs are called the primary blood filters during all these processes the body tissues have been stimulated to produce invisible chemical substances called antibodies these antibodies enhance the capacity of the body to resist further attacks by bacteria of the same kind one typical antibody action is shown here in this single typhoid bacterium we see that the antibody and modify its surface causing it to become sticky here we see typhoid basili moving about freely note how they rebound when they strike each other this is because they have identical electrical charges which mutually repel we shall introduce a drop of serum which contains antibodies to the typhoid bacilli now note how these bacteria have collided and stuck together gradually building up large clumps as a result of this clumping it is likely that more will be engulfed when phagocytosis occurs as we see here a clump of several bacteria is being engulfed at a single contact and here a mass of bacteria engulfed and being digested another type of antibody action such as with these cholera germs is shown here serum containing these antibodies starts a dissolving process on the germs so that they begin to disintegrate and disappear within a short time the defense value of these types of antibody action lies first in checking their drift from the point of infection to other parts of the body and second in directly destroying them without such antibody action bacteria may spread through tissues enter the bloodstream and cause serious disease or even death thus we have seen the body's three defenses first the skin and the mucous membrane which make difficult the entry of bacteria to the body second the lymphatic system with its node filters and third the bloodstream with its primary filters the liver and spleen we have witnessed the destruction of bacteria by special cells whose action is made more effective by antibodies in some cases this antibody production can be induced artificially by the use of vaccines also serums containing the antibodies themselves may be introduced most of these treatments have been developed since 1890 such accomplishments give evidence of the progress which science is making in its unending fight to overcome these microscopic but deadly enemies of the human race you