Bovine biology series
Part - 18 Immune system (1/2)
The word "immune" is derived from the Latin word, immunis, which means "safe". I plainly recall my first glimpse of this concept of "safeness". At a place where the glorious expression of life out of the womb was expressed. In a calving pen, while I watched my grandfather cradle a newborn calf's head in his hands. I have written of this calving pen on many occasions, perhaps for the very reason that it is here, amongst the straw and shavings or maybe grass paddock that the innocence of newborn life so plainly is laid before us. It is here that we appropriately begin the topic of immunity. For very soon after this newborn calf (slender head, therefore a heifer) was free of the life-sustaining umbilical cord, it was on its own. I watched my Grandfather milk colostrum from the cow and nurse it with a nipple bottle into the calf. Thus life begins out of the womb.
Little did I realize at this moment there was a reason for this colostrum, that rich and in this case, blood-colored milk, was necessary for the survival of this calf. Little did I realize that there could be harmful bacteria in this clean and fresh calving pen. Little did I realize that the very sustenance of life was at peril in this newborn calf, because bloodstream molecules we call antigens were not yet there. Little did I realize the reason that in some cases my grandfather would take frozen milk from an older grandma cow and feed a newborn calf, had to do with immunity.
Later on in college and on the farm, the term immunity would take on new meanings and appreciations. As a biologist, I would learn that without immunity, life is not sustained for very long.
So immunity is wisely defined in the Latin language as "safe". So it would be for the calf held by my grandfather’s hands, after drinking colostrum, that it would be safer. So it would later develop an immune system of its own, thus dealing with the sea of harmful bacteria we all live in.
With this introduction, let us take a closer look at immunity and the remarkable immune system that is so important to life, as we know it.
One of the early questions many of us had as we studied biology was what are the body's defenses? Well, the first and most obvious body defenses are the skin and mucous membranes (and their secretions). The second line of body defenses are the phagocytic white blood cells and inflammation (the small blood vessels in the traumatized area increase in diameter (vasodilatation) so that an increased blood volume can bring white blood cells, antimicrobial proteins (such as interferon’s) and blood clotting molecules. One end product of inflammation is pus, which is the accumulated dead cells and lymph material. In milk, we know that inflammation results in high somatic cells in the milk volume). Both of the body defense types described above, skin and inflammation, are what we call non-specific mechanisms. That is, they are readily available in a healthy animal to deal with unwelcome intruders.
You might ask how does the body recognize these unwelcome intruders? Well, the body possesses a unique chemical identity, scribed into each cell at certain locations on the DNA strands that comprise genes that make up the unique chromosomal configuration of each animal. This uniqueness is simply that, one of a kind. So specific is it that all animals can recognize substances, bacteria, viruses, chemicals, and other pathogens that are foreign to its chemical identity. Thus when in contact with one of these unwelcome invaders, the body recognizes it as foreign, not possessing the chemical identity as directed by the linkage of atoms comprising the DNA strand.
In fact every cell in the body has "self markers", specifically a glycoprotein (a protein molecule with a carbohydrate molecule attached) that is embedded into the membrane of every cell. Thus we are truly unique, in that unless we have an identical twin, our body cells, the trillions of them performing a thousand different roles, all have a biochemical fingerprint, the "self marker" in which antibodies cannot be made against them. Quite remarkable, this self-recognition concept. Otherwise we would be soon overrun with invaders.
The third line of body defense is the immune system, and it is specific in that there does exist within the blood volume, certain very specific lymphocytes and antibodies. We call this naturally acquired active immunity.
The concept of specificity refers to the immune systems ability to recognize and eliminate particular unwelcome invaders. These unwelcome invaders are called pathogens in some cases, but are properly called antigens as well. The immune system performs the function of making lymphocytes and specific proteins called antibodies. The antibodies are specific towards a specific antigen.
The term antigen is derived from two words: antibody-generating.
So when an antigen is found to be in the body, the bloodstream for instance, then in most cases there will be a specific antibody to bind with the invading antigen and render it harmless. It is as simple as that. Of course when an antigen is found in the body and no antibody is there to recognize it, then the immune system speeds in fast gear to produce an antibody for it. Meanwhile, various lymphocytes deal with the invading antigen as best as they can, until the more specific antibody can be made.
This, then, is the reason we vaccinate animals. We call this acquired artificial immunity. We inject an antigen into the body, and the body absorbs it into the bloodstream in a concentration usually less than harmful levels. Sometimes this is done without a fetus growing in the reproductive tract. Sometimes this is done during the dry period when the animal is not lactating. But the essence of vaccination is giving the animal a small amount of antigen so that the immune system, the specific part of a body's defense system, can produce an antibody.
If the specific antigen is found inside the body again at a later date, the body says, "oh, I have seen one of you before, I have just the antibody for you!' There is a great deal of variation in terms of lasting ability of these antibodies. For some diseases, like brucellosis, the antibody lasts a lifetime. For some others, like Hemophilus, the antibodies last just a few months. Thus the vaccination program for this disease must be done according to the durability of the antibody. We call this acquired immunity, in that the body acquires a defense to a specific antigen, some of which for a few months and some for the life of the animal.
One note about the relationship of antigen and antibody is a molecular one, in that while the invading antigen may be a very large molecule or bacteria, the antibody that is produced by the immune system is constructed based upon a specific surface configuration of the invader. The specific location (called an epitope), a unique molecular shape or configuration prompts the very specific production of antibody against that specific location.
We must review the relationship between active and passive immunity. A very good place to start is the calving pen.
When the close-up dry cow is put in here, a tremendous number of transformations are taking place. The fetus, now growing at least one pound of weight each day, is being fed by the bloodstream through the umbilical cord. There is more to the blood volume than just nutrients. Here also are the cow's antibodies, moving into the fetal bloodstream. This is passive immunity, in which antibodies created by another animal are used to provide antigen protection. The fetal immune system is not yet fully developed, but the need for protection (remember the word "safe") is minimal. The fetus is bathed in placental fluid, protected in the womb by the warmth of body heat and the containment of the reproductive tract.
At birth, however, the nexus of the umbilical cord is severed; the fetus is now a calf, on its own. Passive immunity can be transferred by feeding colostrum. Certainly the wisdom of my Grandfather helped ensure the chances of his heifer calf. The newborn calf, now exposed to the sea of unwelcome invaders the moment it is born, cannot respond to these antigens quickly enough without this colostrum. Thus the wonderful antibodies found in this bloody thick secretion of an old grandma cow contained antibodies specific for hundreds, perhaps thousands of specific antigens. Whatever unwelcome invaders may be found in the calving pen, the old grandma cow has seen before. She has produced antibodies against them, the process of natural acquired immunity working very well in this healthy cow. They accumulate in the colostrum as protein immunoglobulins. So when my Grandfather feeds this colostrum, he is actually transferring antibodies to the newborn, a process we call passive immunity. Without these antibodies, the chances are slim for the survival of this newborn calf.
Another way passive immunity can occur is the procurement of blood from an animal that has been exposed to a particular disease, the result of unwelcome invaders making their way into the body and causing the immune system to respond with the production of antibodies against the specific antigen.....natural acquired immunity at work. Well, the blood is withdrawn and injected into an animal that may be exposed to these same unwelcome invaders. But in the interest of time, the blood already contains antibodies that can minimize or reduce completely the expression of clinical disease. Hence, the animal survives because of passive immunity, in that a herdmate has already produced specific antibodies.
Sometimes this race against time is best addressed by passive immunity, especially if the unwelcome invader is so unique that no vaccination exists for it. Perhaps a strain has mutated so quickly that it has outraced the vaccine makers in developing a vaccine....the process of artificial acquired immunity cannot be accomplished, so in order to address the time issue, passive immunity via blood transfer is done.
Thus we have immediate immunity, albeit specific for a specific antigen. In this way, animals survive, we as managers of these animals constantly trying to keep within the bloodstream volume a large supply of immune specificity, a component of antibodies that can not only recognize invading antigens, but render them harmless.
There are times, of course, when we do not succeed. So we alter our vaccination programs, change timing of injection, utilize different management techniques, and so on. The bottom line, of course, is a healthy animal that can address the unwelcome invaders with non-specific means until a specific antibody can neutralize it until it can be phagocytized.
Now then, the immune system is actually a dual defense system consisting of the humoral and the cell-mediated system. Humoral immunity results in the production of antibodies from certain lymphocytes (of the lymph system). The cell-mediated immune system is defined as the direct action of certain cells, like lymphocytes called B cells and T cells rather than the humoral antibodies. Both systems, however, are derived from the lymph system, and the primary precursor in the humoral and cell-mediated systems are the lymphocytes, or white blood cells.
These white blood cells originate from stem cells in the bone marrow.
In next month’s biology series, we will examine the humoral, cell-mediated immune systems, and even look into the world of white blood cell formation. The world of stem cells, so much a part of our very existence that without them we would not live long. In the deepness of our bones, those carefully arranged white skeletal stiff things that let us stand and walk (and run of course!) are the bone marrow tissue from which stem cells live. They form all blood cells, including the white blood cells of the immune system.
Next month we continue the journey of immunity, and really, of life itself.
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