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Immunology and the Neonate

Immunology and the neonateI have been surprised recently by the misinformation and misunderstanding about the functioning of the newborn’s immune system. Following is my attempt to end some myths and provide some understanding.

Immunoglobulin:

What is immunoglobulin? Also called “antibodies,” they are proteins produced by white blood cells that are made to recognize specific foreign invaders-- bacteria, viruses, fungi. For the remainder of this discussion the term immunoglobulin will be used and considered synonymous with antibody (ies).

Each immunoglobulin protein has specific parts, receptors, that can recognize and bind to a specific foreign organism. For example, the immunoglobulin that is made to bind to West Nile Virus is different from the immunoglobulin that will recognize and bind to Clostridium tetani.

The other part of the immunoglobulin signals and binds to a white blood cell once it has found the foreign organism which it is meant to recognize. The white blood cell then destroys the invading organism, hopefully before it has reproduced.

Immunoglobulin (Ig) is grouped into classes, the three major ones being IgA, IgG and IgM. Each class of Immunoglobulin performs a different function for the immune system. IgA provides protection around the entrances to the respiratory tract. IgM provides rapid but short- lived immunity through the body. IgG, provides long-lasting protection through the body. When talking about “protective antibodies” we are referring to IgG.

How is immunoglobulin made?

Immunoglobulin is made by a group of white blood cells in the body called B--cells. These cells have DNA that carries code for Ig’s to thousands of different agents and it takes the introduction of one of these foreign agents into the body to stimulate the B--cells to start making Ig’s against the organism.

This process is taken advantage of when whenever we vaccinate. A vaccine contains the immunoglobulin stimulating portions of an infectious organism, which has been killed or attenuated so that it cannot reproduce.

Young animals have very low immunoglobulin levels when they are born, because they have not lived in the world and been exposed any disease causing organisms. Neonates get antibodies from their mothers in 2 ways-- across the placenta from the mother’s bloodstream or, more importantly in the case of ungulates (hoofed animals) by ingesting them from their mother’s milk early in life.

The first milk made by the dam, called colostrum, is very rich in immunoglobulin. For roughly the first 12--24hours of life, the newborn’s intestinal tract allows for absorption of these very big immunoglobulin particles across the protective barrier of the intestinal wall and into the bloodstream.

The neonate’s intestinal wall undergoes a change within the first day of life and cannot absorb molecules as large as immunoglobulin after that change.

Once absorbed, immunoglobulin can interact with the newborn’s immune system and B--cells to provide protection against infectious agents while the newborn’s immune system begins to make its own immunoglobulin as it is exposed to everything in its living environment.

How is immunoglobulin used?

As mentioned above, immunoglobulin binds to foreign substances in the body, marking them as foreign and flagging them for response by the rest of the immune system. Each immunoglobulin particle has a very specific receptor area that allows it to recognize a specific agent. The receptor area on a WNV immunoglobulin particle is different form the receptor area on a Rabies virus Ig particle.

After an immunoglobulin particle binds to a foreign substance it binds another portion of its structure to a white blood cell (WBC). When this binding occurs, it signals the WBC to begin the process of consuming the organism before it can reproduce and cause infection.

If Ig’s are not present, the infectious agent can begin to reproduce in the body and cause an infection before other parts of the immune system can respond.

How long does it take for immunoglobulins to be produced?

When first exposed to a foreign agent (called an antigen-- for “antibody generator”) the immune system’s response is to turn on reproduction of the B--cells that have the ability to make the specific immunoglobulin (antibody) against that antigen. Those cells then start making large quantities of immunoglobulin. The processes of cellular reproduction and immunoglobulin production take about three weeks each.

This process takes so much time, during which an infectious organism can cause much illness. You can understand why it is important for a newborn to have adequate immunoglobulin intake in the colostrum and also understand why we vaccinated repeatedly and well before exposure to an infectious agent.

How can we measure immunoglobulin?

There are a few ways to measure immunoglobulin levels. All require collecting a blood sample from the animal.

Immunoglobulin is one of two large groups of protein that make up the “total protein” levels in the body. Albumin is the other protein that is included in “total protein.”

One way to measure the immunoglobulin levels is to measure the blood total protein levels and subtract a number that we assume is the average amount of albumin. The result is a rough estimate of the immunoglobulin level. The advantages of this method are it is relatively quick and inexpensive. The disadvantage is that it is only a rough estimate and assumes the amount of albumin rather than measuring it. In an animal with low albumin levels (a concern in poor--doing neonates) the globulin levels would be overestimated.

Another way to measure immunoglobulin is with a serum precipitation test. This test uses a chemical reaction to precipitate out the immunoglobulin in serum and compares the amount of precipitated material to a photographed range of standards. This test, too, is only an estimate. It provides you with a range of immunoglobulin levels-- generally 0--200, 200--400, 400--800 and 800--1600. The advantages are that it is more reliable than measuring total protein and can be run within 1--2 hours with minimal lab equipment. The disadvantages are that it too is only an estimate and there is much room for interpretation and error in comparing the result with the photographed standard.

The final way is to run a radial immunodiffusion plate on serum. The big advantage to this method is it provides us with a quantitated result, not an estimate. This is the only method of measurement accepted by insurance companies as proof of adequate passive transfer of immunity. The disadvantages are that it requires specialized equipment and, unless you are running a large volume of tests, can be cost--prohibitive. It also results in a time delay because of the time needed to ship the sample to the appropriate lab and the time taken to incubate the test.

What don’t we know about immunoglobulin?

Immunoglobulin is very specific in structure and function. Immunoglobulin are produced by DNA within the host animal’s white blood cells and react on a molecular level with not only infectious agents but with the host body’s white blood cells.

We don’t know if immunoglobulin from one species of animal is capable of the interacting with and signaling the immune system in another species. The most specific Immunoglobulin test (the RID plate) will only measure immunoglobulin from the species that it was designed for. This suggests that there is enough difference in the molecular structure of immunoglobulin from different species that they are not interchangeable.

We don’t know how long immunoglobulin from one species of animal will last in the body of another species. We do know that human albumin (the other big blood protein) will cause an immune response in non--human species and therefore will not be in the bloodstream for long and will cause a severe immune system reaction if administered more than once. This suggests that non-host species immunoglobulin will similarly be reacted to and destroyed by the host’s immune system.