The immune system contributes to the organism’s defense against invasive pathogens, i.e. bacteria, fungi, protozoa, parasites, and xenobiotics. Besides, it is capable of recognizing and eliminating its own cells if they should undergo pathological alteration. Among other elements, the immune system is composed of:
- mechanical barriers (e.g. skin, mucus membranes) which are designed to prevent the invasion of noxious substances / pathogens
- cells, such as granulocytes, natural killer cells, or T lymphocytes, etc.
- proteins which function as messenger substances, markers, or serve the purpose of defending the body against pathogens (e.g. antibodies).
How does immunity work?
Here is a very simplified scenario: Once pathogens have succeeded in breaking through the body’s mechanical barriers, the further course of the immunological response will depend on whether the body already had contact with the particular pathogen before.
In case of an initial infection, the innate and unspecific branch of the immune system (phagocytes, macrophages, and dendritic cells) will come into force, i.e. everything not belonging to the body will be “destroyed” (depending on the so-called histocompatibility complex, MHC, which is located on the surface of all body cells). The fragments of the destroyed microbes / foreign bodies are then presented to the acquired, specific branch of the immune system (B and T lymphocytes). These cells that belong to the specific branch of the immune system are then activated and produce either substances designed to kill the pathogens directly or antibodies which bind to the pathogen thus rendering it innocuous. After this initial infection the antibodies and so-called memory cells holding the blue print for new antibodies remain in order to defend the body without delay in the event of a new infection.
Passive und active immunity
As opposed to humans, who are already supplied with maternal antibodies in the womb via the placenta, the fetuses of many other mammals are not yet capable of combating pathogens effectively before and shortly after birth. The young animal acquires its passive immunity by means of antibodies (immunoglobulins) from the colostrum. Their transmission in the womb is either impossible (cattle, pigs) or only possible to a limited extent (dogs, cats) due to a variant morphology of the placenta in these species.
Since both the immunoglobulin content and the permeability of the intestinal wall of the young animal rapidly decline within a few hours, an immediate uptake of colostrum right after birth will be essential. This “passive” protection given by the colostrum is only intended to function as an “immunity starter kit” – the young animal must now build up its own active immunity. Its body will be challenged by prevalent microbes and synthesize antibodies on its own.
What stresses the immune system?
The immune system of productive livestock and domestic animals is stressed by the absence of certain vital substances in food (minerals, vitamins, etc.) or by stress factors (reallocation to other pens, feed change, weaning, lack of hygiene in the farrowing or calving pens, change of owners in case of domestic animals).