It is usually considered that vaccination is the most important process resulting in immune protection of a herd. This perception derives from the specificity of this process. However, the current understanding of immune principles implies that other management processes are very relevant in improving productivity via control of immunity. Populational immunity implies investments in nutritional quality, wise use of performance additives, use of antibiotics, biosecurity, training of the workforce, together with a well-structured vaccination protocol.
The mucosal-associated lymphoid tissue (MALT) is very developed in mature swine. The intestinal component of this system is the gut-associated lymphoid tissue (GALT), which corresponds to 80% of the entire MALT. It is composed of a complex organization of primary and secondary lymphoid organs. The Peyer patches (organized lymphoid tissues present in the intestinal wall) are composed of B lymphocytes, most of which secrete IgA into the intestinal lumen. These cells compose 40% of the Peyer patches. Circa 20% of cells are CD4 (helper) lymphocytes, and about 10% are CD8 (cytotoxic) lymphocytes. Between 5-9% of cells in this tissue are dendritic cells and other phagocytes.
Because of the intimate connection of these lymphoid tissues with the intestinal epithelial lining, nutritional components have a large effect over local immune cells. Thus, intestinal integrity is just as relevant as vaccination protocols in inducing adequate immunity to environmental challenges. The more animals are resistant to infections, the lower the lateral transmission of pathogens following an initial case within the herd. The functional development of the intestine as a digestive and absorptive organ is intimately associated with its development as an immune tissue.
Processing of foreign materials (antigens) by the GALT follows a similar sequence to that of systemic lymphoid tissues. However, in the GALT enterocytes also play a role in transporting molecules from pathogens to the reaches of immune cells. In the intestinal epithelial lining, M cells are crucial in performing this task. Both enterocytes and M cells will contribute to pathogen defences. Local intestinal architecture is often changed in response to pathogens, such as alterations in crypt depth, mucus production, lymphoid cell infiltration, increased spacing between cells, and so on. More specific responses will be built in the Peyer patches, where B lymphocytes will begin IgA responses directed against the pathogen.
Among all these factors that are relevant in the natural protection of the herd, optimal diet composition will determine the best possible outcomes in balancing the costs of activating immunity and animal productivity. The microbiota is obviously very relevant in this context. Animals with extremely low environmental challenge (in laboratory conditions) tend to demonstrate improved performance when compared to field conditions, due to the natural costs of responding to the challenge by activating immunity. Germ-free animals have 10-30% lower metabolic demand than conventional swine. Only when the diet is tightly managed, and the microbiota is improved can animals in normal conditions surpass germ-free animals. Obviously, “germ-free” is not an option for commercial farm animals, and therefore controlling the bacteria that interact with mucosal immunity is very important. Among the two possible points for controlling gut immunity (nutrition and microbiota), the latter is much less understood. Responses to “enhancers of the microbiota” are usually unstable and vary widely between different situations. The best responses come from the establishment of high quality microbiota already from the first few weeks of life, when the intestinal bacterial populations are still labile.
The "costs" generated by immunity on the host are of two-fold: 1) Immune cells have high protein demands. Not only that, but the amino acid composition of immune proteins deviates largely from other proteins in the animal – and therefore nutritional demands will vary if immunity is being constantly activated. As an example, valine and threonine are consumed in higher levels for the production of IgA and of mucin (the main constituent of the mucus). 2) The initial phases of immune responses present an “opportunity cost” to the host. Immune protection begins with inflammation. Some inflammatory molecules can reach the hypothalamus and control body temperature, inducing fever. This is a natural anti-pathogen response, but it is highly energy-demanding, even to increase the temperature by just a few degrees. Inflammation also negatively regulates appetite, and feed consumption will decrease until the latter stages of immunity are reached. Later phases of immunity (such as IgA production), while costly, are less demanding in terms of "opportunity" – no fever or changes in appetite occur in these phases.
Therefore, nutrition is especially relevant for animals in challenging environments. Responses to pathogens in these situations will occur primarily at the mucosae, in the intestines and respiratory tract. Controlling immunity, whether via the microbiota or directly by immunonutrition, is vital in preventing productivity losses. A balance must be reached between strong immune responses and performance. This is dependent on interactions involved in the largest immune organ, the INTESTINE.