SPECIAL STORY FEED & ADDITIVE MAGAZINE April 2023 61 Figure 1. Calves immune system maturation and main external stressors from birth to 2 months old (Adapted from Hulbert & Moisá, 2016.). the physical barriers, the innate immunity, and the acquired (also called passive) immunity. 1. The physical barriers prevent any pathogens from entering the body. Some physical barriers can act as an alarm signal for the farmer: coughing, sneezing, vomiting, urinating… And others are invisible at producer level: skin, mucus membranes and gut epithelium, enzymes in the saliva and the intestine and a balanced microbiota. 2. The innate immunity that will respond quickly to the presence of pathogens and try to destroy them. Its activation leads to inflammation. This system uses chemical messengers such as cytokines and activates immune cells (white blood cells, WBC) such as macrophages that can be used as markers of inflammation. 3. The acquired or passive immunity that is specific and has a memory role. It must be acquired and developed through exposure to pathogens along the animal life. It’s the system exploited by vaccination: exposure to disactivated pathogen that will ’teach’ the immune system how to recognize this pathogen to be able to neutralize it at the next encounter. An organism needs these three components working at their optimum to ensure health and growth. The gut plays an important role in the immune defense system as innate and acquired immunity are concentrated within the gastro-intestinal lymphoid tissue (GALT). This GALT gathers up to 70% of the body’s WBC. Using nutritional additives to support the physical barriers such as gut integrity and mucin production in addition to stimulating a balanced immune response at gut level can be a winning strategy to improve pre-weaned calves' performances.
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