Feed Additives and Immunity Boosting for Chickens – A Review

Feed additives are an alternative to antibiotics to support sustainable poultry production, improving health and productivity, while meeting consumer demand for organic and environmentally-friendly products. Antimicrobial resistance has become a major global concern, which is encouraging the poultry industry to adopt nutritional immunomodulation as a strategy to produce healthy birds without antibiotics. This approach relies on nutritional science to replace antibiotics with immune-boosting feed additives, which is necessary because genetic selection of commercial poultry strains has improved productivity but reduced birds' ability to combat diverse disease challenges. A deeper understanding of immune system physiology can help optimize feed additives to enhance immune function without compromising performance. This review explores the poultry immune system's physiology and mechanisms, followed by an analysis of widely used synthetic feed additives: prebiotics, probiotics, enzymes, betaine, and L-carnitine. Preand probiotics modulate gut microbiota composition, favoring beneficial bacteria that produce short-chain fatty acids and competitively exclude pathogens, indirectly bolstering gut-associated lymphoid tissue development and function. Enzymes, either provided as a single enzyme or combination of enzymes, improve feed digestibility stabilize the intestinal microflora. Two further additives are valuable in boosting cellular immunity: betaine and L-carnitine. Betaine is mainly known as an osmotic regulator of cellular function, but it has important immunocompetence strengthening effects. L-carnitine is best known for its effects on mitochondria, but it too can have substantial immunocompetence benefits. Furthermore, many immunomodulatory micronutrients and phytochemicals, including selenium, vitamin E, zinc, and various plant extracts rich in polyphenols and flavonoids, act as powerful antioxidants, mitigate oxidative stress, and directly influence the proliferation and differentiation of immune cells such as T and B lymphocytes, consequently strengthening both humoral and cell-mediated immunity. These alternatives aim to compensate for antibiotic removal while supporting immune modulation and maintaining productivity. A multifaceted approach integrating these additives is essential for poultry health, welfare, and performance.