自由基（FR），也称为活性氧（ROS），通常是在氧气充足时在体内产生的。体内氧化应激和 FR 的建立主要是由高代谢活动、快速生长的需要、鸡群管理不当、接触病毒和细菌微生物以及不利的环境条件引起的。此外，当吞噬细胞依赖 ROS 的作用来杀死吞噬的病原体时，FR 也可以在吞噬细胞的活动过程中产生。 FR 对所有细胞，特别是免疫系统细胞都有非常不利的影响。它们是极其不稳定和反应性的分子，直接损害细胞内的 DNA、细胞蛋白质、脂质和碳水化合物。抗氧化剂是能够消除和中和体内FRs，使身体免受由于FRs积累而产生的氧化应激的物质。许多维生素和矿物质支持免疫系统对抗微生物和癌症的活性和效果，这主要依赖于它们的抗氧化元素来减少体内 FR 的负面影响。例如维生素 C、维生素 E、超氧化物歧化酶、硒、甘氨酸、谷胱甘肽过氧化物酶的辅因子、锰、精油和酚类化合物。

Free radicals (FRs), also known as reactive oxygen species (ROS), are usually established in the body when adequate oxygen depletion occurs. Oxidative stress and the establishment of FRs in the body are mainly caused by high metabolic activity, the need for rapid growth, inadequate flock management, exposure to viral and bacterial microorganisms, and adverse environmental conditions. Furthermore, FRs can also be produced during the activity of phagocytes when they depend on the action of ROS to kill the engulfed pathogen. FRs have very adverse effects on all cells, particularly the cells of the immune system. They are extremely erratic and reactive molecules that directly harm DNA, cellular proteins, lipids, and carbohydrates within cells. Antioxidants are substances that can eliminate and neutralize FRs within the body and free the body from the oxidative stress that occurs due to the accumulation of FRs. Many vitamins and minerals support the activity and effect of the immune system in fighting against microbes and cancer, which mostly depend on their antioxidant elements to diminish the negative impact of FRs in the body. Examples are vitamin C, vitamin E, superoxide dismutase, selenium, glycine, cofactors of glutathione peroxidase, manganese, essential oils, and phenolic compounds.