Humoral and cellular factors of natural resistance of animals are represented by diverse proteins, peptides, lysosomal enzymes, and immunocompetent cells contained in blood. The natural resistance factors possessing antimicrobial properties additionally activate the other humoral and cellular immunity mechanisms. The natural resistance system formed during the process of evolution and microevolution of domesticated animals determines their fitness to biotic and abiotic stresses caused by the action of pathogens, unfavorable natural-climatic conditions that is most importantly for animals under all-year-round pasture keeping conditions.