The avian immune system
The avian immune system is divided into non-specific and specific immune mechanisms.
Non-specific immune mechanisms include the innate or inherent ways in which the chicken resists disease. This protective system is often not considered when designing a poultry health program. Many programs tend to rely primarily on vaccinations and/or antibiotics to maintain flock health. The importance of non-specific immune mechanisms should be realized. Examples include Genetic factors - birds may not have complementary receptors to allow many disease organisms to infect them. For example, some strains of chickens are genetically resistant to the lymphoid leukosis virus.
Body temperature: The high body temperature of the chicken precludes many diseases. Blackleg disease of cattle is not a problem in poultry. If the body temperature of the chicken is lowered, the disease may occur.
Anatomic features: Many disease organisms cannot penetrate intact body coverings (skin and mucous membranes) or are trapped in the mucus secretions. Some nutritional deficiencies (biotin deficiency) or infectious diseases compromise the integrity of the body coverings, allowing penetration of disease organisms.
Normal microflora: The skin and gut normally maintain a dense stable microbial population. This stable microflora prevents invading disease organisms from gaining a foothold. Improper use of antibiotics or poor sanitation can disrupt the balance of the microflora.
Respiratory tract cilia: Parts of the respiratory system are lined with cilia which remove disease organisms and debris. If the air in the poultry house is of poor quality due to high levels of dust or ammonia, the ciliary system may be overwhelmed and become ineffective.
Other factors involved in innate resistance include nutrition, environment (avoid heat/cold stress), age (young/old animals are more susceptible to disease), inflammatory processes, metabolic factors, complement, and interferon.