Poultry immunity is the most important system to maintain good health and prevent diseases of all kinds.

The immune system of poultry is a complex group of responses that are designed to protect the life of animals. These types of defenses are an interaction between molecular, cellular, and tissue mechanisms.

On the other hand, the organic systems (respiratory, digestive, cutaneous, among others) have a conformation adapted to face injuries. These are called physical barriers and are the first alternative of the defense system to prevent the entry of foreign agents.

For example, the skin and feathers have the function of protecting the bird from foreign agents that may enter by contact. For this purpose, they have unique qualities related to their physicochemical and cellular composition. Another important example is in the respiratory system, in which there are physical barriers that protect against foreign agents coming from the breathed air. Among these barriers are the nasal turbinates, secreted mucus, cilia in the tracheal tube and the bronchi.

Likewise, there are defense mechanisms at the cellular and molecular levels that protect the poultry in their different systems.

What is innate immunity in birds?

Innate immunity is the first response of superior organisms to face any challenge. In the environment, multiple hazards are ranging from infectious agents to particles that can cause damage. Therefore, innate immunity constitutes the first barrier of protection against these injury organisms.

The name “innate” comes from its congenital origin, meaning from birth. The poultry are born and grow with an innate immune system that does not need learning or memory to function against invading or foreign agents. These agents have been recognized thanks to the antigens (particles) present in them. However, the disadvantage of this innate immune system is that it only protects against a limited number of antigens, since it has no memory.

Cells of the innate immune system

The cells of the immune system are called leukocytes (white blood cells). They are a group of cells that have different functions but complement each other to generate a rapid response. Some can phagocytize microorganisms and others are responsible for releasing molecules that destroy them.

Monocytes and macrophages

Monocytes are undifferentiated immune system cells. When these cells reach a tissue through the blood, they become macrophages. These cells take on different names depending on the organ where they are located. Five examples of macrophages located in tissues are:

• Kupffer cells: liver.
• Langhans cells: skin.
• Microglia: central nervous system.
• Osteoclasts: bones.
• Alveolar macrophages: lungs.

Their name derives from the mechanism by which they capture and ingest bacteria, viruses, parasites, or foreign agents. This process is called phagocytosis.

Heterophils

Heterophils are the most abundant cells of the innate immune system and are found in the bloodstream. These cells are the first cellular response to external threats. They are found in tissues and when there are inflammatory or infectious processes. In addition, there are molecules called cytokines that are produced to attract more heterophils to the injured area. This process is called chemotaxis.

In poultry, they are called heterophils and are the equivalent of neutrophils in mammals.

Eosinophils

Eosinophils are cells that have enzyme packets called granules inside them. When eosinophils encounter bacteria or foreign cells, they release enzymes to break down their membrane. These leukocytes also circulate in the bloodstream of poultry.

Basophils

Basophils, contrary to the previously mentioned cells, do not ingest microorganisms. These leukocytes contain histamine, a molecule involved in allergic responses. This histamine increases the inflammatory response in the affected tissues.

Natural killer cells

These leukocytes are of the lymphocyte group and are called killer cells because they recognize and rupture infected or cancerous cells of their own. For this purpose, they adhere to these cells, release enzymes to break their membranes, destroying them. They are an important cellular group of innate immunity against virus infections.

Innate Immunity: Which molecules are involved?

In innate immunity, there are also molecular components such as proteins that play an important role in the different phases of processes such as inflammation or the response to infections.

Acute Phase Proteins

Acute-phase proteins are a large set of proteins that participate in and mediate the inflammatory mechanisms of innate immunity. These proteins can increase or decrease several times their normal number.

Among the major acute-phase proteins are several interleukins and tumor necrosis factor (TNF-a).

C-Reactive Protein

This protein is recognized for its participation in infectious processes, as well as in the elimination of damaged tissues, preventing autoimmune problems, and regulating the inflammatory response.

In cases of infections by bacteria and intracellular antigens, it acts as an opsonin to activate the complement system and phagocytosis.

In poultry, infections by Eimeria spp. and Histomonas produce elevated levels of C-Reactive Protein.

Serum Amyloid A

It is an apolipoprotein associated with plasma high-density lipoproteins (HDL). During acute inflammatory processes, serum amyloid A can increase up to 1000-fold. Its role in avian immunity is not entirely clear, but its effect on leukocyte chemotaxis and cholesterol binding has been described. In addition, it has been found to have immunomodulatory activity and is involved in opsonization.

Serum amyloid A is the precursor of amyloid A protein. In chronic inflammatory processes, increased serum amyloid A can generate amyloidosis in poultry. This is considered a major problem in laying hens.

Complement system

The complement system is constituted by more than 30 proteins that act sequentially activating step by step. They circulate in the bloodstream in an inactive form and once a microorganism enters, this complement is activated.

It is of great importance in innate immunity, but also in acquired immunity. In this way, responses ranging from phagocytosis to activation of B and T lymphocytes are activated.

Factors affecting immunity

To maintain the correct functioning of innate immunity in poultry, it is important to know the factors that can affect this. To function properly, the cells involved in immunity need nutrients and energy to protect the poultry.

1. Nutritional status: good nutritional status of poultry is necessary to maintain good innate immunity in them. Multiple immune processes depend on the complete supply of nutrients and energy.
2. Macronutrient supply: main nutrients such as fat, glucose, and protein are very important to ensure the proper functioning of innate immunity.
3. Micronutrients supply: minerals such as Zinc, Copper, and Selenium are essential for the functioning of immunity in poultry. Therefore, these elements should be supplemented.
4. Vitamins: these molecules are basic components of many cellular and metabolic processes of the immune system. A balanced intake of Vitamin A, C, D, and E is recommended. In addition, the B complex plays an essential role in humoral and cellular immunity.
5. Immunostimulants: there are formulations available for poultry that stimulate their immune system. These are called natural immunostimulant pronutrients and strengthen innate and acquired immunity, especially in young animals. In addition, they improve the response to vaccination and are an ideal complement to control infectious processes. In poultry, these components have shown great efficacy against diseases such as Newcastle or infectious Bronchitis (Coronavirus).

Conclusions

Immunity in poultry is an organic system that contemplates a great number of tissues, cells, and molecules. All these interact with each other to constitute defense mechanisms in poultry against external threats.

The first defense is formed by the physical barriers of the different organ systems, such as the skin, the respiratory or the intestinal epithelium, among others. However, these physical barriers only protect from a part of the external aggressors such as microorganisms.

Therefore, innate immunity is a general defense mechanism and the first response of animals when a microorganism enters the poultry’s body. There, multiple cells, and defense molecules are activated to confront them.

Part of these cells are monocytes, heterophils, NK lymphocytes, eosinophils, and basophils. Among the molecules are proteins such as C-Reactive Protein, Serum Amyloid A, the Complement System, and Acute Phase Proteins.

Finally, some factors favor the immune system response and that must be applied to maintain a healthy state in the poultry farm.