Avian immunity 101: The basics

Many factors can impact the performance of the avian immune system, which is critical to poultry health and productivity. Fortunately, certain nutritional interventions can positively affect immune function, as shown through research over the past decade. However, interpreting such scientific findings and knowing what questions to ask for practical guidance in poultry nutrition and health requires a basic understanding of the immune system. Immunity provides ‘protection against disease’ and is ‘mediated by a collection of molecules, cells, and tissues collectively called the immune system’. In birds and other vertebrates, there are 2 complementing units within the immune system – innate and adaptive – that work together to recognise and protect against disease-causing agents, including pathogenic bacteria and viruses.

Innate immunity

Innate immunity is the bird’s first line of immune defence against a wide variety of pathogens. Responses of the innate immune system are ‘non-specific’ and do not distinguish between invaders but respond to features that are common to many types of pathogens. The innate immune system has several components that help provide the initial defence response. These are physical and chemical barriers, blood proteins and cellular components.

Physical and chemical barriers include the skin, the mucosal epithelium, and secretions of the gastric and respiratory tracts that help against possible infection. One specific blood protein called ‘complement’ is a serum protein that works in conjunction with antibodies to help identify certain target cells that ultimately leads to their destruction.

One of the most common signs of activation by the innate immune system is inflammation. Inflammation serves to protect against the initial cause of body cell injury. It removes necrotic cells and cells damaged by the injury and the inflammatory process and initiates tissue repair. Cellular components of the innate immune system include:

• Macrophages
• Natural killer (NK) cells
• Heterophils
• Thrombocytes
• Eosinophils
• Basophils

Many of these innate protector cells – such as macrophages – have a phagocytic role, ingesting and eliminating pathogenic microbes, including bacteria and viruses. A microbe’s pathogenicity refers to its ability to resist the innate immune system. When the non-specific immune response cannot cope with the pathogen, then the adaptive immune system begins to respond.

Adaptive immunity

The adaptive immune system takes a more targeted approach as it recognises specific features on the surface of the pathogen, thus beginning a cascade of events leading to the elimination of the pathogen and creation of ‘memory’ that provides protection from future exposure to that same pathogen. Adaptive immunity involves a specific immune response that distinguishes between even closely related macromolecules or antigens on the surface of the pathogen.

Adaptive immunity makes use of lymphocytes – white blood cells capable of recognising antigens. Lymphocytes include B-cells and T-cells, which initiate a series of events ending with elimination of the antigen. In birds and mammals, the thymus is a primary lymphoid organ where the maturation of T-cells occurs. However, unique to avian species, the Bursa of Fabricius also is a primary lymphoid organ, where the maturation of B-cells takes place.

Adaptive humoral immunity

There are 2 categories of adaptive immunity – humoral and cell-mediated – through which specific types of lymphocytes recognise and eliminate invading pathogens. The humoral response is the adaptive immune system’s primary defence mechanism to eliminate pathogens and their toxins coming from outside the bird’s cells. Blood is the vehicle for humoral immunity, circulating ‘antibodies’ or immunoglobulins produced by B-lymphocytes that can identify and neutralise a specific antigen. In avian species, scientists have identified only 3 types of immunoglobulin molecules – IgM, IgY, and IgA – that may be secreted from several sites in the body to perform primary functions, which might vary depending upon the challenge. For example, IgA synthesis occurs constantly and is primarily located in mucosal areas such as the gastrointestinal and respiratory tracts. The primary role of IgA is to provide mucosal immunity thereby preventing colonisation by invading pathogens.

Adaptive cell-mediated immunity

Cell-mediated immunity involves T-lymphocytes and is the primary defence mechanism for elimination of pathogens that manage to invade body cells and so are not accessible by the circulating immunoglobulins of the humoral response.

The T-cells of the cell-mediated response can be loosely divided into CD4+ ‘helper’ T-cells and CD8+ ‘cytotoxic’ T-cells. The CD4+ and CD8+ designations represent the distinct cellular marker on the surface of these lymphocytes. These T-cells only recognise peptide antigens that are present on the surface of the invading pathogens.

In response to the presence of pathogenic microbes and their antigens, recognition helper T-cells secrete cytokines. The main role of these specialised proteins is to mediate and regulate the various mechanisms of both the innate and adaptive immune systems, including inflammation (innate), macrophage activation (innate) and activation, proliferation, and differentiation of T-cells and B-cells (adaptive).

In the adaptive immune response, once activated by antigen recognition, cytotoxic T-cells break down the infected cell. After exposure to an antigen, B-lymphocytes and T-lymphocytes can further differentiate into ‘effector’ and ‘memory’ lymphocytes, which represents ‘learning’ in the adaptive immune system. If the bird’s immune system encounters the same pathogen again, then these lymphocytes can quickly identify the pathogen and eliminate it. Vaccination makes use of the ‘learned’ adaptive immune response to provide protection against pathogens that the bird has not yet encountered.

Optimising immune function

A number of studies have investigated how interventions via feed or water can help support the innate and adaptive immune responses. Specific influences investigated include effects on innate immunity through natural killer cell activity, complement activity, lysozyme activity and IFN-gamma production.

Adaptive immunity has also been studied and reported in terms of secretory IgA levels and antibody titre levels following vaccination. Poultry’s successful defence against invading pathogens requires a coordinated effort between both the innate and adaptive immune responses and their various components. Much more research is underway to understand how natural products delivered through feed or water can support the optimal, efficient functioning of the bird’s immune system.

References available on request

Author: Hilary Pavlidis, Director, Poultry Research & Technical Service, Diamond V