Innate and adaptive immunity rely on both mechanisms to defend the body against disease agents and achieve proper health status. In the poultry industry vaccines boost the immune response to reach and maintain protective immunity. A successful vaccination program should take in account several elements including age at the first vaccination, number of vaccinations, vaccine type, route and interval between vaccinations. The objective is to reach optimal immunity at the correct age, before encountering any pathogen. Immunological memory (the main characteristic of the adaptive immune response) passes through several phases in its development, of which the recognition step is the most important for activation. Adaptive immune response amplifies with antigen exposure. Completing the primary exposure without clinical signs and with minimal pathological changes ensures an effective and protective response. Secondary exposure to the same antigen usually promotes a stronger, more effective response. Exposure to infection has more destructive effect in naïve birds than immunized ones, therefore it is preferable to train the immune response by a first exposure to less virulent agents to minimize damage and side effects.

The strength of adaptive immune response activation is dependent on the innate immune response. It is well established that a strong innate immune response to an antigen defines the adaptive immunity to that antigen. Many infectious agents can evade immunity or provoke weak immune responses so infection has a high impact because of low or absent innate immune response activation. The innate immune system instructs the adaptive immune response to respond to microbial infection; the major decision to respond or not respond to a particular ligand is decided by the genome-encoded receptors of the innate immune system.

Initiation of immunity must pass several barriers and check points. The presence of natural or external inhibitors at the time of immunological memory development results in serious impacts on immunity so that it may be impossible or very difficult to bring the immune response activation back to the right track. For example, maternal immunity at the time of immune response development must be taken into consideration. Although maternal immunity provides protection against many serous viral and bacterial diseases during the first period of life, it is well known that it limits the neonatal immune response activation to both natural infection and active immunization, probably through negative feedback inhibition. Other factors modulating the avian immune system includes immune suppressive disease such as infectious bursal disease, environmental factors including temperature, humidity, air quality stressors, metabolic stressors due to critical dietary deficiencies and toxic substances such as mycotoxins. Immunosuppression in poultry occurs as a series of interactions between many stressors that usually results in problem exacerbation.

In addition to immune suppressive factors increasing the seriousness of infectious problems in the poultry industry, some infectious agents develop immune evasion mechanisms that enable establishment of infection even in birds with healthy immune responses. Birds faced with these types of infections need assistance to avoid the deleterious effect of disease through successful vaccination programs. Influenza virus is among the viruses that pose threats to wild birds, farmed poultry, mammalian livestock, companion animals and humans. Understanding the interaction of this virus with its avian hosts is critical for better control mechanisms. The innate immune response usually senses the presence of influenza virus. However, influenza can minimize the immune response through resistance to the interferon system by activating host protein p58 that prevents activation of innate immune factors and recognition of the viral single-strand RNA which thus escapes detection. In addition, through viral antigenic shift and drift, the adaptive immune response is left powerless against new infections. In this case, previous exposure to this virus is unable to help achieve a proper and effective secondary immune response.

Finally, it is clear that better understanding of the immune response and mechanisms in health and disease is vital to achieve proper control strategies against many problems. Therefore, the Milestone of success in the poultry industry is to understand the immunology of the flock.

Prof. Mohammad KHALIFEH
Professor of Immunology, University of Science and Technology (JUST), Amman, Jordan

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^{Information from the Avian Flu Forum hosted by Boehringer on April 2017}