There are two types of immune mechanism in birds:
( i ) non-specific ( innate )
( ii ) Specific ( acquired immunity )
The innate immunity of the chicken refers to the natural or inherited ability to resist disease by non-specific mechanisms and this response is systemic.
Examples of innate immune cells: intraepithelial leukocytes, including macrophages, dendritic cells, heterophils, natural killer cells and T-lymphocytes ( Jeurissen and Janse, 1996 ). The innate immune cells have several functions, including the recognition and control of invading pathogens as a first line of immunological response as well as antigen presentation and the activation of the mechanism of acquired immunity.
Innate response is non-specific, the immune cells are not targeted against specific antigen, but instead, it recognises generalised and conserved molecules across many pathogens through pathogen molecular pattern ( PAMP; Humphrey and Klasing, 2004 ). Because of this non-specificity, the innate response is generally faster than the acquired response ( Merlino and Marsh, 2004 ).
Acquired immunity is exquisitely target-specific which has immunological memory.
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