Genetic control of immune responsiveness in the chicken

Disease can be combated by medication, vaccination, hygienic measures, eradication and genetic resistance. Genetic resistance to infectious diseases is advantageous because of its permanent character in contrast with the aforementioned procedures. In the chicken genetic resistance to specific diseases like Marek's disease and lymphoid leukosis is well known. Despite this knowledge improvement of genetic resistance to specific diseases is not included extensively in breeding programmes yet. The need for infection of populations, the lack of knowledge of correlations with resistance to other diseases and production traits and the rare understanding of defence mechanisms are major drawbacks for application.A different approach to genetic improvement of disease resistance is the composition of a series of defence traits, defining general resistance to disease. The major histocompatibility complex, the B-locus in the chicken, may be such a valuable marker especially for resistance to viral oncogenesis. The capacity to produce antibodies to a multideterminant antigen, like sheep red blood cells (SRBC) is another possible marker trait. Selective breeding for this trait in mice has shown, that the effects are non-specific. In this thesis genetic and environmental aspects of the agglutinin antibody response to SRBC in poultry are discussed.The agglutinin antibody response to SRBC was polygenically determined. Heritability estimates for total antibody titre and 2-mercapto-ethanol (2-ME) resistant and sensitive antibody titres varied from non-significant to, usually. levels around .2 to .3 for the primary response in White Leghorn (WL), White Plymouth Rock (WPR) and ISA Warren populations. The heritabilities of a secondary response, measured in WPR and ISA Warren populations, were somewhat lower. The size of these heritability estimates offers a good perspective for mass selection as already shown in the first selection generation for high and low antibody production in our ISA Warren population. Moreover the repeatibility of this trait is very high, above .9.Phenotypic correlations between primary and secondary total antibody titres were not significant in a OR population. This result was explained by a positive relationship between primary 2-ME sensitive antibody titres and secondary 2-ME resistant antibody titres and a negative relationship between primary and secondary 2-ME sensitive antibody titres. Additive genetic correlations between total antibody titres of primary and secondary response were quite negative. If these negative genetic correlations will be confirmed by other research workers, then the choice of the primary antibody response to SRBC as a selection criterion for general disease resistance becomes a major concern.Differences between three stocks: WL, WPR and ISA Warren were found for antibody response to three doses of SRBC and for the cell-mediated response, the swelling of the wingweb post phytohaemagglutinin (PHA) injection. Genetic origin by dose of SRBC interactions were absent. However the response curve was different for each stock, indicating genetic origin by testday post injection interactions. Phenotypic correlations between total antibody titres to SRBC and PHA wingweb swelling were absent, overall and within stocks.The antibody response to SRBC is influenced by many non-genetic factors. Important non-genetic effects were dose of SRBC, age of the chick, primary versus secondary response, vaccination programme, interval between SRBC injection and titre determination. Desirable traits of the defence system in combating infectious diseases are immune competence at an early age, responsiveness to low doses of infectious agents. a quick response post infection and development of memory. The selection criterion, therefore, has to be clearly defined and the effects of selection upon these desired characteristics will have to be evaluated.Considering the heritabilities, the repeatibility and the manifestation at an early age, the haemagglutinin antibody response to SRBC offers a perspective for selection. The value of this trait for general disease resistance will have to be proven in the near future, when our selection lines for high and low antibody responsiveness have been established