Serum antibody responses and haemolytic complement activity were evaluated in White Leghorn (WLH) and Rhode Island Red (RIR) chickens that were vaccinated with live-attenuated vaccines of Newcastle disease virus, or infectious bronchitis virus, or infectious bursal disease virus by means of ocular challenge at 10 times the normal vaccination dose. Complement titres in non-vaccinated birds were significantly higher in WLH birds compared to RIR birds. The lentogenic viral infection resulted in an immediate stimulation of complement activity, followed by a decrease to initial complement levels within 2 weeks post vaccination, when the antibody response took over immune defence. As compared to WLH chickens, RIR birds mounted a faster and significantly higher antibody response to the vaccine viruses used. In WLH hens, significantly higher haemolytic complement activity post vaccination was found as compared to RIR hens. Possible consequences of the observed differences in immune responsiveness of the two breeds to viral vaccines are discussed.

A total of 376 chickens from different ecotypes were immunized with the non-pathogenic multi-determinant antigen sheep red blood cells (SRBC). The ecotypes included indigenous chickens from various locations in Tanzania (n=102), India (n=86) and Bolivia (n=89). In addition, eight German Dahlem Red (GDR) chicken lines with different major genes (dwarf, naked neck and frizzled) of tropical interest were also immunized with SRBC. Immune competence of the breeds was assessed by measuring complement haemolytic activity, both from the classical calcium-dependent complement pathway (CPW) and alternative calcium-independent complement pathway (APW), alongside IgTotal, IgG and IgM antibody responses to SRBC at 7 days post immunization. Large variations in complement activity and antibody responses to SRBC were observed within and between the indigenous breeds. Many indigenous chickens, especially from Bolivia, showed decreased complement activity (APW) following immunization with SRBC. Breeds from India showed the highest CPW activity and humoral (especially IgM) responses to SRBC, suggesting high immune competence. In contrast, Bolivian chickens were characterized by low CPW activity, low APW activity and low antibody levels to SRBC suggesting an overall low immune competence. In the GDR chickens, characterized by high CPW activity and high IgG antibody responses to SRBC, the major genes for naked neck, frizzling and dwarfism had no significant effect on the antibody responses and complement activity to SRBC.

To study the relation between serum complement levels and the chicken MHC (B) complex, complement haemolytic activity was measured in sera from hens from seven pure-bred B-typed White and one Brown Leghorn lines, and three ISA-Warren lines that had been divergently selected for antibody responses to sheep red blood cells (SRBC). Significant differences occurred in the serum haemolytic complement activities, both belonging to the classic (CPW) and the alternative (APW) pathways, among the 11 different haplotyped chicken lines. Hens with high CPW and high APW titres predominantly displayed the B2 or B21 haplotypes. Chickens with low CPW and APW were found in B14 and B15 haplotypes. Haplotype B14 appears to be different in complement levels when present into the pure-bred lines or into the ISA-Warren line selected for low antibody responses to SRBC. Otherwise, the presence of B21 in ISA-Warren line selected for high antibody responses to SRBC does not differ with the B21 in the inbred lines (except in the NL-line for CPW values). In general the haplotypes B2 and B21 are found in chicken lines with enhanced disease resistance, and the B15 haplotype has been connected with enhanced disease susceptibility. Our results suggest that levels of haemolytic complement activity, either from the classical or from the alternative pathways, may underlie part of the immunocompetence ascribed to the MHC (B) complex in chickens.

German Dahlem Red chickens with three different major genes of tropical interest: Nana– (naked neck), Ff– (frizzled) and dw– (dwarf), respectively, were tested for serum haemolytic complement, which is essential in innate host defence against infectious agents. Eight different combinations of genes for body size and feather coverage were evaluated. Significant differences both for both the calcium-dependent (classical, CPW) and the calcium-independent (alternative, APW) complement titres were found between the phenotypes. Phenotype nanaffDw– showed the highest complement status. The frizzled (Ff–) gene had a negative influence on APW titres, whereas the dwarf (dw–) gene had a negative influence on CPW titres. The naked neck (Nana–) gene had various influences on the haemolytic complement status. All tested hens had MHC (B) 21 haplotypes, whereas the gene for dwarfism appeared to be linked with the B19 haplotype. It was concluded that introducing major genes (Nana–, dw–, Ff–) to conquer environmental stress in hot climates can have a negative impact on certain aspects of the innate immunity of poultry.

Titres of classical (CPW) and alternative (APW) complement pathways were measured in clinically healthy local chicken populations (ecotypes) from Africa (Benin, n = 78; Cameroon, n = 299; Tanzania, n = 101), Asia (India, n = 96) and South America (Bolivia, n = 64). A wide variation was found in haemolytic complement levels between the various ecotypes. Distributions of the classical and alternative complement titres were not normal but were skewed to the right. Differences in complement were found both within and between ecotypes. Furthermore, CPW titres of the indigenous chickens were lower than those determined in commercial layer chickens. This suggests that complement levels in indigenous hens are not solely dependent on local antigenic pressure but may also depend on genetic background and husbandry. The relationships between complement levels, the chicken MHC(B) complex, environmental antigenic pressure, and survival of the scavenging local chickens are discussed.