Haematology and blood chemistry
Blood indices are used as an index of the health status of laying birds and they are influenced by dietary factors. Studies by Yalcin et al. (2008b) have found that dietary YC supplementation supports serum protein, lipids and aspect of liver enzymes in layers. Similar findings were also reported by Sanaa (2013) and Yalcin et al. (2014) who reported that addition of 0.3% and 0.6% dried yeast in chickens support the production of serum protein, lipids, and aspect of liver enzymes. In similar studies, Wakwak et al. (2003) and Maziar et al. (2007) observed no significant effect of yeast on serum proteins and cholesterol, respectively while inclusion of 0.2% of YC in laying hens ration containing either soybean meal or sunflower meal had comparable serum aspartate transaminase (AST) and alanine transaminase (ALT) (Yalcin et al. 2008a), and this implies that there was no deamination of amino acids. This result corroborated Hewida et al. (2011), who reported that YC had a similar influence on serum proteins and serum creatinine. Others studies have noticed the nonsignificant effect of yeast on blood cholesterol in laying hens (Stanley et al. 2004; Bageridizaj et al. 2006; Pinar et al. 2013). A similar study in quails fed with 1 or 2% yeast supplementation, Ghally and Abd El-Latif (2007) recorded improved blood constituents (serum proteins, AST and ALT). Contrary to these findings, some investigators have shown that addition of yeast on hen diets significantly reduce the concentrations of serum cholesterol (Mahdavi et al. 2005; Yalcin et al., 2008a, 2010, 2014; Hassanein and Soliman 2010) and serum triglycerides (Yalcin et al. 2014). The observed variations in these studies may be attributed to the variety of the yeast forms or different experiment conditions (etc: environmental stress).

Immune status of laying chickens
The gut and its inhabiting microorganisms play a vital role in shaping the body defense system in poultry (Diarra et al. 2011). Yeast cell wall products (chitin, mannan, and glucan) are known to stimulate the gut defense systems, increase immunoglobulin levels (Abaza et al. 2008), and white blood cell count (Abdollahi et al. 2002). The immune (Toms and Powrie, 2001; Cotter et al., 2002) stimulation potential of yeast products in breeders and laying hens have been reported. Kabir et al. (2004) and Maziar et al. (2007) investigated the influence of yeast on the body defense system of chickens and observed increased antibody production (p

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