It is therefore always difficult to understand the broader modulating influence for instance on the productive parameters. The ability of certain fatty acids to influence the immune system and the function of its various cellular components have been recognized for nearly 30 years. The first review of this field was published by Meade and Mertin (1978). During this same period, several researchers demonstrated that consuming diets high in fat tends to suppress immune responses, such as phagocytosis and infectious disease resistance (Palmblad and Gyllenhammar, 1988). Another hypothesis that emerged during this early period was that certain dietary fats induced changes in immune cell membranes. Consumption of n-3 PUFA, resulted in incorporation of these fatty acids into lipid membrane of all tissues, including cells of the immune system (Cinader et al., 1983; Huang and Fritsche, 1992). Changes in membrane dependent functions, such as phagocytosis and cell signaling, were thought to be a direct consequence of alterations in membrane composition and “fluidity” (Peck 1995). 

Kinsella et al., (1990) in his review indicated that dietary fat source affects both cellular fatty acid composition and subsequent eicosanoid biosynthesis. Eicosanoids are fatty acid metabolites which act as a mediators of inflammation and immune cell function. Eicosanoids are primarily derived from arachidonic acid (AA), typically the most abundant PUFA in immune cell membranes. In the 1990s, researchers conducted work on cytokines, which are protein-based regulators of inflammation and immune cell function (Hill and Sarvetnick  2002). It was noted that association existed between FA modulation of inflammatory and immune responses with changes in cytokine production (Blok et al., 1996, Calder,  1997, Endres and  Schacky, 1996). The existing evidence indicates that dietary FA modulate immune responses through one or more of three major molecular mechanisms (Fritsche, 2006) : (a) altered membrane composition and function, (b) modified eicosanoid production, and (c) changed cytokine biosynthesis.

Long chain n-3 PUFA showed to improve the immune response and reduce inflammation in different species such as chicken, mice and fish (Calder, 2006; Wall et al., 2010). Recently, there has been some concern that diets enriched  with n-3 PUFA may have  detrimental effects on chicken immunity and impair resistance to infection. However, it is not clear whether this concern is justified, given that some studies show no effect (Puthpongsiriporn and Scheideler, 2005), some show a detrimental effect (Fritsche et al., 1991a, Babu et al., 2005), and some show an improvement (Phipps et al., 1991; Korver and Klasing, 1997; Parmentier et al., 1997; Sijben et al., 2000; and Scheideler, 2005; Yang and Guo, 2006).

Recent investigations on the effect of nutrients on immunity are related to the increase in antibody (Ab) responses of young chickens, which have poor immunity against some diseases even with vaccination. Most of these studies have focused on dietary PUFA (Sijbenet al., 2000; Puthpongsiriporn and Scheideler, 2005). Two major classes of immunomodulating PUFA are n-6  and n-3 fatty acids. PUFAs of the n-3 series are considered to be beneficial because eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are major precursors of eicosanoids that are less inflammatory than those of the n-6 series. The principal precursor of n-3 family is α-linolenic acid (ALA; C18:3), whereas linoleic  acid (C18:2) and arachidonic acid (AA; C20:4) are the forms of n-6 PUFA. These PUFA compete to react with eicosanoid synthesis enzymes. Eicosanoids are important regulators and influence various immune responses (Kinsella, 1991; Sijben et al., 2000). Linoleic acid and ALA may induce immunomodulatory functions after elongation and desaturation to AA and docosahexaenoic acid (DHA), respectively, which are incorporated into phospholipid membranes in immune cells. Phospholipase causes the release of these long-chain PUFA, which change to active communicators of the immune system such as prostaglandins, leukotrienes, and thromboxanes (Guo et al., 2004). Fish oil is rich in n-3 PUFA and consists mainly of eicosapentaenoic acid (EPA) and DHA; conversely, flaxseed is rich in ALA n-3 PUFA. Calder (1996) fed mice dietary FO and observed decreased production of interleukins and tumor necrosis factor that are required for lymphocyte development. However, production of IgG and IgE was enhanced by using high levels of FO (EPA and DHA) in rat diet. It has been demonstrated in chickens that in vitro spleen lymphocyte proliferation was significantly suppressed through n-3 PUFA, ALA, or EPA and DHA (Fritsche et al., 1991). In broiler chickens, feeding fish oil produced more Ab in response to SRBC compared with maize oil supplementation diets rich in n-6 PUFA (Fritsche et al., 1991).

Al-Khalifa et al. (2012) also suggested that eicosanoids regulate the production of cytokines (Miles and Calder, 1998). If n-3 PUFA are present, EPA compete with AA, inhibit arachidonic acid’s oxygenation and direct the metabolic pathway toward leukotrienes, hydroperoxy-eicosatetraenoic acid, hydroxyl-eicosatetraenoic acids, and lipoxins by the action of 5-, 12-, or 15-lipoxygenase (Yaqoob and Calder, 1995; Calder, 1997, 1999, 2005, 2006, 2008; Calder et al., 2002; Yaqoob, 2010). Some of the immunomodulatory effects of n-3 PUFA may result from effects on intracellular signaling pathways and transcription factor activity (Calder, 1998a, 2002, 2005; Calder and Burdge, 2004) and on lipid rafts (Cheng et al., 1999; Katagiri et al., 2001; Heerklotz, 2002), and suggested that the immunomodulatory effect of n-3 PUFA may be mediated by their effect on lipid raft structure and composition.

Supplementation of dietary n-3 PUFA has shown to increase immunoglobulin, suggesting an immune-stimulating property of n-3 PUFA. Immunoglobulins production by beta cells and interferon gama is facilitated by interlukin-2. This elevation is more pronounced in DHA rich diet, indicating positive response of PUFA enrichment of diets (Sugano et al., 2000). Puthpongsiriporn and Scheideler (2005) used flaxseed and maize oil to formulate n-6:n-3 ratios. Their results showed that dietary ratio of 2 had greater Ab responses against Newcastle disease (ND) virus vaccine compared with those fed diets with ratios of 4, 8, and 17. Ebeid et al. (2008) indicated that dietary FO levels below 3.5% increased the Ab titer in laying hens. Likewise, Yuming et al. (2004) proved that the Ab levels were higher in hens fed oils rich in n-3PUFA (FO or LO) than in hens fed maize oil rich in n-6PUFA. The apparent discrepancy in findings may be attributable to the result of interaction between n-6 and n-3 PUFA, different actions of dietary PUFA between antigens, and the nature of the antigens (Sijben et al., 2001). Because the metabolisms of n-3 and n-6 are intertwined, the effect of one PUFA depends on the level of all diet fatty acids.

The main immune organs in poultry are the thymus, spleen, and bursa of Fabricius. During an immune response, mature lymphocytes and other immune cells interact with antigens in these tissues. Consequently, immune tissue mass can in some cases indicate immune status (Grasman, 2002). Wang et al. (2000) reported that layers chicks fed sunflower oil, animal oil, linseed oil, or fish oil at 5% (wt/wt) had significantly higher weights of the thymus, spleen, and bursa compared with those of chicks fed the diet with animal oil. Wang et al. (2000) also indicated that feeding laying chickens diets rich in n-3 PUFA promoted the growth of the thymus, spleen, and bursa up to 4 wk of age. However, from the age of 4 wk onward, immune tissue weights began to decline, and the bursa degenerated between 4 and 8 wk of age. Nevertheless, they suggested that changes in the weights of the thymus and spleen did not correlate with the immune function. Simliar findings were reported in the thymus and spleen of mice fed n-3 PUFA diets (Ellis et al., 1986; Huang et al., 1992). Studies in chickens report inconsistent effects of dietary n-3 PUFA on subsets of immune cells (Wang et al., 2000, Yang and Guo, 2006, Yang et al., 2008). Al-Khalifa et al., (2012) reported that increasing levels of FO did not affect the weights of the spleens of broiler chickens. They further added that chickens fed diets containing 50 g/kg of FO had significantly greater thymus weights compared with chickens fed 0, 30, or 60 g/kg of FO. Higher levels (50 and 60 g/kg) of FO had significantly lowered the bursa weights than those of chickens fed diets containing no or 30 g/kg of fish oil (Al-Khalifa et al., 2012).

Effects of n-3 PUFA on phagocytosis are inconsistent, with some studies showing enhanced phagocytosis, some showing a decrease, and some showing no effect. There are no published studies investigating the effect of dietary n-3 PUFA on phagocytosis in chickens. However, Al-Khalifa et al., (2012) reported non-significant effect of FO on phagocytosis by heterophils. The suppressive effects of dietary n-3 PUFA on lymphocyte proliferation are also reported in mice (Pompos and Fritsche, 2002; Barber et al., 2005; Kim et al., 2008), rats (Jeffery et al., 1996; Peterson et al., 1998a;1998b), and chickens (Fritsche et al., 1991a; Wang et al., 2000; Yang et al., 2008). The enhanced phagocytic activity in rodent was explained on the basis that the FA composition of cell membranes affects their fluidity as well as changes in membrane-bound enzymes and receptor functions (Calder and Newsholme, 1993). Calder et al. (1990) and Calder (1998b) on the basis of in vitro experiments suggested that membrane fluidity is important in determining macrophage adhesion and phagocytic activity upon enrichment with FO derived FA. However, some studies report that n-3 PUFA  incorporation into cell membranes following dietary supplementation does not affect membrane fluidity (Yaqoob, 1993; Grimble and Tappia, 1995).

Al-Khalifa et al., (2012) noted immunosuppressive effect of feeding broiler chickens diets containing 50 g/kg of fish oil on the proliferative response of splenocytes and thymocytes. The similar findings of immunosuppressive effect were also reported by 5 other studies in chickens (Cassity et al., 1990; Fritsche et al., 1991a; Wang et al., 2000; Yang et al., 2008). However, three studies indicated increased lymphocyte proliferation

after feeding chickens diets rich in FO (Sijben et al., 2000, 2001; Babu et al., 2005) and two studies indicated no effect (Korver and Klasing, 1997; Puthpongsiriporn and Scheideler, 2005). The contradictory results might be because of different species and strains, age of birds, basal diet, level and type of oils used, type of stimulant, immune status of the animal under study, cell type, ratio of n-6 to n-3 PUFA, and different fatty acids of n-3 PUFA used for various studies. Yaqoob (2010) also attributed the inconsistent results in human studies to the doses of fatty acids used, levels of EPA and DHA in different preparations of FO, differences in methodological approaches used, and the different range of immune function parameters under investigation.

In light of the above paragraphs, it may be concluded that poultry diets enriched with n-3 PUFA might have the potential to modulate the avian immune response, and thus, affect the bird’s ability to resist invading pathogens. However, feeding chickens diets rich in n-3 PUFA suppresses some aspects of the immune response that are considered to be important lines of defense against tumor, viral, bacterial, and other infections. More work is required in animal models of pathogen exposure to evaluate this in a relevant pathological setting. This further highlighted the need for the poultry industry to consider the risk of infection when poultry products are being enriched with PUFA.