Journal of Life Science and Biomedicine  
J Life Sci Biomed, 9 (5): 144-150, Sept 2019  
License: CC BY 4.0  
ISSN 2251-9939  
Systematic review on avian immune systems  
Mastewal BIRHAN1  
College of Veterinary Medicine and Animal Science, Department Veterinary Paraclinical Studies, University of Gondar, Ethiopia  
Corresponding author’s Email: ; ORCID: 0000-0002-0984-5582  
Review Article  
PII: S225199391900023-9  
Aim. The aim of this review paper is too summarized and compares avian immune systems  
to the other domestic animals as comparative immunology type of review. Appreciation of  
the avian immune systems and their functions are very critical for disease diagnostics and  
Rec. 06 June 2019  
Rev. 25 August 2019  
Pub. 25 September 2019  
new vaccine developments. Some of the avian immune systems are differ from mammalian  
immune systems, based on their production sources of immune cells like B-cells production  
site bursa of fabrics, but in mammalian is bone marrow. When we see the antibody type of  
birds; there are three principal classes of antibodies: IgM, IgG, IgY and IgA. Antibody  
diversity is achieved by gene re-arrangement. The other effector immune cell of birds is T  
cells. There are two distinct pathways that are α/β and γ/δ, avian T-cell diversity is probable  
made through combinatorial and junctional mechanisms. Recently, genes of several avian  
cytokines have been cloned and expressed. A number of naturally occurring viruses cause  
immunosuppression in chickens. Conclusion. There is much current interest in  
T cells,  
understanding the mechanisms of immunosuppression and developing strategies to Vaccine  
enhance immune responsiveness in commercial poultry.  
One of the wonderful rules in the poultry industry is to hardly working on disease and predator control, good  
institutional linkage, and with good management from the healthy birds it is possible to increased high  
productive efficiency, capacity and with it, economic profitability” [1]. Scientific research on poultry  
immunology and the diseases affecting avian species is not a new concept [2]. But, more recently, the chicken  
was the first agricultural species as an income sources for which indicted by a genome sequence map [3].  
Meaningful what specific immune molecules are encoded in the chicken genome delivers an outstanding  
background to form and magnify our information on the avian immune systems [2]. Comparable other avian  
immune systems, the immune system of chickens is made up of two types of mechanisms non-specific and  
specific [4]. The potential pathogen and other risks facing mechanisms are slight different from those come  
across by mammals. It is therefore essential that mechanisms be available to combat invading bacterial, viral  
and parasitic pathogens and to destroy neoplastic or other altered cells. It is also essential in birds, as in  
mammals, that the resulting immune response be regulated to ensure that it is adequate in quantity and quality  
We need to understand the chicken immune system, to familiarize you with those defense mechanisms.  
The bursa of Fabricius and the thymus organs are the central lymphoid organs in the chicken, essential to the  
development of adaptive immunity [6]. In bird’s poor of all bursal lymphoid tissue, but still holding a normal  
thymus, no circulating antibody was detected after challenge with different antigens. Delayed hypersensitivity  
reactions to tuberculin or vaccinia virus (VACV) were nearly completely inhibited [7].  
From the pronounced important avian organs, gut-associated lymphoid tissue is one of the organ that  
contains functionally immature T and B lymphocytes at hatch, and that function is achieved during the first 2  
weeks of age as demonstrated by mRNA expression of both ChIL-2 and ChIFNγ confirmed by Bar-Shira et al. [8].  
The gut is a vital organ system which makes up two equally important functions, that are digestion systems  
and host defiance [9]. When we address the chickens immune systems, the innate immunity includes physical  
barriers (skin, mucus coat of the GI tract), specific molecules (agglutinins, precipiacute phase proteins,  
lysozyme), phagocytic function of phagocytes (macrophages and neutrophils), and lysing activity of a class of  
lymphocytes called natural killer (NK) cells [10].  
In females birds, may improve their reproductive victory by mediating brotherly competition and growth  
of offspring by means of differential hormone transfer to the egg yolk [11] [12] and [13]. For example, differential  
Citation: Birhan M. 2019. Systematic review on avian immune systems. J. Life Sci. Biomed. 9(5): 144-150;  
transfer of steroids to eggs within the same grasp may alleviate or intensification the effect of hatching  
asynchrony as yolk steroids enhance nestling growth and competition [14] and [13]. Yolk testosterone was also  
present in the eggs of female canaries that were kept without a male, indicating that it is of maternal origin [11].  
Birds are born with an imperfect immune system and young chicks have to rely on maternal antibodies and the  
innate immune defiance system to fight off pathogens [15].  
While the avian system shares several similarities with mammalian systems, there are differences in the  
genes and molecules involved, the cells and organs involved, as well as the functional mechanisms. Chickens, for  
example, have a different assortment of Toll-like receptors, defensins, chemokines and antibodies. Birds do not  
have eosinophils though the functional corresponding to the mammalian neutrophil is the avian heterophil.  
Birds do not have lymph nodes, but do have a Bursa of Fabricius, which mammals do not. The mechanisms by  
which the different receptors are generated are also fundamentally different [16]. Therefore, the aim of this  
review paper are compering and analyzing of how the avian immune systems, structures organization, cells and  
organs differ from the other domestic animal immune systems.  
Studies that comprehensive study type, as a comparative method to the immunology with an gratefulness for  
physiological ecology and evolution are defining an important new field in biology-ecological immunology [17].  
Though in wide-ranging terms the avian immune response is strangely similar to that of mammals, when one  
looks at specifics birds have a different repertoire of immune organs, cells and molecules compared to those  
characterized in mammals.  
The unique structures of chickens are adversely distresses by heat stress, so, due to this impressions  
reeducations of productive performance, immune response, survival and profitability of fast growing chickens  
[18]. Beyond the beneficial features, the risk regarding the development of antimicrobial resistance and  
transference of antibiotic resistance genes from animal to human microbiotaled the European Union to ban the  
application of antibiotics as growth promoters since1st January 2006, which was followed by the other parts of  
the world including North America [19].  
Avian are extremely vulnerable to varies infection by opportunistic pathogens during the first few days  
after hatching [20]. In avian species, adaptive immunity encompasses both humoral and cell-mediated immune  
(CMI) responses [21]. The avian embryo provides numerous compensations for studies on development of the  
immune system [22]. The bird egg is worthily adapted to house, feed, and protect the developing embryo. The  
outer lime-flavored shell and adherent shell membranes provide a physical barrier that excludes most  
microorganisms, but permits free exchange of respiratory gases [23]. Interior to the shell membranes is a thick  
zone of albumen that provides a sterile fluid medium for the free growth and morphogenesis of the embryo and  
the extra embryonic membranes. In the center of the egg is the yolk mass that will nourish the embryo through  
the incubation period [24].  
Commonly, Birds are lack organized lymph nodes, yet have the Bursa of Fabricius. Birds lack neutrophils  
and functional eosinophils, yet have a distinct group of polymorph nuclear granulocytes known as heterophil.  
Birds also have a different repertoire of cytokines, chemokines, Toll-like receptors, defensins and integrin’s [25].  
Innate cells  
The innate immune system develops in the bone marrow (BM) from common myeloid progenitors (CMPs).  
Due to the expression of AR in hematopoietic progenitors, there is reason to believe testosteronemay play an  
important role in shaping the immune cell repertoire even prior to the cells leaving the BM [26].  
Macrophages instigate from bone marrow stem cells by differentiating into monoblasts, promonocytes,  
and monocytes. However monocytes establish foremost phagocytic cellular component in chicken blood, tissue  
macrophages are extensively dispersed and present in almost every organ. Monocyte cultures from peripheral  
blood leukocytes can be established by incubating the leukocyte fraction on a solid substrate such as Petri  
dishes or glass coverslips. The adherent blood monocyte cultures can then be established by incubation and  
washing off the non-adherent cell fractions [27]. The two most commonly used avian macrophages cell lines are  
Citation: Birhan M. 2019. Systematic review on avian immune systems. J. Life Sci. Biomed. 9(5): 144-150;  
MQ-NCSU. and HD11, an avian myelocytomatosis virus (MC29) transformed chicken macrophage-like cell line  
[28]. The MQ-NCSU cell line was established from spleen of a broiler-type chicken experimentally challenged  
with the JM/102W strain of Marek’s disease virus. The cultural, morphological and functional characteristics of  
the MQ-NCSU cell line imply that this is a malignant-transformed chicken cell line belonging to the  
mononuclear phagocyte lineage. Avian macrophages harvest chemotactic cytokines of both macrophage  
inflammatory protein (MIP) families. The chicken MIP-1 and MIP-2 chemokines have the identical amino acid  
motifs as mammalian chemokines: adjacent cysteine’s (CC) in the MIP-1 chemokines and cysteine’s separated by  
another amino acid (CXC) in the MIP-2 family. The chicken MIP-2 family chemokine is currently designated as  
9E3/CEF4. It has high homology to mammalian interleukin (IL)-8 and is abundantly expressed by activated  
peripheral blood monocytes ([29]; [30] and [31].  
Pattern recognition receptors (PRRs) are a serious component of pathogen recognition in both mammals  
and chickens [32]. Toll-like receptors (TLRs), a main family of PRRs, are expressed in chicken intestinal tissues  
and the local immune cells have been shown to respond to bacterial ligands [33]. Influxes of heterophil as well as  
increases in cytokines and chemokines are evident [34] and [35] and are thought to contribute to the pathology  
detected. However, once chicks are more than a few days old, S. Typhimurium persistently colonizes their  
intestines in the absence of pathology [36], signifying that maturity of host defenses contribute to the deficiency  
of clinical signs  
In chickens, it has been established that heterophil constitutively express TLR2A, TLR2B, TLR1/6/10 mRNA  
and that heterophil isolated from neonatal chicks and exposed to LTA undergo an oxidative burst [37]. There are  
also data to suggest that CD14 and TLR2 mediate LTA-stimulated oxidative burst in heterophil [37]. Chicken  
TLR3 expression pattern appears to be similar to what is observed in mammals [33]. For example, chicken  
heterophil express TLR3 and are approachable to poly I:C, demonstrated by an induced oxidative burst and  
degranulation of the stimulated heterophil, which may be mediated by a signalling pathway involving  
phospholipase C, phosphatidylinositol 3-kinase and intracellular Ca2++ [38]. In contrast, others have confirmed  
the poor ability of poly I:C to stimulate nitric oxide (NO) production in chicken monocytes, while HD11cells, a  
chicken macrophage cell line, were readily stimulated to produce NO by poly I:C [34] and [39].  
The first groups of TLR are expressed on the cell surface and recognize primarily cell-surface PAMPs. They  
include TLR1, TLR2, TLR46 and TLR10 in human and TLR11 in mice. There are direct chicken orthologous of  
mammalian TLR4 and TLR5 [40] and [33]. In mammals, there is a single TLR2 gene, and the genes encoding  
TLR1, 6 and 10 lie in a single locus. Mammalian TLR2 forms functional heterodimers with at least TLR1 and  
TLR6, allowing recognition of a wider panel of pathogen associated molecular patterns (PAMPs). At the  
equivalent locus to the mammalian TLR1, 6 and 10 locus, the chicken genome encodes only two genes, TLR1LA  
and TLR1LB [41] and [42]. Avian TLR repertoire and the response to various agonists [43]. Toll-like receptors  
(TLRs) are important for eliciting innate immunity in animals by playing an essential role as pattern  
recognition receptors that detect infectious pathogens by recognizing the conserved molecular structures  
known as pathogen associated molecular patterns [44]. There are ten avian toll-like receptors and that five of  
these, TLR2a, 2b, 3, 4, 5 and 7, are clear orthologous to TLRs found in mammals [45]. The non-mammalian TLR21  
exists in many species of birds, fishes, and frogs [46] and [47]. As a homologue of mammalian TLR9, TLR21 can  
recognize synthetic oligo-deoxy-ribo-nucleotides (ODN) and DNA viruses that contain CpG motifs, which  
further trigger the innate immune response [46] and [48]. The RLR family encompasses three members: RIG-I,  
melanoma differentiation-associated gene 5 (MDA5) and research laboratory of genetics and physiology 2  
(LGP2), which are located in the cytoplasm  
Cell mediated immunity and humeral immunity  
Chicken αβ T cells express either CD4 or CD8 accessory molecules, whereas most of the γδ T cells do not  
[49]. The cytotoxic T lymphocyte response can decrease viral shedding in mildly pathogenic avian influenza  
viruses, but provides doubtful protection against HPAI. Influenza viruses can directly affect the immune  
response of infected birds, and the role of the Mx gene, interferon’s, and other cytokines in protection from  
disease remains unknown [50]. Avian T cell progress has emerged with the use of monoclonal and functional  
antibodies to elucidate T cell differentiation antigens and molecular and functional explanations of mammalian  
Citation: Birhan M. 2019. Systematic review on avian immune systems. J. Life Sci. Biomed. 9(5): 144-150;  
T cell receptors (TCRs) [51]. Avian T cells bearing a γδ TCR are the first to be generated during ontogeny and  
they comprise up to 50% of the recirculating T-cell pool in mature birds [52].  
Progress of B cells in chickens proceeds via a series of disconnected developmental phases that includes  
the maturation of committed B cell progenitors in the specialized microenvironment of the bursa of Fabricius  
[53]. Three classes of chicken immuno globulins have been identified immunochemically [54] and genetically  
[55] as homologues to the mammalian IgM, IgA and IgG, and their organizational properties have been reviewed  
in more detail elsewhere [56]. The intestine is a complex tissue that includes a major immune constituent.  
Indeed, the numbers of immune cells found in intestinal tissues exceed the numbers found in the rest of the  
body [57] and [58].  
Expression of selected genes involved in pathogen detection and the innate immune response were  
profiled in caecal tissues by quantitative RT- PCR. TLR4 and TLR21 gene expression was transiently increased  
in response to both bacterial species [59] and [60]. Defense of the intestinal mucosal surface from enteric  
pathogens is initially mediated by secretory IgA (SIgA) [61].  
Three classes of chicken immunoglobulin’s have been identified immunochemically [62, 63]and genetically  
[64] and [65] as homologues to the mammalian IgM, IgA and IgG and their structural properties have been  
reviewed in more detail elsewhere [66]. Chicken IgM is structurally and functionally homologous to its  
mammalian counterpart, being current in serum as a high molecular weight pentamer of m2L2 units and being  
the first antibody generated during a primary antibody response. IgM is also the major class of immunoglobulin  
expressed on the surface of chicken B lymphocytes [67].  
Chemokines and Cytokines  
Interferon’s (IFNs) are a family of multifunctional cytokines with significant roles in cellular resistance  
against viral infection [68]. In response to virus invasion, host pattern recognition receptors (PRRs) detect  
pathogen associated molecular patterns (PAMPs) and subsequent activation of innate immune system through  
retinoic acid inducible gene I (RIG-I) like receptors (RLRs)-MAVS-dependent IFN signaling or toll-like receptors  
(TLRs)-TRIF/MyD88-dependent IFN signaling [69], eventually, inducing the expression of type I IFNs. IFNs then  
bind their cognate receptors, triggering a signaling cascade that outcomes in the expression of abundant  
interferon-stimulated genes (ISGs) by the JAKSTAT signaling pathway, various of which possess antiviral  
properties [70] and [71]. Interferon regulatory factors (IRFs), a family of transcription factors, play authoritative  
roles in the regulation of IFN expression during viral infection [72]. To date, 9 IRF genes (IRF1-9) have been  
described in mammals, a tenth (IRF10) is present in numerous avian species and a total of 11 IRFs (IRF1- 11) have  
been identified in fish [68].  
The chicken, perhaps surprisingly, has made several influential contributions towards our understanding of  
immune responses as comparable way. Notwithstanding this, before the chicken genome sequence, our ability  
to study immune systems in detail in birds is appropriate and also in our thoughtful of the immune gene  
catalog. There are still gaps, both in the chicken immuno systems and their catalog. In comprehensive study is  
well important by comparing of birds immune systems with the other animals. Both innate and adaptive  
immune responses, with the latter including both cell-mediated and humoral immune responses, leading to  
address and increases our knowledge about chicken’s immune activity. However, looking at the organs, cells,  
and molecules of the immune response in birds, it appears that mammals and birds accomplish the equivalent  
overall responses-often in quite different ways. Instead, we concentrate on the basic immune response, as well  
as a description of the major cell types and major areas where the cells and molecules of the immune response  
differ from those of mammals. Generally, the immune system of the chicken is very helpful in avoiding disease  
and helping to insure maxi-mum productive potential is realized. We must learn how to take advantage of all  
parts of the system when designing health programs. Based on the above information’s the following  
recommendation will be forwarded:  
Researcher should be focus on the avian immune systems and their role of contributions, and the  
delineation of the bursal and thymus-derived arms of the immune system.  
The genes of several avian cytokines have been cloned and expressed; so that scientists would be given  
an attention for new disease resistant gene formed.  
The researcher should be emphasis on current attentiveness in thoughtful the mechanisms of  
immunosuppression and developing approaches to advance immune responsiveness in commercial poultry.  
Citation: Birhan M. 2019. Systematic review on avian immune systems. J. Life Sci. Biomed. 9(5): 144-150;  
The authors’ heartfelt thanks to University of Gondar, research and community service v/president office,  
college Veterinary Medicine and Animal sciences for the financial and resource supporting  
Authors' contributions  
Mastewal Birhan conceived the review, coordinated the overall activity, and write and submit the  
Availability of data and materials  
Data will be made available up on request of the primary author  
Consent to publish  
Not applicable.  
Competing interests  
The authors declare that they have no competing interests.  
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Citation: Birhan M. 2019. Systematic review on avian immune systems. J. Life Sci. Biomed. 9(5): 144-150;