Dendritic cells (DCs) are the main antigen-presenting cells, capable of endocytosis, exocytosis, and cytokine production. In humans, DCs are composed of two major populations: conventional DCs (cDCs), previously called myeloid dendritic cells, which are further subdivided into two major subpopulations, cDC1 and cDC2, and plasmacytoid DCs (pDCs). cDCs are the professional antigen-presenting cells of the innate immune system. They reside in tissues, and after tissue infection or injury, they become activated and migrate to lymph nodes to promote adaptive immune responses (41). pDCs are distinct from cDCs in terms of their development and function. They arise from the lymphoid progenitor and their main function in the human immune response is type I and type III interferon secretion upon acute or chronic viral infection (42).

In both chickens and humans, DCs act as a bridge between the innate and the adaptive immune responses, though many differences are described in these heterogeneous cell types. For instance, a specific subset of DCs, present in birds but not in mammals, exist: the bursal secretory DCs, located in the bursa of Fabricius (43). Overall, chicken DCs have high plasticity, and all of these cell subsets share structural and functional differences with humans, from regulating central tolerance to shaping the adaptive immune response (44). A comprehensive review previously summarized the ontogeny, cytoarchitecture, and immunophenotype of avian DCs (45). To this day, knowledge of avian DCs still appears to be limited compared to that of mammals. Nevertheless, many aspects of mature DCs, including their morphology, their surface receptors, their response to stimuli, and their respective functions, seem to be highly similar between chickens and humans (45–47).

Chicken cDCs are the most thoroughly studied dendritic cell subset. Indeed, comparative gene expression profiling has been performed in chickens and humans, which demonstrated that cDC subsets are homologous (48, 49). A recent study also revealed the enrichment of cDC lineage in the chicken spleen (50). Still, a key aspect should be further investigated: the location of antigen presentation to lymphocytes. Indeed, while this phenomenon generally appears in vertebrates’ secondary lymphoid organs, it occurs mostly in lymph nodes in humans, which chickens lack as mentioned earlier. This location is still not properly identified in chickens yet, although it is hypothesized that lymphoid nodules, and several novel lymphoid structures and aggregates, might be involved in antigen presentation (46).

Macrophages are an essential component of the innate immune system. Like dendritic cells, macrophages can present antigens to T cells. Their main function is to phagocytose cellular debris, pathogens, and damaged cells, but they also play a crucial role in tissue homeostasis and the host immune response by mediating inflammation (51). Under normal physiological conditions or during inflammation, circulating monocytes, the precursors of macrophages, migrate into an appropriate location according to chemotactic signals. After cytokine-mediated differentiation, mature macrophages are then capable of destroying a pathogen, producing cytokines capable of modulating other inflammatory cells, and secreting various growth factors to promote angiogenesis and tissue repair (52). Depending on the various mediators and cytokines present in their environment, immature macrophages from humans and chickens can be differentiated schematically into two main types of macrophages: M1 and M2. However, macrophages being extremely plastic, it is not possible to simply divide them into two phenotypes but are rather characterized by a range of intermediate phenotypes between the extremes (53, 54). In both species, while M1 macrophages stimulate inflammation, mainly by secreting IL-1β, TNF-α, and IL-6, M2 macrophages instead stimulate cell reparation and angiogenesis through secretion of IL-10, TGF-β, and VEGF (55–58).

Similar to humans, functional studies on chicken macrophages indicate the presence of scavenger receptors, complement receptors, Fc receptors, C-type lectins, and mannose receptors, all mediating the recognition of antigens before phagocytosis (59–62). Moreover, the production and release of nitric oxide, an important microbicidal mechanism of activated macrophages in humans, has been described in chicken macrophages with high homology (63–65). Finally, as in humans, IFN-γ can induce polarization of immature chicken macrophages into the pro-inflammatory M1 phenotype, while IL-4 can induce it into the anti-inflammatory M2 phenotype (53, 54, 66). However, although the mammalian peritoneal cavity contains non-activated resting macrophages, these cells are mainly absent in chickens. Instead, avian macrophages must be recruited from the bloodstream into the cavity by inflammatory stimuli (46). Interestingly, a recent study characterized several different macrophage subsets in the chicken spleen that may play an important role in antigen presentation and immune responses (50).

Granulocytes are described as non-specific immune cells containing granules (with defensins, lysozymes, histamines, etc.) in their cytoplasm. They are derived from common myeloid progenitors in the bone marrow, and they also contribute to the initial innate immune response against pathogens. In mammals, they can be divided into three subsets, with structural and functional differences: neutrophils, eosinophils, and basophils (67). Neutrophils are highly heterogeneous and the most abundant type of myeloid cells (68, 69), representing 50-70% of the total circulating human white blood cells. Neutrophils are released into the bloodstream with as a main function to phagocytose infected cells and destroy pathogens through degranulation (70). Eosinophils, however, represent a small number of circulating leukocytes (1-6%) and are mainly involved in fighting parasitic infections, but also in allergic diseases (71). Finally, basophils, accounting for less than 1% of total blood leukocytes, can release histamine and other mediators, which contribute to the inflammatory response (72). All these cells’ subsets migrate to peripheral and lymphoid tissues during inflammation.

Even though granulocytes represent essential cells of the chicken immune system, they do differ from mammals. Indeed, chickens lack neutrophils, but they do have a functional counterpart called heterophils. These cells are highly phagocytic as well, but in contrast to mammalian neutrophils they lack myeloperoxidase, they have a lower bactericidal activity through oxidative burst, and their granule components seem to differ (73). It is however still unknown whether heterophils represent a single set of cells with identical functions, or whether they encompass functionally different subsets of cells (74). Despite these differences, the biological functions of avian heterophils seem to be highly similar to mammalian neutrophils. They are the first cell type recruited to the inflammation site in response to chemokines. They express pattern recognition receptors (mostly Toll-like receptors) at their surface to recognize antigens, and they can eliminate pathogens through oxidative burst, degranulation, and extracellular traps (47, 75–77).

Even though eosinophils are present in chickens, they have yet to be demonstrated to be functional. For instance, IL-5 expression and IgE production, both essential for the response and function of mammalian eosinophils, are absent in chickens. Furthermore, eotaxins (CCL11, CCL24, and CCL26) and their receptor (CCR3), which are vital for eosinophils migration in mammals, are absent in the chicken genome (27, 78). Earlier studies also showed that avian eosinophils are less responsive to antigenic stimulation than in mammals (79). Basophils are present in chickens as well, and appear to have a function similar to their mammalian counterpart, with the induction of a hypersensitivity reaction through histamine release (80, 81).

Natural Killer (NK) cells represent a lymphoid lineage that shares many features with cytotoxic T lymphocytes. Indeed, they are both defined as large granular lymphocytes, and they play an essential role in the innate immune response by eliminating infected and damaged cells. Even though NK cells are derived from a common lymphoid progenitor, their development is independent of the thymus in both species and, for chickens, of the bursa of Fabricius as well. For both mammals and chickens, the maturation process instead relies on the bone marrow (73, 82). NK cells’ functions rely mainly on a combination of their inhibitory and activating receptors (83, 84), which is a similar system in humans and chickens. Several homologs exist between the two species, namely CD107, CLEC-2, 2B4, and DNAM-1 (84–87). Furthermore, human NK cells can express KIR, NKp46, LILR, and PIR, whereas chickens cannot. They instead express the chicken Ig-like receptor (CHIR) family that may be used as a functional homolog of these receptors (27, 88). Likewise, chickens do not express NKR-P1 but have a functional equivalent, B-NK (87, 89). This suggests that chicken NK cell biology is close to mammalians’, although some other mammalian receptors do not seem to be expressed in chickens, and orthologs have yet to be found.

NK cells bear three main roles in both humans and chickens, although the underlying mechanisms can be slightly different. The first is a direct cytolytic activity, mainly through the secretion of perforin and granzyme, and surface expression of Fas ligand and TRAIL (90, 91). Another essential role of NK cells in both species is antibody-dependent cell-mediated cytotoxicity (ADCC), a mechanism activated by cross-linking of an activating Fc receptor with the Fc region of an antigen-bound antibody, leading to secretion of perforin, granzymes, and cytokines (92, 93). In humans, the activating Fc receptor mainly expressed by NK cells is FcγRIIIa (CD16a), a receptor specific for IgG (94). However, as described earlier, chickens are not able to produce IgG, and therefore do not express FcγRIIIa. To perform ADCC, they instead use another receptor, CHIR-AB1, a high-affinity Fc receptor for IgY (88, 95). Finally, the third role of NK cells is cytokine secretion, and more importantly of IFN-γ, involved in inflammation and macrophage activation in both species (96, 97).

While NK cell biological functions appear to be highly similar between humans and chickens, it is still unsure how much these cells contribute to the chicken immune response. Indeed, earlier studies showed that the frequency of NK cells in blood, spleen, and cecal tonsils was remarkably low (0.5 to 1% of blood lymphocytes) (98), whereas 5-20% of circulating lymphocytes appear to be NK cells in humans (99). However, other publications still observed an important NK-like activity in chickens (73, 100). One of the most plausible explanations resides in the difficulty to properly identify them in chickens. Indeed, in contrast to B and T cells, NK cells do not appear to have a unique specific marker. It is possible that the frequency of NK cells is far higher in peripheral tissues, and that the panel of markers used in the above-mentioned study (TCR^- IgL^- CD3^- CD8⁺ lymphocytes) may be inappropriate or insufficient (86). There is therefore a need to identify additional markers for NK cells, like the analog of CD56 that is used to identify NK cells in humans and chickens (101, 102). Another explanation could be an actual lack of NK cells, compensated with other functionally similar T cells. For example, the high proportion of γδ T cells in chickens, as described further below, might potentially be involved in maintaining an NK-like activity, since they share common features with NK cells (103, 104). Further studies are then needed to properly understand how much NK cells contribute to the chicken immune system.

While all the above-mentioned classical immune cells play essential and distinctive roles in the general immune response, other cells are also involved. Among them, we find the mast cells, described as tissue-resident hematopoietic cells with cytoplasmic granules. In humans, these cells are capable of histamine release and are involved in a wide range of physiological processes, including homeostasis, tissue repair, and angiogenesis. Additionally, they are associated with innate and adaptive immune processes, including immune tolerance and inflammation (105–107). Even though they seem to be quite similar to basophils, in both structure and function, they differ in their development, their reaction to stimuli, and their ability to release immunomodulating cytokines (108). In chickens, mast cells have been greatly studied. Many aspects, including the general ontogeny, the morphology, and the biological functions, appear to be similar to their mammalian analogs (109). Several studies have also demonstrated that avian mast cells can mediate the inflammatory response, infiltrate contaminated tissues and release antimicrobial substances after pathogen infection, similar to human mast cells (110, 111).

In the circulating blood, two other essential types of cells can be found: the thrombocytes, and the erythrocytes. They are respectively involved in coagulation (112) and oxygen delivery (113), though they also appear in humans to be both potentially involved in the mediation of immune responses (114–116). While human thrombocytes and erythrocytes are both enucleated, they appear different in chickens as they kept their nucleus. Many aspects, such as morphology, biology, and ontogeny, are different in both species. For instance, avian thrombocytes arise from a stem cell whereas mammalian thrombocytes arise from megakaryocytes (112, 117). In chickens, thrombocytes have a more defined role in the immune response: even though they share similar biological functions to their mammalian equivalents (e.g., initiation of coagulation), various papers report how their role in the mediation of the inflammatory response is important. Indeed, thrombocytes are not only able to recognize pathogen-associated molecular patterns through Toll-like receptors (TLRs), but can also perform phagocytosis, produce nitric oxide, and release pro-inflammatory cytokines (118–120). Likewise, in contrast to their mammalian counterparts, avian erythrocytes have conserved their nucleus and mitochondria, allowing the cell to produce reactive oxygen species that might be used as an antimicrobial defense (121). Nucleated erythrocytes are believed to have a direct role in immune responses, as they also express TLRs and are capable of regulating the expression of different immune-related genes (e.g., TLRs, CCL4, and α-interferon) (122). Thus, thrombocytes and erythrocytes are believed to play a more important role in the chicken immune response than their mammalian counterparts.

T cells are a key component of the cell-mediated adaptive immune response and can be distinguished from other lymphocytes by the presence of T cell receptors (TCR) on their surface. They are differentiated from hematopoietic progenitors and matured in the thymus, before being released into the bloodstream. These immune cells are then able to recognize antigens presented by major histocompatibility complex (MHC) molecules through their TCR. T cell development and conservation between avian and mammalian models have been investigated for more than two decades and are now well-described: central features, such as the general TCR structures, the T cell subpopulations, and their respective biological functions, seem to be similar between chickens and humans (123, 124).

As in humans, chicken T cells use the TCR, a heterodimeric surface receptor, for antigen recognition. Each TCR chain is composed of two immunoglobulin super-family domains: a variable region with an extensive sequence diversity, and a constant domain, anchored into the plasma membrane. Both birds and mammals have similar TCR chains which can form either TCR-αβ or TCR-γδ heterodimers, defining the two major T cell lineages. TCR-αβ⁺CD4⁺ T cells are often described as T helper cells, TCR-αβ⁺CD8⁺ T cells as cytotoxic T cells, and TCR-γδ⁺ T cells as a cytotoxic lymphocyte subset that can bind to many different ligands, independently of MHC recognition (124–127). γδ T cells are mainly enriched in peripheral tissues, where they make key contributions to immune responses with essential functions such as immune surveillance and protection that cannot be compensated by αβ T cells (128, 129). Overall, the TCR complexes are quite similar between humans and chickens, and their assembly, their surface expression, and their signal transduction are all controlled and regulated by CD3, a transmembrane receptor (124, 130).

Despite the similarity in T cell structures and functions, there are interesting differences between the two species as well. For instance, in mammals, the CD3 complex consists of three chains: CD3γ, CD3δ, and CD3ϵ. These chains can then form two different heterodimers, CD3ϵγ and CD3ϵδ, before assembly with the TCR. In chickens, however, only two CD3 genes exist: a CD3ϵ homolog, and a single CD3γ/δ gene, which has an equal homology to both mammalian CD3γ and CD3δ. Both subunits form two identical dimers (CD3ϵγ/δ) before complex assembly (124, 131). This difference might be explained by CD3γ/δ being an evolutionary precursor before CD3γ and CD3δ genes were split in mammals. Even though CD3 complexes share similar functions in both species, chicken TCR/CD3 components are not able to replace the human molecules in T cells, whereas this is possible for sheep and mouse TCR chains (132). Another notable difference between the two species resides in the TCR expression. Indeed, while γδ T cells have a relatively low expression in humans (around 2 to 10%), frequencies of γδ T cells in chickens can reach up to 50% of the circulating T cell population. This can be explained by the fact that chicken γδ T cells develop exclusively in the thymus, whereas development in mammals can also occur in periphery organs where this T cell subpopulation is enriched (124, 126, 128, 133).

While most T cells’ functions are related to cell-mediated immunity, B cells are instead mainly known to be involved in the humoral component of the adaptive immune system through the production of antigen-specific antibodies, a secreted form of the B cell receptors (BCR), following exposure to a pathogen. B cells, and the generation of the BCR repertoire, have been conserved throughout evolution and are relatively similar between chickens and humans (134). Even though the cells themselves share a great resemblance to both species, the main difference resides in their maturation. As mentioned earlier, while B cells mature in the bone marrow in humans, their development in chickens occurs in an avian-specific organ: the bursa of Fabricius. This organ’s function has been discovered as early as 1956 through a series of bursectomy experiments on chickens, where Glick et al. demonstrated that the antigen-specific antibody response was bursa-dependent (135). The general nature and development of B cells, analogous between humans and chickens despite the different organs, have then been thoroughly described in the literature (136, 137).

Succinctly, key differences between the two species are linked to the generation of the antibody repertoire. Indeed, in mammals, this process happens mostly through gene rearrangement, which is ongoing throughout life. Each chain of immunoglobulins, heavy and light, is composed of a V (variable) region, a D (diversity) region for heavy chains, and J (joining) segments, joined together with a C (constant) region to produce functional immunoglobulins with VJ light chains and VDJ heavy chains. This production of variable regions leads to a vast antibody repertoire of more than 10¹¹ different immunoglobulins that can be generated (31, 137, 138). This process is different in chickens due to their very limited number of variable genes. Because they only have a single copy of functional V and J segments for both chains, the chicken immune system relies on another mechanism known as somatic gene conversion, a process taking place during bursal development (134, 136). Thus, the clusters of pseudogenes, upstream of the immunoglobulin loci, are critical for antibody diversity in chickens (139, 140). After a low-efficiency V(D)J rearrangement in the bone marrow, immature B cells migrate to the bursa of Fabricius where gene conversation, a process in which V sequences are replaced with the pseudogene sequences, takes place. This contributes to the chickens’ robust immune response, with a mammalian-like BCR repertoire (31). However, while the generation of the mammalian antibody repertoire continues throughout life, gene conversion in chickens is only active until the bursa involutes, around 6 months after hatching. It is believed that afterwards, the adult chicken might use, as a source of B cells, post-bursal stem cells mainly from the spleen (31, 134, 136).

Finally, while chicken and mammalian immunoglobulins are also similar in their general structure and functionality, there are notable differences between the two species as well. Indeed, there is some confusion in the literature: although chickens are not able to produce IgG per se, they do secrete another immunoglobulin, IgY. Mammalian IgG and avian IgY functions are equivalent, but the main difference resides in their structure: the chicken IgY heavy chain contains one V domain and four C domains, whereas mammalian IgG contains only three C domains (136, 141). Furthermore, even though chickens have homologs for mammalian IgM and IgA, they are not able to generate IgD and IgE at all. It is nonetheless believed that mammalian IgE functions may be replaced by avian IgY (142, 143).