High-affinity IgE Research Articles

The B Cell Response and Formation of Allergenic and Anti-Allergenic Antibodies in Food Allergy.

Food allergies are a growing public health concern worldwide, especially in children and young adults. Allergen-specific IgE plays a central role in the pathogenesis of food allergies, but their titers poorly correlate with allergy development. Host immune systems yield allergen-specific immunoglobulin (Ig)A, IgE and IgG subclasses with low or high affinities and differential Fc N-glycosylation patterns that can affect the allergic reaction to food in multiple ways. High-affinity IgE is required to induce strong mast cell activation eventually leading to allergic anaphylaxis, while low-affinity IgE can even inhibit the development of clinically relevant allergic symptoms. IgA and IgG antibodies can inhibit IgE-mediated mast cell activation through various mechanisms, thereby protecting IgE-positive individuals from allergy development. The production of IgE and IgG with differential allergenic potential seems to be affected by the signaling strength of individual B cell receptors, and by cytokines from T cells. This review provides an overview of the diversity of the B cell response and the diverse roles of antibodies in food allergy.

Evidence that High-Affinity IgE Can Develop in the Germinal Center in the Absence of an IgG1-Switched Intermediate.

High-affinity allergen-specific IgE is essential for the severe allergic anaphylaxis response. High-affinity Abs are formed by successive rounds of selection of Ag-specific B cells in the germinal center (GC); however, several studies have shown that IgE+ GC B cells are impaired in their ability to undergo selection in the GC. A pathway, known as the "indirect switching pathway" for IgE, has been described whereby Ag-specific B cells initially switch to the IgG1 isotype and undergo affinity selection in the GC, with a secondary switch to the IgE isotype after affinity selection. In previous work, using a food allergy model in mice, we investigated how high-affinity IgE develops in the GC, but we did not test the indirect switching model. In this study, we analyzed the importance of the indirect switching pathway by constructing IgG1-cre Bcl6-fl/fl mice. In these mice, once B cells switch to IgG1, they delete Bcl6 and thus cannot enter or persist in the GC. When we tested IgG1-cre Bcl6-fl/fl mice with our food allergy model, we found that, as expected, IgG1 Abs had decreased affinity, but unexpectedly, the affinity of IgE for allergen was unchanged. IgG1-cre Bcl6-fl/fl mice underwent anaphylaxis in response to allergen, consistent with the formation of high-affinity IgE. Thus, in a food allergy response, high-affinity IgE can be efficiently formed in the absence of indirect switching to IgG1, either by direct selection of IgE+ GC B cells or indirect selection of IgM+ GC B cells that later switch to IgE.

Anti-Allergic Activity of Fucoidan Can Be Enhanced by Coexistence with Quercetin.

In recent years, the incidence of type I hypersensitivity including hay fever has been increasing year by year in Japan. Our previous study using mice showed that only oral, but not intraperitoneal, administration of fucoidan extracted from seaweed (Saccharina japonica) suppressed type I hypersensitivity by secretion of galectin-9, which has a high affinity for IgE in the blood. However, the amount of seaweed required to achieve this activity is quite high (12 g dry weight per person per day). Therefore, the purpose of this study was to search for food ingredients in vegetables that enhance type I hypersensitivity suppression effect when consumed together with fucoidan. As a result, the enhanced effect was observed in extracts from Welsh onions and onions among vegetables. When we compared the polyphenols in the vegetables that showed activity with those that did not, flavonols such as quercetin and kaempferol were found as candidates. When quercetin or kaempferol (100 μg each) were orally administered at the same time, even at amounts where fucoidan alone showed no anti-allergic activity, anti-allergic effects were observed. More interestingly, when both flavonols were combined and administered simultaneously at half the amount of each of the above flavonols (50 μg), while the fucoidan amount remained the same, a similar effect was observed as when each flavonol (100 μg) was administered alone. The simultaneous intake of fucoidan and vegetables containing high contents of quercetin or kaempferol may reduce fucoidan intake while maintaining the allergy suppression effect, suggesting the importance of food combination.

FAK inhibitor PF-431396 suppresses IgE-mediated mast cell activation and allergic inflammation in mice

Mast cells (MCs) initiate and maintain allergic inflammation. Upon being stimulated with immunoglobulin (Ig)E and antigen (Ag), MCs exhibit FcεRI (high-affinity IgE) receptor-mediated degranulation, cytokine secretion, and increased focal adhesion kinase (FAK) activity. The aims of this study were to examine mechanisms of FAK regulation in IgE-mediated MC activation and the effects of FAK inhibition on MC-mediated allergic responses. FAK activity was manipulated with short hairpin RNA (shRNA) knockdown, FAK overexpression, and the FAK inhibitor PF-431396 (PF). Gene expression and kinase activation were analyzed with quantitative molecular biology assays. PF effects were tested in the passive cutaneous anaphylaxis (PCA), active systemic anaphylaxis (ASA), and allergic conjunctivitis (AC) mouse models. Our results showed that FAK overexpression increased IgE-mediated degranulation and reduced the dexamethasone inhibitory effect on MCs activation. The FAK inhibitor PF diminished MC release of β-hexosaminidase (β-hex), histamine, and inflammatory cytokines, via a mechanism that involves MAPK and NF-κB signaling pathways. CaMKII was identified as a robust FAK-associating protein. Inhibition of CaMKII activation by KN-93 suppressed FAK activity and its downstream pathway. PF attenuated inflammatory responses in our PCA and ASA models, and relieved signs of allergic disease in AC model mice. In conclusions, MC degranulation and production of inflammatory mediators in allergic disease may be consequent to FcεRI crosslinking inducing CaMKII-mediated activation of FAK activity. FAK inhibition may represent a new MC-suppressing treatment strategy for the treatment of allergic diseases.

VAMP8 mediates TLR4-dependent TNF secretion in RBL-2H3 pro-inflammatory cells

Abstract Mast cells (MCs) participate in allergic diseases by producing pro-inflammatory mediators and regulatory cytokines. After high affinity IgE (FceRI) receptor activation, they release tryptase, b-hexosaminidase and other preformed pro-inflammatory substances by anaphylactic degranulation. Also, upon stimulation through Toll-like receptor 4 (TLR4), they release pro-inflammatory mediators (such as Tumor Necrosis Factor, TNF), by piecemeal degranulation, orchestrating protective reactions against pathogens. Reactions involved in anaphylactic degranulation require specific vesicular-associated membrane proteins (VAMPs) involved in the latest step of exocytosis. However, the particular roles of VAMP proteins on piecemeal secretion observed after TLR4 receptor activation are not known. Utilizing RBL-2H3 cells differentiated to a pro-inflammatory phenotype, we investigated the role of VAMP8 and VAMP3 on lipopolysaccharide (LPS)-induced mediator secretion in MCs. Confocal microscopy images showed that, in non-stimulated cells, TNF localizes in VAMP3 and VAMP8-positive vesicles. After TLR4 stimulation, we observed an increase in TNF and VAMP8 co-localization, without any change on TNF and VAMP3 co-localization. Remarkably, VAMP8 blockade with specific siRNA impaired TNF secretion after LPS-challenge, while VAMP3 siRNA-mediated down regulation did not. Our data indicate that, although TNF is maintained in VAMP3-containing vesicles, TLR4 triggering leads to the secretion of TNF in VAMP8-containing carriers. Our results contribute to the description of the molecular events involved in late steps of TNF secretion induced by innate immune stimuli in that important immune cell type.

Peripheral IgE Repertoires of Healthy Donors Carry Moderate Mutation Loads and Do Not Overlap With Other Isotypes.

IgE-mediated allergic disease represents an increasing health problem. Although numerous studies have investigated IgE sequences in allergic patients, little information is available on the healthy IgE repertoire. IgM, IgG, IgA, and IgE transcripts from peripheral blood B cells of five healthy, non-atopic individuals were amplified by unbiased, template-switching, isotype-specific PCR. Complete VDJ regions were sequenced to near-exhaustion on the PacBio platform. Sequences were analyzed for clonal relationships, degree of somatic hypermutation, IGHV gene usage, evidence of antigenic selection, and N-linked glycosylation motifs. IgE repertoires appeared to be highly oligoclonal with preferential usage of certain IGHV genes compared to the other isotypes. IgE sequences carried more somatic mutations than IgM, yet fewer than IgG and IgA. Many IgE sequences contained N-linked glycosylation motifs. IgE sequences had no clonal relationship with the other isotypes. The IgE repertoire in healthy individuals is derived from relatively few clonal expansions without apparent relations to immune reactions that give rise to IgG or IgA. The mutational burden of normal IgE suggests an origin through direct class-switching from the IgM repertoire with little evidence of antigenic drive, and hence presumably low affinity for specific antigens. These findings are compatible with a primary function of the healthy IgE repertoire to occupy Fcε receptors for competitive protection against mast cell degranulation induced by allergen-specific, high-affinity IgE. This background knowledge may help to elucidate pathogenic mechanisms in allergic disease and to design improved desensitization strategies.

SLAP Is a Negative Regulator of FcεRI Receptor-Mediated Signaling and Allergic Response.

Binding of antigen to IgE-high affinity FcεRI complexes on mast cells and basophils results in the release of preformed mediators such as histamine and de novo synthesis of cytokines causing allergic reactions. Src-like adapter protein (SLAP) functions co-operatively with c-Cbl to negatively regulate signaling downstream of the T cell receptor, B cell receptor, and receptor tyrosine kinases (RTK). Here, we investigated the role of SLAP in FcεRI-mediated mast cell signaling, using bone marrow derived mast cells (BMMCs) from SLAP knock out (SLAP KO) mice. Mature SLAP-KO BMMCs displayed significantly enhanced antigen induced degranulation and synthesis of IL-6, TNFα, and MCP-1 compared to wild type (WT) BMMCs. In addition, SLAP KO mice displayed an enhanced passive cutaneous anaphylaxis response. In agreement with a negative regulatory role, SLAP KO BMMCs showed enhanced FcεRI-mediated signaling to downstream effector kinases, Syk, Erk, and Akt. Recombinant GST-SLAP protein binds to the FcεRIβ chain and to the Cbl-b in mast cell lysates, suggesting a role in FcεRI down regulation. In addition, the ubiquitination of FcεRIγ chain and antigen mediated down regulation of FcεRI is impaired in SLAP KO BMMCs compared to the wild type. In line with these findings, stimulation of peripheral blood human basophils with FcεRIα antibody, or a clinically relevant allergen, resulted in increased SLAP expression. Together, these results indicate that SLAP is a dynamic regulator of IgE-FcεRI signaling, limiting allergic responses.

IgE Test in Secretions of Patients with Respiratory Allergy.

IgE is a key player in multiple inflammatory airway diseases. Ample literature demonstrates its presence in mucosa of patients with allergic rhinitis (AR), local allergic rhinitis (LAR), asthma, or chronic rhinosinusitis with nasal polyposis (CRSwNP). Current evidence shows that high-affinity IgE in blood stream of allergic individuals derives mainly from the mucosae. Also, mucosal synthesis of IgE can occur in the absence of systemic atopy, and may be relevant in atopic and non-atopic phenotypes of rhinitis as demonstrated in LAR. Specific IgE (sIgE) detection varies depending on technique used for sample collection and its measurement. sIgE detection is highly specific for diagnosis of LAR. Moreover, measurement of sIgE in secretions could be useful in monitoring response to allergen-specific immunotherapy in both AR and LAR phenotypes. This review will focus on recent developments in the role of IgE in respiratory diseases, and the clinical implications of its measurement in secretions.

IgG1 memory B cells keep the memory of IgE responses

The unique differentiation of IgE cells suggests unconventional mechanisms of IgE memory. IgE germinal centre cells are transient, most IgE cells are plasma cells, and high affinity IgE is produced by the switching of IgG1 cells to IgE. Here we investigate the function of subsets of IgG1 memory B cells in IgE production and find that two subsets of IgG1 memory B cells, CD80+CD73+ and CD80−CD73−, contribute distinctively to the repertoires of high affinity pathogenic IgE and low affinity non-pathogenic IgE. Furthermore, repertoire analysis indicates that high affinity IgE and IgG1 plasma cells differentiate from rare CD80+CD73+ high affinity memory clones without undergoing further mutagenesis. By identifying the cellular origin of high affinity IgE and the clonal selection of high affinity memory B cells into the plasma cell fate, our findings provide fundamental insights into the pathogenesis of allergies, and on the mechanisms of antibody production in memory B cell responses.

The discovery of IgE 50 years later

In 1906, Von Pirquet1 used the term supersensitivity without immunity to describe (1) symptoms of inhalant allergy, (2) positive immediate skin test results, and (3) the fact that the results of other tests of immunity were not positive in these patients. The other tests he referred to were precipitin tests and complement fixation. Thus, he opened up the question of serum tests for supersensitivity. In 1919, Ramirez2 reported that a patient who had received a blood transfusion from a horse allergic donor became allergic to horse dander. At this point, Prausnitz and Kustner3 set out to investigate whether the serum of allergic individuals contained a factor or factors that could sensitize the skin. In 1921, they reported that the local injection of serum from a fish allergic individual, Kustner, to an individual who was only allergic to pollen, Prausnitz, would transfer specific sensitivity.3 This transfer of sensitivity came to be known as the P-K test and was used widely to study sensitivity not only to common allergens but also to extracts as diverse as those obtained from schistosomes.4,5 Furthermore, in New York, Cooke et al6 identified that there were other antibodies in the serum that increased during desensitization treatment and could block the skin-sensitizing activity. By the 1950s, it was clear that the transferred sensitivity was specific, that it could be diluted extensively, and that the skin remained locally sensitive for days if not weeks after the injection of serum.7,8 It was also already clear that the ability to sensitize the skin was lost after moderate heating of the serum.7 Several studies had also been reported on the physical properties of P-K activity. Indeed, Heimlich et al9 at Cal Tech reported on the sedimentation properties of skin-sensitizing antibodies in 1960. However, the studies before 1964 had not succeeded in defining the nature of these antibodies.

Gender-related effects of sex steroids on histamine release and FcεRI expression in rat peritoneal mast cells.

Mast cells (MCs) are versatile effector and regulatory cells in various physiologic, immunologic, and pathologic processes. In addition to the well-characterized IgE/FcεRI-mediated degranulation, a variety of biological substances can induce MCs activation and release of their granule content. Sex steroids, mainly estradiol and progesterone, have been demonstrated to elicit MCs activation. Most published studies have been conducted on MCs lines or freshly isolated peritoneal and bone marrow-derived MC without addressing gender impact on MC response. Our goal was to investigate if the effect of estradiol, progesterone, testosterone, and dihydrotestosterone (DHT) on MCs may differ depending on whether female or male rats are used as MCs donors. Our results demonstrated that effect of sex steroids on MCs histamine release is dose- and gender-dependent and can be direct, synergistic, or inhibitory depending on whether hormones are used alone or to pretreat MCs followed by substance P-stimulation or upon IgE-mediated stimulation. In contrast, sex steroids did not have effect on the MC expression of the IgE high affinity receptor, FcεRI, no matter female or male rats were used. In conclusion, MCs degranulation is modulated by sex hormones in a gender-selective fashion, with MC from females being more susceptible than MC from males to the effects of sex steroids.

Omalizumab in an allergology clinic: real life experience and future developments

Omalizumab is a recombinant humanized monoclonal antibody that reduces levels of circulating IgE and expression of IgE high affinity receptor (FCɛRI) on mast cells and basophils. Its role in the therapy of allergic asthma and urticaria is well established. According to GINA guidelines, omalizumab should be considered as an important alternative to systemic corticosteroids in uncontrolled asthma. Several ongoing trials will evaluate omalizumab efficacy in the treatment of other allergic diseases and conditions. Further studies are needed to answer several practical questions on the optimal duration of treatment and possible biomarkers to predefine a cohort of responders to this therapy.

Persistent molecular microchimerism induces long‐term tolerance towards a clinically relevant respiratory allergen

Development of antigen-specific preventive strategies is a challenging goal in IgE-mediated allergy. We have recently shown in proof-of-concept experiments that allergy can be successfully prevented by induction of durable tolerance via molecular chimerism. Transplantation of syngeneic hematopoietic stem cells genetically modified to express the clinically relevant grass pollen allergen Phl p 5 into myeloablated recipients led to high levels of chimerism (i.e. macrochimerism) and completely abrogated Phl p 5-specific immunity despite repeated immunizations with Phl p 5. It was unclear, however, whether microchimerism (drastically lower levels of chimerism) would be sufficient as well which would allow development of minimally toxic tolerance protocols. Bone marrow cells were transduced with recombinant viruses integrating Phl p 5 to be expressed in a membrane-anchored fashion. The syngeneic modified cells were transplanted into non-myeloablated recipients that were subsequently immunized repeatedly with Phl p 5 and Bet v 1 (control). Molecular chimerism was monitored using flow cytometry and PCR. T cell, B-cell and effector-cell tolerance were assessed by allergen-specific proliferation assays, isotype levels in sera and RBL assays. Here we demonstrate that transplantation of Phl p 5-expressing bone marrow cells into recipients having received non-myeloablative irradiation resulted in chimerism persisting for the length of follow-up. Chimerism levels, however, declined from transient macrochimerism levels to persistent levels of microchimerism (followed for 11months). Notably, these chimerism levels were sufficient to induce B-cell tolerance as no Phl p 5-specific IgE and other high affinity isotypes were detectable in sera of chimeric mice. Furthermore, T-cell and effector-cell tolerance were achieved. Low levels of persistent molecular chimerism are sufficient to induce long-term tolerance in IgE-mediated allergy. These results suggest that it will be possible to develop minimally toxic conditioning regimens sufficient for low level engraftment of genetically modified bone marrow.

Insights into Cell Membrane Microdomain Organization from Live Cell Single Particle Tracking of the IgE High Affinity Receptor FcϵRI of Mast Cells

Current models propose that the plasma membrane of animal cells is composed of heterogeneous and dynamic microdomains known variously as cytoskeletal corrals, lipid rafts and protein islands. Much of the experimental evidence for these membrane compartments is indirect. Recently, live cell single particle tracking studies using quantum dot-labeled IgE bound to its high affinity receptor FcϵRI, provided direct evidence for the confinement of receptors within micrometer-scale cytoskeletal corrals. In this study, we show that an innovative time-series analysis of single particle tracking data for the high affinity IgE receptor, FcϵRI, on mast cells provides substantial quantitative information about the submicrometer organization of the membrane. The analysis focuses on the probability distribution function of the lengths of the jumps in the positions of the quantum dots labeling individual IgE FcϵRI complexes between frames in movies of their motion. Our results demonstrate the presence, within the micrometer-scale cytoskeletal corrals, of smaller subdomains that provide an additional level of receptor confinement. There is no characteristic size for these subdomains; their size varies smoothly from a few tens of nanometers to a over a hundred nanometers. In QD-IGE labeled unstimulated cells, jumps of less than 70 nm predominate over longer jumps. Addition of multivalent antigen to crosslink the QD-IgE-FcϵRI complexes causes a rapid slowing of receptor motion followed by a long tail of mostly jumps less than 70 nm. The reduced receptor mobility likely reflects both the membrane heterogeneity revealed by the confined motion of the monomeric receptor complexes and the antigen-induced cross linking of these complexes into dimers and higher oligomers. In both cases, the probability distribution of the jump lengths is well fit, from 10 nm to over 100 nm, by a novel power law. The fit for short jumps suggests that the motion of the quantum dots can be modeled as diffusion in a fractal space of dimension less than two.

Sequential class switching is required for the generation of high affinity IgE antibodies

IgE antibodies with high affinity for their antigens can be stably cross-linked at low concentrations by trace amounts of antigen, whereas IgE antibodies with low affinity bind their antigens weakly. In this study, we find that there are two distinct pathways to generate high and low affinity IgE. High affinity IgE is generated through sequential class switching (μ→γ→ε) in which an intermediary IgG phase is necessary for the affinity maturation of the IgE response, where the IgE inherits somatic hypermutations and high affinity from the IgG1 phase. In contrast, low affinity IgE is generated through direct class switching (μ→ε) and is much less mutated. Mice deficient in IgG1 production cannot produce high affinity IgE, even after repeated immunizations. We demonstrate that a small amount of high affinity IgE can cause anaphylaxis and is pathogenic. Low affinity IgE competes with high affinity IgE for binding to Fcε receptors and prevents anaphylaxis and is thus beneficial.

Adjuvants determine the contribution of basophils to antigen sensitization in vivo

PurposeBasophils expressing FcɛRIα, the alpha chain of the total IgE high affinity receptor, appear to play a role in antigen (Ag) sensitization, although dendritic cells (DCs) are the principal Ag-presenting cells (APCs). To investigate whether these two types of APCs are involved differentially in Ag-sensitization when distinct types of adjuvants are utilized. MethodsTo investigate whether basophils and DCs serve as APCs in vitro, whole splenocytes, FcɛRIα+ basophil-depleted splenocytes, and CD11c+ DC-depleted splenocytes from naïve DO11.10 mice were stimulated in vitro with ovalbumin (OVA) or OVA peptide 323–339 to evaluate Ag-induced proliferation. To investigate whether basophils function as APCs in vivo, BALB/c mice were actively immunized with ragweed (RW) emulsified in alum or Complete Freund's Adjuvant (CFA) followed by an RW challenge in eye drops. An anti-FcɛRIα antibody (Ab) or a control Ab was injected intraperitoneally during the sensitization phase. Twenty-four hours after the RW challenge, conjunctivas and spleens were harvested for histological analysis to evaluate conjunctival eosinophilia and cytokine production, respectively. ResultsDepletion of basophils or DCs from naïve DO11.10 splenocytes significantly suppressed proliferative responses to either OVA or OVA peptide. Treatment with the anti-FcɛRIα Ab suppressed the conjunctival eosinophilia when alum, but not CFA, was utilized as the adjuvant. Similarly, the anti-FcɛRIα Ab inhibited cytokine production by splenocytes when alum was used as the adjuvant. ConclusionAdjuvants determine which APCs are utilized in Ag-sensitization.

Erratum to: Specific post-transcriptional inhibition of mRNA for ligand binding chain of IgE high affinity receptor

Erratum to: Specific post-transcriptional inhibition of mRNA for ligand binding chain of IgE high affinity receptor

Specific post-transcriptional inhibition of mRNA for ligand binding chain of IgE high affinity receptor

IgE high affinity receptor (FcεRI) plays an important role in triggering type I allergic reactions. In this study, we have investigated the ability of four synthetic and sequence-specific RNA interfering antisense oligodeoxynucleotides (AS-ODNs) to reduce the expression of FcεRIα gene in granulocytes of allergy sufferers in vitro. Only AS1 out of four AS-ODNs specifically inhibited the FcεRIα gene expression and the dose response assay revealed that AS1 was capable of specific inhibition of target mRNA expression over a linear concentration range without affecting the expression of house keeping genes such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Together, these results indicate that sequence-specific RNA interfering ODNs can be effectively used to silence the expression of key genes like IgE high affinity receptor that are involved in chronic inflammatory diseases.

Trimolecular Complex Formation of IgE, FcεRI, and a Recombinant Nonanaphylactic Single-Chain Antibody Fragment with High Affinity for IgE

IgE is a central molecule in allergic disease. We have isolated cDNAs coding for the heavy and light chains of a murine mAb specific to human IgE and expressed a recombinant single-chain variable fragment (ScFv) derived thereof in Escherichia coli. The purified recombinant ScFv has a molecular mass of 28 kDa as measured by mass spectrometry and shows a beta-sheet fold as determined by circular dichroism. In biosensor-based studies it was demonstrated that the ScFv rapidly and stably binds to human IgE with an affinity of K(D) of 1.52 x 10(-10) M, which is almost as high as the affinity of IgE for FcepsilonRI, and that the ScFv is able to recognize FcepsilonRI-bound IgE and to prevent IgE binding to FcepsilonRI. The ScFv reacts specifically with IgE but not with other isotypes, allows the measurement of allergen-specific IgE in serum samples, and specifically targets cells that contain FcepsilonRI- or FcepsilonRII-bound IgE or that secrete IgE. Using negative-stain electron microscopy we demonstrated the formation of bimolecular complexes consisting of two ScFv molecules and one IgE and trimolecular complexes consisting of IgE, FcepsilonRI, and ScFv in which only one ScFv is able to bind to IgE. Accordingly, we found that the ScFv does not cross-link basophil-bound IgE and hence does not induce histamine release or activation of basophils as demonstrated by FACS analysis of CD203c expression and by histamine release experiments. In vivo skin testing confirmed the lack of allergenic activity of the ScFv. The recombinant ScFv may represent a universal tool for the IgE-targeted treatment of allergies.

Avian IgY Binds to a Monocyte Receptor with IgG-like Kinetics Despite an IgE-like Structure

An ancestor of avian IgY was the evolutionary precursor of mammalian IgG and IgE, and present day chicken IgY performs the function of human IgG despite having the domain structure of human IgE. The kinetics of IgY binding to its receptor on a chicken monocyte cell line, MQ-NCSU, were measured, the first time that the binding of a non-mammalian antibody to a non-mammalian cell has been investigated (k(+1) = 1.14 +/- 0.46 x 10(5) mol(-1)sec(-1), k(-1) = 2.30 +/- 0.14 x 10(-3) s(-1), and K(a) = 4.95 x 10(7) m(-1)). This is a lower affinity than that recorded for mammalian IgE-high affinity receptor interactions (Ka approximately 10(10) m(-1)) but is within the range of mammalian IgG-high affinity receptor interactions (human: Ka approximately 10(8)-10(9) m(-1) mouse: Ka approximately 10(7)-10(8) m(-1). IgE has an extra pair of immunoglobulin domains when compared with IgG. Their presence reduces the dissociation rate of IgE from its receptor 20-fold, thus contributing to the high affinity of IgE. To assess the effect of the equivalent domains on the kinetics of IgY binding, IgY-Fc fragments with and without this domain were cloned and expressed in mammalian cells. In contrast to IgE, their presence in IgY has little effect on the association rate and no effect on dissociation. Whatever the function of this extra domain pair in avian IgY, it has persisted for at least 310 million years and has been co-opted in mammalian IgE to generate a uniquely slow dissociation rate and high affinity.