Beyond Allotypes: The Influence of Allelic Diversity in Antibody Constant Domains

Polymorphic diversity in antibody constant domains has long been defined by allotypic motifs that cross react with the sera of other individuals. Improvements in sequencing technologies have led to the discovery of a large number of new allelic sequences that underlie this diversity. Many of the point mutations lie outside traditional allotypic motifs suggesting they do not elicit immunogenic responses. As antibodies play an important role in immune defense and biotechnology, understanding how this newly resolved diversity influences the function of antibodies is important. This review investigates the current known diversity of antibody alleles at a protein level for each antibody isotype as well as the kappa and lambda light chains. We focus on evidence emerging for how these mutations perturb antibody interactions with antigens and Fc receptors that are critical for function, as well as the influence this might have on the use of antibodies as therapeutics and reagents.

Immunoglobulin allotypes and RFLPs in disease association

Immunoglobulin heavy chain (Gm) and light chain (Km) allotypes have been extensively studied as possible markers of susceptibility to a range of immune-related diseases including malignancies, infectious diseases and autoimmune disorders. This review is concentrated on susceptibility to multiple sclerosis, systemic lupus erythematosus and insulin-dependent diabetes mellitus.

HLA, Gm, and Km polymorphisms and immunity to infectious diseases in South Amerinds

Isolated South Amerinds, a population at very high risk from infectious disease, mount good immune responses to pneumococcal polysaccharides, viral antigens and other immunogens. No unusual immunoglobulin allotype or HLA antigen, which might explain the high risk from infectious disease, was found among them. Responses are examined in relation to the immunogenetic markers that are most prevalent. Amerinds with Gm 1,2,17,21 responded less well than persons without this haplotype to 10 of 12 pneumococcal polysaccharides, and those who were homozygous at the HLA class I loci responded less well to viral antigens. However, these differences are not strong and there are few such findings relative to the number of possibilities examined. The most distinctive immunogenetic characteristic of these populations is their low level of polymorphism at all tested loci. Their susceptibility to infectious agents can be attributed to this genetic uniformity and a consequent ability of pathogens to adapt to the population.

Alloantibodies to human platelet glycoprotein antigens (HPA) and HLA class 1 in a cross section of Nigerian antenatal women

The prevalence of antibodies to human platelet antigens (HPA) and human leukocyte antigens (HLA) class 1 antigens among Nigerian pregnant women has not been reported in our country. This study was therefore aimed at screening the obstetric population for evidence of alloimmunization due to human platelet and HLA class 1 antigens. One hundred and forty four (144) pregnant women attending the obstetric clinic of Military Hospital, Port Harcourt, participated in the study. Their sera were tested for antibodies to HPA and HLA class 1 antigens using GTI PakPlus solid phase ELISA Kit. The total prevalence rate of antibody production was 60.5% (87 out of 144). Among the positive samples, 60 had platelet glycoprotein specific antibodies (41.7%) and 27 had HLA class 1 antibodies (18.8%). In 39.6% of the pregnant women, both platelet specific antibodies and HLA class 1 antibodies appeared. The prevalence of platelet specific glycoprotein antibodies were obtained as follows: GP 11b/111a 12 (8.3%), GP 1a/11a 35 (20.8%), GP Ib/IX 18 (12.5%) and GP IV 9 (6.3%). The prevalence of each platelet antibody subgroup was obtained as follows: anti-HPA-1a,-3a,-4a (4.2%), anti-HPA-1b,-3b,-4a (4.2%), anti-HPA-30 5a and anti-GP Ib/IX (12.5% each), anti-HPA-5b (8.3%) and anti-GP IV (6.3%). A high prevalence rate of human platelet arid cytotoxic antibodies has been observed in our obstetric population. There is need to establish platelet serology laboratory for the proper antenatal and postnatal management of pregnant mothers in this region.

Anaphylactic transfusion reaction associated with a possible anti-A2m(1)

A 42-year-old leukaemic black male developed shortness of breath, itching, hot flushes, substernal pain, marked hypotension, and cardiorespiratory arrest after the infusion of 50 ml of pooled platelet concentrates. The patient had had one previous and uneventful red blood cell transfusion. Serologic studies of this patient's post-transfusion serum showed the presence of an antibody against one denatured IgA2 protein with the phenotype A2m(1 + 2-). All of the platelet donors were A2m(1) positive. The patient was successfully transfused with platelets from donors with the phenotype A2m(1 - 2+). Although 35.8% of Blacks and 30.0% of Chinese are A2m(1) negative, this is the first reported transfusion reaction associated solely with the occurrence of a possible anti-A2m(1) in an individual of the type A2m(1 - 2+).

Immunoglobulin allotypes and IgG subclass antibody response to Aspergillus fumigatus in cystic fibrosis patients

BACKGROUND

A majority of patients with cystic fibrosis (CF) become colonised with Aspergillus fumigatus (Af.), but only a minority develops allergic bronchopulmonary aspergillosis (ABPA). ABPA is associated with increased levels of specific immunoglobulin G (IgG) anti-Af. antibodies with a characteristic IgG subclass distribution. We examined whether this characteristic immune response was under the influence of GM and KM allotypes, which are genetic markers (antigenic determinants) on gamma- and kappa-light chains, respectively.

METHODS

Sera from 233 CF patients were typed for seven GM determinants and two KM determinants. The types were correlated to IgG subclass anti-Af. antibody levels and to the presence or absence of Af. colonisation as well as ABPA.

RESULTS

The IgG2 antibody level was significantly higher in heterozygous GM (1,2,17 23 5,21 and 1,3,17 23 5,21) compared to homozygous GM allotypes (p = 0.02). Patients with the same allotypes tended to have higher IgG1 (p = 0.051). In patients with ABPA, being heterozygous for G1M and G3M was linked to higher IgG4 and lower IgG3 as compared to the other genotypes. The KM markers did not influence the antibody levels. The allotype GM(3 23 5), associated with atopic bronchial asthma, tended to make a relatively larger group in ABPA patients compared to non-ABPA and patients not colonised with Af. (p = 0.09).

CONCLUSIONS

An influence of the GM allotypes on the immune response to Af. and on the development of ABPA in patients with CF is suggested.

Demonstration of specific antibodies against infliximab induced during treatment of a patient with ankylosing spondylitis

Therapeutic proteins, such as infliximab, have revolutionized the treatment of many diseases during the last decade and more than 80 therapeutic proteins are currently approved for clinical use. However, all exogenous proteins have the potential to cause antibody formation. In order to ensure patient safety and the efficacy of therapeutic proteins, careful monitoring of the immunogenicity of therapeutic proteins is therefore necessary not only during preclinical trials, but also during the treatment of patients. Here, we report a clear-cut demonstration of the induction of anti-infliximab antibodies during the treatment of a patient with ankylosing spondylitis (AS). Assessment of anti-infliximab antibodies in sera obtained at various time periods were performed using a highly specific double antigen assay system developed in our laboratory. Immunoreactivity was found to be solely specific for infliximab. Because all sera obtained from the patient were found to be negative for the presence of human anti-mouse antibody (HAMA) and anti-human antibodies. The loss of effect of infliximab, as judged by observing the relapse of signs and symptoms of disease in the patient, seemed to be related with the appearance of antibodies. This study clearly demonstrates that monitoring for the induction of specific antibodies during clinical trials is an important issue for therapeutic proteins.

A Newly Discovered Fc Receptor that Explains IgG-Isotype Disparities in Effector Responses

The known IgG FcRs primarily mediate inflammation by interacting with IgG1, even though IgG2 isotypes tend to be more pathogenic. A novel FcgammaR that binds IgG2 but not IgG1 has just been identified, potentially explaining differences in biological activity that are seen with various IgG isotypes.

Positive selection of the peripheral B cell repertoire in gut-associated lymphoid tissues

Gut-associated lymphoid tissues (GALTs) interact with intestinal microflora to drive GALT development and diversify the primary antibody repertoire; however, the molecular mechanisms that link these events remain elusive. Alicia rabbits provide an excellent model to investigate the relationship between GALT, intestinal microflora, and modulation of the antibody repertoire. Most B cells in neonatal Alicia rabbits express V_Hn allotype immunoglobulin (Ig)M. Within weeks, the number of V_Hn B cells decreases, whereas V_Ha allotype B cells increase in number and become predominant. We hypothesized that the repertoire shift from V_Hn to V_Ha B cells results from interactions between GALT and intestinal microflora. To test this hypothesis, we surgically removed organized GALT from newborn Alicia pups and ligated the appendix to sequester it from intestinal microflora. Flow cytometry and nucleotide sequence analyses revealed that the V_Hn to V_Ha repertoire shift did not occur, demonstrating the requirement for interactions between GALT and intestinal microflora in the selective expansion of V_Ha B cells. By comparing amino acid sequences of V_Hn and V_Ha Ig, we identified a putative V_H ligand binding site for a bacterial or endogenous B cell superantigen. We propose that interaction of such a superantigen with V_Ha B cells results in their selective expansion.

Epistatic effects of immunoglobulin GM and KM allotypes on outcome of infection with hepatitis C virus

Immunoglobulin GM and KM allotypes-genetic markers of gamma and kappa chains, respectively-are associated with immune responsiveness to several infectious pathogens and with survival in certain viral epidemics. We hypothesized that GM and KM allotypes affect the outcome of hepatitis C virus (HCV) infection. To test this hypothesis, we serologically allotyped 100 persons with well-documented clearance of HCV infection and 198 matched persistently infected persons. None of the GM or KM phenotypes by itself was associated with the clearance or persistence of HCV infection. Particular combinations of these phenotypes, however, were significantly associated with the outcome of HCV infection. Subjects with GM 1,17 5,13 and KM 1,3 phenotypes were over three times (odds ratio [OR], 3.57; 95% confidence interval [CI], 1.44 to 8.87) as likely to clear the infection as the subjects who lacked these phenotypes. This GM phenotype had a similar association with clearance in the absence of KM 3 (OR, 2.75; 95% CI, 1.21 to 6.23). The presence of GM 1,3,17 23 5,13 phenotype (in the absence of KM 3) was associated with persistence (OR, 0.21; 95% CI, 0.06 to 0.77), while its absence (in the presence of KM 1,3) was associated with the clearance of infection (OR, 2.03; 95% CI, 1.16 to 3.54). These results show epistatic interactions of genes on chromosomes 14 (GM) and 2 (KM) in influencing the outcome of an HCV infection. Further investigations involving candidate genes (GM, KM, HLA, and Fcgamma receptors) and cellular and humoral immune responses to HCV epitopes are needed to understand the mechanisms underlying these associations.

Human/mouse cross-reactive anti-VEGF receptor 2 recombinant antibodies selected from an immune b9 allotype rabbit antibody library

Vascular endothelial growth factor (VEGF) and its receptors have been implicated in promoting solid tumor growth and metastasis via stimulating tumor-associated angiogenesis. Models of murine tumor angiogenesis and receptor-specific antibodies are required to evaluate roles of VEGF receptors in mouse models of human cancer. Human VEGFR2 (also known as KDR) and murine VEGFR2 (or Flk-1) share 85% amino acid sequence identity in their extracellular domain. We describe here the development of antibodies that cross-react with mouse and human VEGFR2. High-affinity, species cross-reactive, Fabs specific for KDR/Flk-1 were selected from an antibody phage display library generated from an immunized rabbit of b9 allotype. The selected chimeric rabbit/human Fabs were found to bind to purified KDR and Flk-1 with nanomolar affinity. Three of the selected Fabs detected KDR expression on human endothelial cells as well as Flk-1 on murine endothelial cells. The availability of anti-VEGFR2 Fab with species cross-reactivity will help to decipher the functional role of KDR/Flk-1 in tumor biology as well as facilitate the preclinical evaluation of the suitability of KDR/Flk-1 for drug targeting. This report underscores our earlier finding that b9 rabbits are excellent sources for high-affinity cross-reactive antibodies with therapeutic potential.

Immunoglobulin GM genes and IgG antibodies to cytomegalovirus in patients with systemic sclerosis

OBJECTIVE

To determine the role of GM and KM genes--genetic markers of immunoglobulin gamma and kappa chains, respectively--in humoral immunity to human cytomegalovirus (HCMV) in patients with systemic sclerosis (SSc; scleroderma).

METHODS

A total of 137 Caucasian patients with SSc and 145 ethnically matched controls were genotyped for GM f/3, z/17, n+/23+, n-/23-, KM 1, and KM 3 alleles by polymerase chain reaction-restriction fragment length polymorphism and direct DNA sequencing methods. IgG antibodies to HCMV were measured by an enzyme-linked immunosorbent assay.

RESULTS

In SSc patients, GM f,z genotypes were strongly associated with the occurrence of anti-HCMV IgG antibodies. The frequency of the GM f homozygotes was lower (42.2 vs 62.2%; p= 0.02; OR=0.4) and the frequency of the GM f,z heterozygotes was higher (51.1 vs 26.7%; p = 0.006; OR = 2.8) in SSc patients with IgG antibodies to HCMV than in subjects who lacked these antibodies. This association was not observed in the control group. KM and GM n genotypes were not significantly associated with the prevalence of these antibodies.

CONCLUSION

GM f,z alleles or alleles in linkage disequilibrium with them influence the generation of IgG antibodies to HCMV in patients with SSc.

Rabbit immune repertoires as sources for therapeutic monoclonal antibodies: the impact of kappa allotype-correlated variation in cysteine content on antibody libraries selected by phage display

The rabbit immune repertoire has long been a rich source of diagnostic polyclonal antibodies. Now it also holds great promise as a source of therapeutic monoclonal antibodies. On the basis of phage display technology, we recently reported the first humanization of a rabbit monoclonal antibody. The allotypic diversity of rabbit immunoglobulins prompted us to compare different rabbit immune repertoires for the generation and humanization of monoclonal antibodies that bind with strong affinity to antigens involved in tumor angiogenesis. In particular, we evaluated the diversity of unselected and selected chimeric rabbit/human Fab libraries that were derived from different kappa light chain allotypes. Most rabbit light chains have an extra disulfide bridge that links the variable and constant domains in addition to the two intrachain disulfide bridges shared with mouse and human kappa light chains. Here we evaluate the impact of this increased disulfide bridge complexity on the generation and selection of chimeric rabbit/human Fab libraries. We demonstrate that rabbits with mutant bas and wild-type parental b9 allotypes are excellent sources for therapeutic monoclonal antibodies. Featured among the selected clones with b9 allotype is a rabbit/human Fab that binds with a dissociation constant of 1nM to both human and mouse Tie-2, which will facilitate its evaluation in mouse models of human cancer. Examination of 228 new rabbit antibody sequences allowed for a comprehensive comparison of the LCDR3 and HCDR3 length diversity in rabbits. This study revealed that rabbits exhibit an HCDR3 length distribution more closely related to human antibodies than mouse antibodies.

Secondary heavy chain rearrangement: a mechanism for generating anti-double-stranded DNA B cells

The chronic graft-versus-host (cGVH) reaction results in a syndrome that closely resembles systemic lupus erythematosus (SLE). It is induced in nonautoimmune mice by the transfer of alloreactive T cells. The availability of anti-DNA transgenes allows us to study the genetic origins of autoantibodies in this model. We induced cGVH in two anti-DNA H chain site-directed transgenic mouse strains. This resulted in clonal expansion and selection of specific mutations in the anti-double-stranded (ds) DNA B cell population. These data, together with a high frequency of anti-dsDNA B cell clones recovered as hybridomas, suggested that anti-dsDNAs are the product of an antigen-driven immune response. Genetic analysis associated this response with the generation of anti-dsDNA B cells through secondary rearrangements that replaced the site-directed transgene (sd-tg) with endogenous VH genes.

Structural basis of allotypes of ecto-nucleotide pyrophosphatase/phosphodiesterase (plasma cell membrane glycoprotein PC-1) in the mouse and rat, and analysis of allele-specific xenogeneic antibodies

Ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPPs) have been implicated in bone calcification, type II diabetes, control of purinergic signalling and tumour invasion. The gene for the plasma cell membrane glycoprotein PC-1 in the mouse (Enpp1) has been known since 1970 to exist in two allelic forms, but their structural basis was heretofore unknown. We show that the Enpp1a and Enpp1b alleles differ by only two amino acids, at positions 650 and 679 in the C-terminal nuclease-like domain. Histidine 650 but not arginine 679 forms an essential part of the Enpp1a epitope recognized by monoclonal antibody IR-518. Sequences of LEW and LOU rats and the rat glioma cell line C6 differ from that of the mouse by about 60 amino acids. The LOU and C6 cell line sequences differ by only three amino acids, but differ from the LEW sequence by 10 amino acids. All three rat strains possess the mouse Enpp1b allele at positions 650 and 679. Despite numerous other differences from the mouse, rats immunized with Enpp1a mouse cells have generated monoclonal antibodies specific for the Enpp1a allele, suggesting that amino acids 650 and 679 may be particularly immunogenic. The cytoplasmic tails of the mouse and rat are highly conserved, but are significantly different from human cytoplasmic tails.

A structural perspective on MHC class I recognition by killer cell immunoglobulin-like receptors

Killer cell immunoglobulin-like receptors (KIR) play a critical role in the regulation of natural killer (NK) cell activity through their recognition of class I MHC molecules expressed on target cells. KIR recognition provides vital information to NK cells about whether a target cell should be lysed or spared. Understanding the molecular mechanism of this recognition has remained a strong focus of investigation. This has resulted in the crystal structures of several members of the KIR family and more recently the determinations of the three dimensional structures of KIR2DL2 and KIR2DL1 complexed with their respective ligands, HLA-Cw3 and HLA-Cw4. A strong structural conservation has been revealed both in the receptor design and in the overall mode of KIR binding to class I molecules. Nevertheless, distinct differences in the receptor binding sites allow for high specificity between ligands. Furthermore, unexpected similarities with T-cell receptor (TCR) recognition of MHC molecules are also observed. The detailed interactions between KIR and HLA-C molecules and their functional implications will be reviewed here.

Anti-fibrillin-1 autoantibodies in systemic sclerosis are GM and KM allotype-restricted

GM and KM allotypes--genetic markers of immunoglobulin (Ig) gamma and kappa chains, respectively--have been shown to play an important role in genetic predisposition to some autoimmune diseases. To determine their role in susceptibility to systemic sclerosis (SSc; scleroderma) and in the generation of anti-fibrillin-1 antibodies, 148 SSc patients and 191 controls were typed for several GM and KM allotypes by a standard hemagglutination inhibition method. IgG and IgM antibodies to fibrillin-1 were measured by radioimmunoassay. GM and KM phenotypes were not significantly associated with SSc. However, these determinants significantly influenced the production of anti-fibrillin-1 antibodies in SSc patients. In Caucasians, GM1,3,17 23 5,13,21 and GM3 23 5,13 phenotypes were associated with the presence and absence of IgG autoantibodies, respectively. The production of these autoantibodies was also associated with KM allotypes, KM1,3 heterozygosity being associated with response and homozygosity for the KM3 allele with nonresponse to fibrillin-1. In African-Americans, the KM1 homozygotes were associated with the absence of anti-fibrillin-1 antibodies and the KM3 homozygotes with the presence of autoantibodies. In this ethnic group, the GM1,17 5,13 phenotype was associated with the absence of IgM autoantibodies. This represents the first description of genetic control of autoimmunity to fibrillin-1 in scleroderma.

Immunoglobulin GM and KM allotypes in systemic lupus erythematosus

Genetic variation in immunoglobulin gamma (GM) and kappa (KM) chains was associated with systemic lupus erythematosus (SLE) in some studies. However, the data are conflicting, and only one study examined associations in African-Americans. We examined GM and KM allotypes, by race, in a population-based case-control study of SLE. Sera from patients (n = 222) and controls (n = 273) were typed for GM and KM allotypes by a hemagglutination inhibition method. GM phenotypes were not significantly associated with SLE in African-Americans or Caucasians. However, the frequency of KM phenotypes in Caucasian patients was significantly different from that in controls (p = 0.032). KM3,3 was associated with an increased risk, whereas KM1,3 was associated with a lower relative risk of SLE. In African-Americans, however, the pattern of associations with KM phenotypes differed from that in Caucasians, and the overall difference between patients and controls was not statistically significant.

Overcoming the poor immunogenicity of a protein by DNA immunization as a fusion construct

The production of antibodies against poorly immunogenic proteins is problematic. Often there is a failure to generate such antibodies. Furthermore, antibodies against other specificities are frequently induced. We describe a simple approach, analogous to conjugation to a protein carrier, whereby immunization with naked DNA was used to raise antibody to a highly homologous and poorly immunogenic allotypic protein. Deoxyribonucleic acid encoding the protein of interest was fused to DNA encoding the Fc region of a foreign Ig, resulting in increased immunogenicity. The potential applications of this approach include the production of antisera and mAb to allotypic variants, mutant proteins, and proteins that are highly conserved between species.

Central tolerance induction in natural immunoglobulin-allotype-specific T cells

While self toleance is induced to IgG(b)(2a) in Igh(b / b) mice, an anti-IgG(b)(2a) T cell activity emerges in their Igh(a / a) congenic counterparts. This activity is revealed by postnatal transfer of Igh(a / a) T splenocytes into Igh(a / b) F(1), in which total suppression of IgG(2a)(b) expression is established. Here, we sought to determine whether the natural T cell unresponsiveness to IgG(2a)(b) in Igh(b / b) mice involved a central tolerance. Based on the kinetics of postnatal thymic C(gamma2a)(b) gene expression in Igh(b / b) mice, we transplanted thymi from Igh(b / b) donors of diverse ages into tolerogen-free Igh(a / a) nu / nu recipients. The state of T cell tolerance or responsiveness to IgG(2a)(b) in these reconstituted nu / nu hosts was determined by monitoring the capacity of their splenocytes to induce suppression in Igh(a / b) F(1). These experiments demonstrated that: (i) in the Igh(a / a) nu / nu recipients of adult Igh(b / b) thymi, 33 to 65 % T splenocytes were from nu / nu recipient origin, but these peripheral Igh(a / a) T cells were rendered tolerant to IgG(2a)(b) during their differentiation through the adult Igh(b / b) thymi, (ii) circulating IgG(2a)(b) was not a prerequisite for this tolerance induction, (iii) Igh(b / b) thymic epithelium was unable to induce tolerance to IgG(2a)(b) and (iv) IgG(2a)(b)-producing / presenting cells, colonizing the Igh(b / b) thymi, were certainly responsible of full tolerance induction to IgG(2a)(b).

Diverse endogenous light chains contribute to basement membrane reactivity in nonautoimmune mice transgenic for an anti-laminin Ig heavy chain

Basement membrane proteins are targeted in a variety of pathologic autoimmune responses, yet little is known regarding the origins and regulation of this subset of pathogenic lymphocytes. To examine the generation and fate of B cells reactive with a matrix autoantigen, nonautoimmune C57BL/6 mice were rendered transgenic for a nephrotropic lupus anti-laminin immunoglobulin (Ig) H chain, termed LamH-Cmu. We previously reported recovery of two distinct phenotypes among LamH-Cmu-transgenic mice: progeny of founders M6 and M29 contained abundant transgene-expressing B cells but little anti-laminin Ig, whereas spontaneous autoreactivity was readily recovered from the M7 lineage that expressed minimal B-cell mIgM. To explore the spectrum of autoreactivity generated in vivo by different LamH-Cmu-endogenous L-chain combinations, we determined in vitro and in vivo antigen reactivity and L-chain V-region sequences of 17 LamH-Cmu-transgenic anti-laminin Igs. The results reveal a heterogeneous population of anti-laminin Igs with different fine specificities encoded by diverse endogenous L chains, encompassing nine different Vk gene families, 11 Vk genes, and three Jk genes. Many of the L chains are identical to known or putative unmutated germline Vk genes used to encode Igs reactive with self and foreign antigens in nonautoimmune and genetically autoimmune-prone mouse strains. These observations confirm that the LamH-Cmu H chain plays a dominant role in determining anti-laminin reactivity, and indicate that nonautoimmune B6 mice are fully capable of generating a diverse pool of basement-membrane-reactive B cells using unmutated Ig genes. When interpreted in the context of the divergent M6/M29 and M7 transgenic mouse phenotypes, our findings further suggest that these matrix-reactive lymphocytes are not spontaneously activated in vivo under normal circumstances.

Binding of bovine IgG2a and IgG2b allotypes to protein A, protein G, and Haemophilus somnus IgBPs

Immunoglobulin binding proteins (IgBPs) are thought to be virulence factors which enable pathogens to evade the host's immune response. Since bovine IgG2 is important in protection against pyogenic infections, the binding characteristics of Staphylococcus aureus protein A (PrA), streptococcal protein G (PrG), or Haemophilus somnus high molecular weight IgBPs to the two bovine IgG2 allotypes were examined. For PrA or PrG binding of IgG2, guinea pig red blood cells coated with specific IgG2a or IgG2b antibodies were used in a competitive binding inhibition assay with unlabeled and horseradish peroxidase-labeled PrA or PrG. To determine which sizes of H. somnus. IgBPs bind to the two IgG2 allotypes, immunoblots with H. somnus culture supernatant were probed with anti-DNP IgG2a and IgG2b. This detects only Fc binding because anti-DNP does not cross-react with H. somnus antigens. Both IgG2 allotypes bound equally well to PrA and PrG. However, IgG2b but not IgG2a bound to H. somnus high molecular weight IgBPs. The lack of differential binding of bovine IgG2 allotypes to PrA and PrG means that these IgBPs can be considered to be unbiased reagents in assays for detection of bovine IgG2 or for immunoaffinity purification. The differential binding of H. somnus IgBPs to the IgG2 allotypes indicates that animals having one allotype may be more resistant to H. somnus infection than animals having the other allotype.