Biochemical characterization of the fibronectin binding sites for IgG

Plasma fibronectin (Fn) is a constituent of cryoglobulins and has been shown to interact with immune complexes. In a previous report we demonstrated that Fn specifically bound to IgG immobilized on a solid matrix. To localize and biochemically characterize the sites on the Fn molecule involved in this interaction, Fn was enzymatically cleaved with subtilisin and subjected to IgG affinity chromatography. Three major polypeptide fragments of 16 kDa, 22 kDa, and a triplet of 26- to 29-kDa bound IgG. They were localized to three separate regions of the molecule by Western blot analysis using antisera to specific regions of the Fn molecule, by amino acid sequencing, and by their previously described heparin binding affinities. The 22-kDa fragment interacted with IgG under physiologic conditions and it is localized at the N-terminal of the Fn molecule. The 16-kDa and 26- to 29-kDa fragments bound to IgG under conditions of lower ionic strength; the former commences at residue 588, carboxyl-terminal to the collagen binding region and the latter begins at residue 1597, carboxyl-terminal to the cell binding domain. The interaction of Fn with Ig has significant implications in host defense and also in immune complex disease where basement membrane Fn may sequester immune complexes from the circulation.     

IgM expressed by leukemic CD5(+) B cells binds mouse immunoglobulin light chain

Mouse immunoglobulin (Ig) molecules have previously been shown to bind to the surface of CD5(+) B cells from patients with B-cell chronic lymphocytic leukemia (B-CLL). The results indicated that surface IgM was involved in the interaction and suggested the phenomenon was an example of the polyreactive binding capacity of the surface Ig (sIg) expressed by these malignant cells. This article describes the further characterization of the interaction between human IgM and mouse Ig molecules and subunits. Mouse Ig molecules of both kappa and lambda light chain classes bound to the B-CLL cell surface. The dissociation constant for the interaction of mouse IgG1 (K121) with the B-CLL cell surface was 3.6 x 10(-7) M. To confirm the involvement of the human IgM expressed by the B-CLL cells in the interaction, the malignant cells were stimulated in vitro to induce secretion of human IgM. Enzyme immunoassay was used to show that secreted human IgM bound to intact mouse Ig, as occurred with the cell surface analysis. The mouse Ig epitope recognized by the purified secreted human IgM was shown by Western blot analysis to be located on the light chain of the mouse Ig molecule and to be conformationally dependent. K121 light chain was cloned and expressed in E. coli and the recombinant light chain bound to the surface of CLL B cells. The results confirm that human IgM is the reactive ligand in the interaction with mouse Ig and indicate that the interaction of polyreactive IgM with mouse IgG occurs via the light chain component of IgG.     

beta-Glucuronidase release from human monocytes induced with aggregated immunoglobulins of different classes

We studied beta-glucuronidase release from human monocytes induced with aggregated immunoglobulins of the nine different human classes and subclasses. Release was induced in a time and dose-dependent manner by all aggregated IgG subclasses. Aggregated IgA1 caused a greater beta-glucuronidase release than aggregated IgM, IgD, and IgE, but the difference was not statistically significant. Release of beta-glucuronidase was not phagocytosis dependent since inhibition of phagocytosis by cytochalasin B or dihydrocytochalasin B did not diminish enzyme release. On the contrary, cells incubated with cytochalasin B prior to addition of aggregated IgG released approximately twice as much enzyme compared to untreated controls. Enzyme release induced by latex particles, a non-Fc receptor mechanism, was decreased by cytochalasin B. Monomeric IgG1, IgG2, IgG3, and IgG4 inhibited aggregated IgG1 enzyme release in a dose-dependent manner. The ability of monomeric IgG to inhibit beta-glucuronidase release correlated with previous reports describing the binding affinities of monomeric IgG to monocytes, i.e., IgG2 was relatively ineffective compared to the other subclasses. Monomeric IgA, IgE, and pentameric IgM were unable to diminish IgG-induced enzyme release. The data indicate that normal peripheral blood monocytes express predominantly Fc receptors for IgG and that all four IgG subclasses induce the release of the lysosomal enzyme beta-glucuronidase.     

Idiotypic analysis of a B cell clone with anti-intermediate filament specificity in a patient with Sj?gren's syndrome: involvement of five subpopulations producing different immunoglobulin isotypes

An IgM paraprotein from patient LP with Sj?gren's syndrome exhibited an antibody activity to intermediate filaments (IMF) of cells from all vertebrates examined, and appeared to recognize several classes of IMF (i.e., vimentin, desmin, and keratin). A mouse monoclonal anti-idiotype (Id) antibody, K4A, was prepared against the IgMk (LP) and used as a specific probe in two-color immunofluorescence to examine the extent of clonal involvement in the patient's blood and bone marrow mononuclear cells (MNC). Twenty to 30% of MNC in her blood samples were IgMk+ plasmablasts with morphologic similarity to Waldenstr?m's macroglobulinemia cells. IgG+ and IgA+ plasmablasts were demonstrated in lower frequencies (approximately 2%). Almost all of the IgM+ cells and approximately 80% of the IgG+ cells and IgA+ cells in the blood were reactive with the K4A anti-Id antibody. Immunoglobulin (Ig) subclass analysis revealed that the K4A Id was expressed by IgG1+, IgG3+, IgA1+ and IgA2+ plasmablasts. Similar observations were obtained with bone marrow samples, although the proportion of Id+ cells among IgG+ or IgA+ cells was lower in marrow than in blood. IgG and IgA fractions isolated from the patient's serum were also shown to contain anti-IMF activity. Ig biosynthetic analysis of blood MNC revealed that the K4A anti-Id antibody precipitated not only IgM but also IgG and IgA. Because cells simultaneously producing two different Ig isotypes were not detected, these results indicate the presence of five separate subpopulations of the K4A Id+ neoplastic clone. The data thus suggest the occurrence of a neoplastic or pre-neoplastic transformation event before the switching of Ig heavy chain isotypes, and imply a role for the IMF antigen in the exaggerated proliferation and differentiation along five of the nine potential intraclonal pathways.     

Suppression of polyclonal human B cell activation by IgG binding factors: interference with the maturation of Ig-containing cells into Ig-secreting cells

Human IgG binding factors (IgG BF) were prepared by immunopurification on IgG immunosorbents from cell-free supernatants of unstimulated peripheral blood mononuclear cells (PB MNC). The suppressive effects of IgG BF was studied using PB MNC stimulated by pokeweed mitogen or by nocardia delipidated cell mitogen. At the end of the culture three parameters of B cell activation were measured: (1) the numbers of IgM-, IgG-, or IgA-containing cells (CC) using direct immunofluorescence, (2) the numbers of IgM, IgG, or IgA plaque-forming cells (PFC) using a Protein A hemolytic plaque assay, and (3) the concentrations of IgM, IgG, or IgA in culture supernatants using an enzyme-linked immunosorbent assay. Addition of IgG BF at the third day of culture resulted in a selective decrease of IgG CC, while IgM CC and IgA CC were increased or unchanged. Conversely, IgG BF induced a nonselective diminution of the number of PFC and of the amount of secreted Ig of the three major Ig classes. Therefore the results demonstrate two distinct effects of IgG BF: (1) an isotype-specific suppression of cells producing IgG, demonstrated by the parallel decrease of IgG CC and IgG PFC, and (2) a blocking of the late stages of B cell maturation evidenced by the discrepancy between normal or elevated Ig CC and decreased Ig PFC of the IgM and IgA classes.     

Degalactosylated and/or denatured IgA, but not native IgA in any form, bind to mannose-binding lectin

Mannose-binding lectin (MBL) is reported to bind to agalactosyl IgG, but not to normally galactosylated (native) IgG. It was recently reported that serum polymeric IgA in its native form reacts with MBL, whereas a more recent report has claimed that native IgD and IgE, and possibly IgM, do not. This led us to investigate whether IgA is truly reactive with MBL. To accomplish this, we collected purified human Igs, of various classes, subclasses, and allotypes, and tested their ability to bind to MBL using an ELISA method. Among these preparations, only one (monoclonal IgA2m(2):Kur) exhibited significant MBL binding. In particular, polymeric or monomeric forms of our normal serum IgA preparation lacked any ability to bind to MBL whatsoever. However, all the Ig preparations which had not bound to MBL became able to do so when they were degalactosylated with a galactosidase treatment, and the binding was further enhanced by acidic denaturation of the Igs. Among the degalactosylated and/or acid-denatured IgA, the IgA2 subclass exhibited a higher level of MBL binding than did IgA1. Our results suggest that MBL does not bind to native Igs (viewed in principle as "self" components), and that only Igs with abnormal glycosylation (degalactosylated forms) and/or denaturation would be MBL reactive.     

伤寒Vi多糖结合疫苗免疫小鼠后的抗体类型及动态分析

伤寒Vi多糖结合疫苗和Vi多糖疫苗分别免疫小鼠,分离血清,采用间接ELISA法测定不同时点血清中特异性IgA、IgM、IgG及其亚类(IgG1、IgG2a、IgG3)的抗体滴度。结果显示,免疫一针后,Vi多糖结合疫苗组的IgG抗体GMT值明显升高,第二针有加强效应(P<0.01);所测3种IgG亚型中IgG2a抗体滴度升高明显;Vi多糖和结合疫苗免疫小鼠后,血清中IgA和IgM抗体滴度均有显著升高,但无加强应答。显示Vi多糖结合疫苗在诱导小鼠血清IgG应答方面有加强效应。     

Recombinant, truncated CD4 molecule (rT4) binds IgG

CD4 is a cell surface glycoprotein that identifies the subset of human T lymphocytes that induces sIg+ B lymphocytes to differentiate and secrete Ig after intimate T-B cell contact. In the course of studying a recombinant, truncated form of CD4 (rT4) we noticed that goat antibodies of apparently irrelevant specificities bound to immobilized rT4. To directly study whether rT4 interacts with Ig molecules, purified human IgG was added to rT4-coated wells and a dose-dependent interaction between IgG and rT4 was observed by ELISA. Purified myeloma IgG proteins bound to immobilized rT4 with the same avidity as polyclonal IgG that suggests that rT4-IgG binding was not due to the presence of anti-rT4 antibodies in the IgG fraction. IgG from 6 sera bound to rT4 in concentration dependent manner similar to purified IgG. Immobilized rT4 specifically bound IgG, and not IgM, IgA, IgD, or beta 2-microglobulin. The specific interaction of rT4 and IgG was also observed when IgG or IgM were immobilized, demonstrating that IgG binding was not a unique property of immobilized rT4. As with low affinity receptors for IgG, rT4 bound heat aggregated IgG with increased avidity. Neither anti-CD4 mAb nor dextran sulfate inhibited rT4-IgG binding. rT4 bound Fc but not F(ab)2 fragments. Each of the purified IgG subclasses; IgG1, 2, 3, and 4 bound to rT4 with similar avidity. Taken together, these data suggest that rT4 specifically interacts with a public structure on IgG Fc.     

Many group A streptococcal strains express two different immunoglobulin-binding proteins, encoded by closely linked genes: characterization of the proteins expressed by four strains of different M-type

Most group A streptococcal strains are able to bind immunoglobulin (Ig) in a non-immune manner, and the majority of these strains bind both IgA and IgG. Using molecular cloning and immunochemical techniques, we have purified and characterized the Ig Fc-receptors expressed by four such strains. Two of the strains express a novel type of receptor, designated protein Sir, which binds IgA and IgG of all subclasses, and therefore has broader reactivity than any Fc-receptor previously described. The other two strains express protein Arp, a receptor that binds IgA of both subclasses, and also binds polyclonal IgG weakly. Characterization of the weak IgG-binding ability of protein Arp shows that it binds only some monoclonal IgG proteins, in particular those of the IgG3 subclass. The four strains studied here were unexpectedly found to also express a second Ig-receptor, called protein Mrp, encoded by a gene closely linked to the gene for the first receptor. The Mrp protein does not bind IgA, but it binds IgG molecules of the IgG1, IgG2 and IgG4 subclasses, and it also binds fibrinogen. Binding of fibrinogen has been reported to be a characteristic property of streptococcal M proteins, which suggests that the Mrp protein may be an M protein that also binds Ig. Taken together, all available evidence now indicates that most strains of group A streptococci express two different Ig-binding proteins, encoded by closely linked genes.     

The Fc portion of intact IgG blocks stimulation of human PBMC by anti-T3

The means by which normal human serum inhibits the activation of PBMC by OKT3 has been investigated. The Fc portion of intact IgG is shown to be the major inhibitor in human serum of this OKT3-induced stimulation. Furthermore, inhibition by IgG subclasses correlated with their ability to bind to the monocyte Fc receptor, i.e., IgG1 and IgG3 produced greater inhibition than IgG2 and IgG4, and this inhibition was competitive. In contrast, hypogammaglobulinemic serum, IgA, IgM, and F(ab')2 of IgG were not inhibitory relative to intact IgG or Fc of IgG. Because of the similarities between T3 and the idiotype-defined T cell receptor for antigen, these investigations suggest that IgG might modulate the interactions between the T cell recognition complex and anti-idiotype antibody, thus regulating the idiotype network.     

Shared N-terminal sequences in monclonal IgMkappa and IgGkappa proteins from a patient with a complex multiple paraprotein disorder.

The N-terminal sequence analyses were performed on the heavy (H) and light (L) chains of the idiotypically identical IgM kappa and IgG kappa paraproteins isolated from the serum of patient, Cam. The N-terminal 39 residues of the kappa chains of the IgM and IgG were identical and belonged to the human V kappa III subgroup. This sequenced stretch included the first L chain hypervariable region. The N-terminal 27 residues of the variable regions (VH) of the respective mu and gamma heavy chains were also identical and belonged to the human VHIII subgroup. These identical VH sequences were unique with lysine residues at positions 13 and 19. This dual lysine substitution has not been seen in 37 other human VHIII sequences reported in the literature. This N-terminal sequence homology in the V-regions of Cam IgM kappa and IgG kappa paraproteins and the shared idiotypy expressed by Cam IgM, IgG, and IgA proteins strongly suggest the existence of complete structural homology in the variable regions of the and L chains of these Ig molecules of three separate Ig classes. At the cellular and genetic level, these results point toward a common clonal origin for the idiotypically related Ig molecules and suggest that identical V-region (VH and VL) genes were utilized by the Cam lymphoid clone in the biosynthesis of the respective IgM, IgC, and IgA proteins.     

Age related patterns of immunoglobulin serum levels in the Quechua Indians of Andean Mountains

Age-dependent changes of IgA, IgG, IgM, and IgD serum levels in a population of Quechua Indians of Peruvian Andes at 4 300 m were investigated. A first increase and a subsequent decrease in IgA and IgM levels were observed with advancing age. IgG and IgD only display an increase during development. More or less pronounced sex-related changes were also found in all Ig classes, the sex dependent pattern of IgA being the more evident one. It has been suggested that sexual, genetic and environmental influences strongly superimpose to high altitude related changes in Ig profile during ageing.     

Normal human sera contain antibodies directed at Fab of IgA

Serum samples from 26 normal volunteers were evaluated by isotype-specific ELISA for the presence of IgG and IgM antibodies directed at IgA. Although there were wide variations in antibody levels, anti-IgA antibodies of both isotypes were found in all individuals tested. The anti-IgA activity was detected against a variety of polymeric and monomeric IgA1 and IgA2 myeloma proteins containing both kappa and lambda light chains. By using Fab and Fc fragments generated by incubation of an IgA1 myeloma protein with IgA1 protease, it was shown that the anti-IgA activity was specific for the Fab portion of the IgA molecule. It was also demonstrated that the serum of two individuals contained both IgG and IgM activity directed at autologous affinity-purified IgA. IgM antibody levels against both whole IgA and Fab of IgA were significantly higher than IgG antibody levels. Cells producing anti-IgA antibodies of both isotypes were detected in lipopolysaccharide-stimulated human spleen.     

Activated T lymphocytes resist the toxic effects of the adenosine deaminase inhibitor, 2-deoxycoformycin

Tonsil lymphocytes from three adults and three children were examined for immunoglobulin (Ig) production before and after Epstein-Barr virus (EBV) transformation. T-cell depletion was required to obtain cell lines from EBV-seropositive individuals. Cytoplasmic Ig was mainly IgG in adult lymphocytes before and after transformation; IgA and IgM were more prominent after than before. IgM and IgG predominated in lymphocytes of children before and after transformation; IgA was more prominent after than before. Cytoplasmic Ig of peripheral blood lymphocytes from these individuals was mainly IgM. Secreted Ig from tonsil lymphocytes was mainly IgA or IgG; after transformation IgM predominated with adult cell lines, and IgG or IgM with cell lines from children. IgE was consistently sparse in spite of ragweed and/or grass allergies of the adults.     

Secretory IgA specific for Toxoplasma gondii

Secretory IgA specific for Toxoplasma gondii was identified in intestinal secretions of mice infected perorally with bradyzoites (encysted in brain) of the Me49 strain of T. gondii by using immunofluorescence microscopy and an ELISA. This activity was absorbed with tachyzoites of T. gondii but not mouse brain. To determine whether increased total amount of intestinal IgA might cause a nonspecific reaction in the ELISA for T. gondii-specific IgA, mice were immunized perorally with cholera toxin. This immunization produced intestinal IgA antibody to cholera toxin and increased the total amount of intestinal IgA, but there was no reactivity of intestinal secretions in the ELISA for T. gondii-specific IgA. Experiments in which ELISA were performed with monoclonal IgA, IgG, or IgM and antisera to IgA, IgG, or IgM demonstrated that the ELISA was specific for each Ig class. In addition, monoclonal IgA competed with the anti-Toxoplasma IgA activity of intestinal secretions obtained from mice infected with T. gondii.     

Differentiation of B lineage cells from liver of neonatal mice: generation of immunoglobulin isotype diversity in vitro

The development and differentiation of B cells expressing different immunoglobulin (Ig) isotypes was studied in cultures of murine neonatal liver cells. Before culture, 5 to 15% of the liver cells were mu + pre-B cells; 1 to 3% had surface IgM and less than 0.1% had slgG. During 4 days in culture the number of pre-B cells declined, whereas the number of IgM B cells increased greater than 20-fold; IgG B cells also increased in number. Of the four subclasses, IgG3+ and IgG2b+ cells predominated, each representing 3 to 10% of the total B cells at day 4. IgG1+ and IgG2a+ cells were present in lower numbers, representing 1 to 5% and 0.3 to 2.5% of B cells, respectively. Most IgG+ cells also expressed sIgM. Only a minority (less than 10%) of the sIgM+ cells were sIgD+, and most sIgG+ cells were sIgD-. Few T cells were present in these cultures (less than 0.5% in newborn liver), and sIgG+ cells were generated in normal frequencies in cultures of cells from nude mice. The numbers of B cells expressing each IgG subclass were similar in cultures from athymic nu/nu mice, nu/+ heterozygous littermates, and normal BALB/c mice. Plasmablasts and plasma cells appeared over a 14-day culture interval, and these expressed cytoplasmic IgM, IgG3, IgG1, IgG2b, IgG2a, and IgA. Measurable amounts of the first four isotypes were detected in the culture supernatants by radioimmunoassay. These results indicate that neonatal B cells can undergo isotype switching in the absence of T cell help, and that the expression of sIgD may not be a prerequisite for cells to switch Ig isotypes.     

IL-6 is a potent cofactor of IL-1 in IgM synthesis and of IL-5 in IgA synthesis

In these studies we determined the capacity of IL-6 to act as a differentiation cofactor for murine Peyer's patch B cells producing different Ig classes and subclasses. In preliminary studies we determined that sufficient endogenous IL-6 was produced in LPS-induced cell systems to obscure responses to exogenous IL-6. We therefore studied IL-6 effects on Peyer's patch B cells (T cell-depleted cell populations) in the absence of LPS, relying on responses of in vivo-activated cells. rIL-1 alpha or purified IL-6 only slightly enhanced synthesis of IgM over minimal baseline levels in Peyer's patch T cell-depleted cell cultures; however, when IL-6 was added to cultures also containing rIL-1, IgM synthesis was very substantially increased. In addition, rIL-5 alone gave rise to a modest increase in IgM synthesis and its effect was not enhanced by either rIL-1 or IL-6. IgG production (mainly IgG3) followed a similar pattern. In contrast, IgA production was only modestly increased above baseline by rIL-1, rIL-5, or IL-6 alone or by rIL-1 and IL-6 in combination, but was greatly increased by rIL-5 and IL-6 in combination. The effect of IL-6 on Ig synthesis in the above studies was not due to an effect on cell proliferation. In summary, these data indicate that B cells differ in respect to the cytokines supporting maximal terminal differentiation and thus the class of Ig produced may depend on the presence of a particular combination of cytokines and lymphokines.     

Effect of normal and immune IgG, IgM and IgA on the intestinal microflora of mice with experimental dysbacteriosis

Human IgG, IgM and IgA produce a pronounced protective effect, preventing enterobacteria from penetration into the mucous membrane of the proximal section of the small intestine of mice in antibiotic-induced dysbacteriosis. Normal mouse IgG and IgM, in contrast to IgA, are effective against mucosal enterobacteria of the small intestine. Immune mouse IgG, IgM and IgA show greater activity in protecting the mucous membrane than normal immunoglobulins of these classes.     

The IgE and IgG subclass responses of mice to four helminth parasites

To investigate whether the formation of IgE is linked in vivo to an IgG subclass, mice were infected with four helminth parasites, Nippostrongylus brasiliensis (Nbr), Mesocestoides corti, Taenia crassiceps and Trichinella spiralis, and the changes in the serum levels of the different Ig isotypes as well as the antibody response to M. corti and T. crassiceps antigen extracts were determined by radioimmunoassays. All four parasites induced a concomitant increase of the IgE and IgG1 serum levels and usually a decrease of the IgG2a level. They also induced an increase of the IgM level but had little effect on the IgG2b, IgG3, and IgA serum levels. The specific antibodies to an M. corti antigen extract were mainly of the IgG1 subclass, whereas it was of both IgG1 and IgG2a subclasses to T. crassiceps. Injections of dead M. corti induced an increase of all IgG subclasses and similar levels of IgG1 and IgG2a anti-parasite antibodies. Subcutaneous instead of intraperitoneal infection with T. crassiceps induced higher IgG2a than IgG1 levels and 10-fold lower IgE levels than the natural ip infection; however, despite the greater IgG2a polyclonal response, anti-parasite antibodies were predominantly of the IgG1 subclass. The data demonstrate that natural infection with four different helminth parasites induces a concomitant polyclonal IgG1 and IgE response. These in vivo observations corroborate the recent in vitro findings demonstrating that interleukin-4 induces lipopolysaccharide-activated murine B cells to secrete both IgG1 and IgE, suggesting that the regulation of these two isotypes is linked.     

B cell response to T helper cell subsets. II. Both the stage of T cell differentiation and the cytokines secreted determine the extent and nature of helper activity.

Helper activity of several murine CD4+ T cell subsets was examined. Effector Th, derived from naive cells after 4 days of in vitro stimulation with alloantigen, when generated in the presence of IL-4, secreted high levels of IL-4, IL-5, and IL-6, and low levels of IL-2 and IFN-gamma, and induced the secretion of all Ig isotypes particularly IgM, IgG1, IgA, and IgE from resting allogeneic B cells. Effectors generated with IL-6 secreted IL-2, IL-4, IL-5, IL-6, and IFN-gamma, and induced similar levels of total Ig, 25 to 35 micrograms/ml, but with IgM, IgG3, IgG1, and IgG2a isotypes predominating. Helper activity of these Th was significantly greater than that of effectors generated with IL-2 (10-15 micrograms/ml Ig) and of 24-h-activated naive and memory cells (2-4 micrograms/ml), both of which induced mainly IgM. Unlike other isotypes, IgE was induced only by effector Th generated with IL-4. Blocking studies showed that secretion of all isotypes in response to IL-6-primed effectors was dependent on IL-2, IL-5, and IL-6. IL-4 was required for optimal IgM, IgG1, and IgA secretion, but limited secretion of IgG2a, whereas IFN-gamma was required for optimal IgG2a secretion, and limited IgM, IgG1, and IgA. In contrast, secretion of all isotypes in response to IL-4-primed effectors was dependent on IL-5, although IL-4 and IFN-gamma were also essential for IgE and IgG2a, respectively. Addition of exogenous IL-5 to B cell cultures driven by IL-6-primed effectors did not obviate the requirement for IL-2, IL-4, and IL-6, suggesting that interaction of IL-4-primed effectors with B cells was qualitatively different from that of IL-6-primed effectors, driving B cells to a stage requiring only IL-5 for differentiation. Addition of exogenous factors to IL-2-primed effector Th, particularly IL-4 in the presence of anti-IFN-gamma, resulted in levels of Ig, including IgE, comparable to those induced with other effectors. These results show that functionally distinct Th cell subsets can be generated rapidly in vitro, under the influence of distinct cytokines, which vary dramatically in their levels of help for resting B cells. The cytokines involved in responses to distinct Th cells differ depending on the quality of interaction with the B cell, and the extent of help is strongly determined by the quantity and nature of cytokines secreted by the T cells.