Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Complement C3c”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 847 records · Page 47Linked to original sources
PubMedDate not supplied

Purification and use of the C3d subunit C3.

A method is described for the preparation of C3d from fresh-frozen CPD plasma. A redissolved EDTA euglobulin precipitate formed from defibrinated plasma is chromatographed on DEAE cellulose. The C3-rich peak is further chromatographed by stepwise elution from hydroxy-apatite. The highly purified C3 and C3b so obtained are then treated with commercial C3b inactivator. G-200 sephadex filtration of this material produces a low molecular weight peak containing C3d greater than 95 per cent purity. The purified C3d, which contains negligible carbohydrate, has a molecular weight of 28,000 (based on SDS polyacrylamide gel electrophoresis). When added at a final concentration of less than 70 mug per ml, it completely inhibits the anti-C3d hemagglutinin reactivity of a potent anti-C3d antiglobulin serum but does not inhibit sera with reactivities against C3c, C4b, and C5b. Two of three rabbits hyperimmunized with purified C3d produced antisera exhibiting only anti-C3d precipitin activity. After absorption of heterologous antibodies, the antisera strongly agglutinated C3d-coated RBC, failed to agglutinate RBC glutinated C3d-coated RBC, failed to agglutinate RBC coated only with C4, and reacted weakly against strongly anti-Rh coated RBC. The latter reactivity was readily absorbed by whole IgG. Preliminary studies with 125I-labeled C3d indicate its potential value in assessing potency of anti-C3d reagents.

Animals↗
PubMed1982

Biotinylation of human C3.

Purified human C3 was biotinylated using the biotinyl-N-hydroxysuccinimide imidoester (BNHS). Depending on the input of BNHS, from three to six molecules of biotin were incorporated per C3 molecule. The biotinyl-C3 retained over 90% of its specific hemolytic activity and when bound to sheep erythrocytes maintained its ability to adhere to human C3b receptors. These functions could be blocked by avidin. The biotinyl-C3 was fragmented normally to C3c and C3d in human serum and adsorption with avidin-Sepharose indicated that biotin moities were present in both fragments. Fluorescein-conjugated avidin reacted well with cell-bound biotinyl-C3b and was useful for quantitating C3 fixation by flow cytometry. Ferritin-conjugated avidin was used as a marker to characterize the distribution of biotinyl-C3b on erythrocytes by electron microscopy. These results suggest that biotinyl-C3 and avidin derivatives may be very useful tools for studies of many of the biological functions of C3.

Avidin↗
PubMed1997

Clinical study of cryoglobulinaemia in Chinese patients with chronic hepatitis C.

Cryoglobulinaemia is the most common immunological disorders seen in patients with chronic hepatitis C virus (HCV) infection. We evaluated the incidence and clinical significance of cryoglobulinaemia in 122 Chinese patients with chronic hepatitis C. The pathogenic roles of HCV genotypes and viraemia in this phenomenon were also evaluated. Fifty-four (44%) of the 122 patients with chronic hepatitis C had cryoglobulinaemia. Eleven (20%) of the patients with cryoglobulinaemia had symptoms and signs of cutaneous vasculitis, arthralgia, neuropathy and renal involvement. The patients with cryoglobulinaemia were predominantly female and had a significantly higher mean serum level of rheumatoid factor and a lower mean serum C4 level compared with patients without cryoglobulinaemia (50 vs 29%, 23 vs 15 IU/mL, 25 vs 31 mg/dL, respectively, P < 0.05). The mean serum HCV RNA level, HCV genotype, the presence of serum auto-antibodies, and the rate of cirrhosis were not significantly different between the two groups. Univariate logistic regression analysis showed female serum levels of alanine aminotransferase (> 90 U/L), rheumatoid factor (> 15 IU/mL), C3c (< 100 mg/dL) and C4 (< 20 mg/dL) to be significant predictors of cryoglobulinaemia in chronic hepatitis C patients. However, multivariate analysis showed only serum C4 levels (< 20 mg/dL) as a significantly independent predictor. We concluded that 44% of Chinese patients with chronic hepatitis C had cryoglobulinaemia. Serum C4 levels were significantly lower in chronic hepatitis C patients with cryoglobulinaemia and the serum C4 level was the only clinical independent predictor associated with this phenomenon. Hepatitis C virus genotype and serum viral load were not clinical independent predictors.

Age Factors↗
PubMed1986

Elevated serum immune complex levels in Pogosta disease, an acute alphavirus infection with rash and arthritis.

Circulating immune complexes (CIC) were studied in Pogosta disease, an acute alphavirus infection with fever, rash and arthritis. The disease is caused by a virus antigenically closely related to Sindbis virus. 75 serum specimens from 25 patients with serologically verified infection were obtained from 1-87 days after the onset. Six different CIC detection methods were used and CICs were observed in all patients at least with one test. Tests based on CIC binding onto human platelets followed the natural course of the disease and maximal values were observed between 10-15 days after onset. Slightly elevated levels were observed 2-3 months after onset. The mean conglutinin binding test values were slightly elevated during the whole follow-up period. The severity of arthritis did not directly correlate to CIC levels. C3c and C1q-binding test were positive only in a few cases. Latex and enzyme immunoassay tests for rheumatoid factors gave low positive values in some of the sera. Agarose gel electrophoresis of serum proteins revealed non-specific changes in alpha 1-alpha 2 interzone characteristic of an acute infectious disease. The presence of CIC in the sera of patients with Pogosta disease may indicate body's natural clearange mechanisms of viral antigens. CIC may have a pathogenic role in the prolonged arthritis, even though no direct correlation with CIC levels and severity of arthritis was observed.

Acute Disease↗
PubMed1983

Suppression of T lymphocyte functions by human C3 fragments. I. Inhibition of human T cell proliferative responses by a kallikrein cleavage fragment of human iC3b.

Cleavage of human iC3b by kallikrein isolated from human plasma generates a fragment, C3d-K, which is capable of inhibiting mitogen-, antigen-, and alloantigen-induced T lymphocyte proliferation. Native C3, C3a, C3b, and C3c-K had no effect on lymphocyte proliferative responses. In addition to being a potent suppressor of mitogen- and antigen-induced proliferation, C3d-K is capable of inducing leukocytosis in both mice and rabbits. Intravenous injection of C3d-K, but not C3, C3a, C3b, or C3c-K, results in a twofold to threefold increase in the number of circulating leukocytes. Thus, C3d-K exhibits two apparently independent functions, namely suppression of T cell proliferation and leukocytosis. Cleavage of iC3b by kallikrein results in the production of only two fragments. The larger fragment, C3c-K, is 144,000 m.w. and has a chemical structure analogous to that of C3c obtained from the cleavage of C3 by trypsin or elastase. The smaller fragment, C3d-K, is 41,000 m.w. and contains the metastable binding site of C3. It is through this site located in the C3d region of the molecule that C3 attaches covalently to target cells. Analysis of the amino terminal region of C3d-K provided a sequence that fails to overlap with any sequence yet reported for other characterized C3 fragments, including C3d originally obtained from elastase digestion. A revised model of the C3 molecule is proposed, with locations of the C3e and C3d fragments assigned on the basis of chemical analyses.

Adult↗
PubMed1993

Association of Sjögren's syndrome with pulmonary hypertension: report of two cases and review of the literature.

We report two autopsy cases of Sjögren's syndrome associated with pulmonary hypertension. The pulmonary muscular arteries of both cases showed concentric fibrocellular intimal proliferation, medial hypertrophy, and plexiform lesions. To determine the significance and pathogenesis of this rare association, we carried out morphometric and immunofluorescent studies and reviewed the seven similar cases reported in the literature. Depositions of immunoglobulin G, Clq, C3c, C4, and C5 were observed in the pulmonary arterial walls of both of our cases. Morphometric studies revealed increased medial thickness to radius ratios and intimal thickness to radius ratios of the pulmonary muscular arteries in both cases. Previously reported patients were all female, and those cases were frequently associated with Raynaud's phenomenon. This report provides additional and convincing evidence for an association of Sjögren's syndrome and plexogenic pulmonary hypertension based on a detailed study of two cases and a review of the literature. The significance and pathogenesis of this association were examined, but not clarified. However, our studies add to the accumulating data suggesting a link between autoimmune diseases and chronic pulmonary hypertension.

Adult↗
PubMed1991

Proteolysis of C3 on U937 cell plasma membranes. Purification of cathepsin G.

Covalent binding of C3 fragments to U937 cell membranes involved a cell surface-associated proteolytic activity. Two proteases able to cleave C3 were purified from U937 plasma membranes. Purification involved solubilization of the membranes and ion exchange chromatography. One of the purified proteases was identified as elastase, based upon a substrate specificity for benzyloxycarbonylalanine-o-nitrophenyl ester and complete inhibition by elastatinal and methoxysuccinyl-alanyl-alanyl-prolyl-valyl-chloromethyl-ketone. The other protease (m.w. 28,000) is cathepsin G, as deduced from the amino acid composition, the amino-terminal sequence, and the substrate specificity for succinyl-alanyl-alanyl-phenylalanine-p-nitroanilide. These two lysosomal proteases are present on the U937 cell surface, as confirmed by immunofluorescence analysis. Plasma membrane elastase and cathepsin G from U937 cells cleave C3 into C3a- and C3b-like fragments; further incubation leads to C3c- and C3dg-like fragments, as judged from SDS-PAGE analysis of the digests. Sequencing of the C3b-like fragment purified by reverse phase chromatography indicates that initial cleavage of C3 by purified cathepsin G occurs at two positions in the amino-terminal part of the alpha-chain, at a Arg-Ser bond located between residues 748 and 749 and at a Leu-Asp bond between residues 751 and 752. These proteases are, thus, able to generate, on the U937 surface, active fragments of C3, which are likely to be involved in cell-protein and cell-cell interactions.

Amino Acid Sequence↗
PubMedDate not supplied

Activation of intravascular macrophages within myocardial small vessels is a feature of acute vascular rejection in human heart transplants.

BACKGROUND: We investigated the pathogenesis of acute vascular rejection by performing immunofluorescent screening on frozen sections for C1q, C3c, and immunoglobulin M in endomyocardial biopsy specimens from all new heart transplants. METHODS: Immunofluorescence for C4c, C5, immunoglobulin G, and immunoglobulin A was performed on all positive endomyocardial biopsy specimens. Twenty-eight positive endomyocardial biopsy specimens from six patients were identified, and 22 of those were studied with transmission electron microscopy. RESULTS: Endothelial hyperplasia and myocyte necrosis were prominent in the five female patients with positive immunofluorescence. In addition, macrophages with ultrastructural cytologic features of activation were seen filling capillaries and venules in intimate contact with endothelium and exiting those vessels. Activated macrophages were large cells with abundant cytoplasm and ruffled borders and contained numerous lysosomes, rough endoplasmic reticulum, and mitochondria. Intravascular activated macrophages were identified in five of six patients with positive immunofluorescence but were not seen in any of the endomyocardial biopsy specimens with negative immunofluorescence, including multiple examples of moderate (grades 2 to 3B) and severe (grade 4) acute cellular rejection. In the five female patients with activated macrophages, acute vascular rejection recurred multiple times with one fatality. Review of the files showed three additional, similar cases. The one male patient with positive immunofluorescence but without activated macrophages had only a single episode of acute vascular rejection. CONCLUSIONS: Complement and antibodies can activate macrophages, so this finding is not surprising. To the best of our knowledge, this is the first report of the intravascular activation of macrophages, and the first association of this process with acute vascular rejection. Activated macrophages may contribute to myocyte necrosis in acute vascular rejection by compromising blood flow in small vessels.

Acute Disease↗
PubMed2002

Membranous glomerulonephritis, antiphospholipid syndrome, and persistent low C3 levels associated with meningococcal disease.

A young male patient with a recent history of meningococcemia was referred to our hospital in his recovery period. He had signs suggesting deep venous thrombosis in the legs but no other abnormalities on physical examination at admission. Laboratory results showed proteinuria (3.1 g/day), prolonged activated partial thromboplastin time (56.3 s), low level of C3c (0.19 g/l), high titers of both IgM (27.04 MPLU/ml) and IgG (74.88 GPLU/ml) anticardiolipin antibodies and recanalized thrombotic changes in the deep veins of the lower extremities on venography. Histopathological diagnosis of the kidney disease was membranous glomerulonephritis. He was started on an angiotensin-converting enzyme inhibitor to reduce proteinuria and an oral anticoagulant to prevent thromboembolic events. Since no reduction in proteinuria was observed at the 10th month of therapy, the angiotensin-converting enzyme inhibitor was discontinued. On his last follow-up, approximately 1.5 years after meningococcemia, he had no complaints and no abnormal findings on physical examination. While both IgM and IgG anticardiolipin antibody titers returned to the normal range, he still had persistent proteinuria and hypocomplementemia.

Adult↗
PubMed1987

Anti-idiotypic antibodies in antisera against human C3 and factor H and their application in the enrichment of antibodies specific for H-binding domains of C3.

Antisera separately raised against C3 and factor H should contain antibodies against the binding domains by which these factors interact. Because these interacting domains are likely to be sterically complementary to each other, antibodies specific for these domains should also be sterically complementary or of anti-idiotypic specificity. To test this hypothesis, rabbit anti-human C3 IgG antibodies which bound to Sepharose-coupled rabbit anti-factor H IgG (anti-H-binding anti-C3) were separated by affinity chromatography. In a control experiment, the anti-human factor H column was found to bind 10 times more anti-C3 antibodies than a Sepharose column coupled with nonimmune rabbit IgG. The binding specificity of the control eluate was indistinguishable from the original anti-C3 preparation, whereas the anti-H-binding anti-C3 was shown to be enriched in regard to antibodies directed against the 42 kd fragment of the C3 alpha-chain of C3c, as demonstrated by immunoblotting of electrophoretically separated polypeptides of C3c and C3d in polyacrylamide gels. In competition binding studies, factor H was 25 times more potent as inhibitor of the anti-H-binding anti-C3 than of the original anti-C3 preparation. This similarity in binding specificity also paralleled a functional similarity in that anti-H-binding anti-C3 in the presence of factor I effected a 20% degradation of the alpha'-chain of C3b into fragments of 65, 45, and 42 kd which are the normal degradation products in the presence of factors I and H. These results suggest that the affinity procedure described resulted in the enrichment of anti-C3 antibodies with selective specificities for factor H-binding domains of C3, from a polyclonal antibody source.

Antibody Specificity↗
PubMed1987

Characterization of three monoclonal antibodies against C3 with selective specificities.

We raised 15 mouse monoclonal antibodies against human C3 from two separate fusions. Mouse plasmacytoma cells were fused with spleen cells from mice immunized either with a mixture of C3, C3b and C3c, or with a mixture of C3dg and C3d. Three of the 15 monoclonals were characterized in detail. N-7A reacted with native C3 as well as C3b and C3c. Two other monoclonals, C-5G and G-3E, recognized neoantigenic determinants on C3c and C3dg, respectively. In ELISA, C-5G reacted with C3b and C3c but not with native C3 nor with C3dg; and on the other hand G-3E reacted only with C3dg. The selective specificities of these monoclonals were further confirmed in a binding assay to C3 fragments formed on cellular intermediates. C-5G bound exclusively to EC3b, and G-3E bound to EiC3b, EC3dg and EC3d. N-7A bound only very poorly to EC3b. C-5G inhibited both the haemolytic activity of C5 convertase and also the CR1-mediated rosette formation of B-enriched peripheral mononuclear cells with EC3b. G-3E inhibited the CR2-mediated EC3dg-rosette formation of Raji cells. These monoclonals, with selective specificities, can distinguish the state of activation and degradation of the C3 molecule.

Antibodies, Monoclonal↗
PubMed1985

Factor I co-factor activity of CR1 overcomes the protective effect of IgG on covalently bound C3b residues.

We have shown previously that C3b resides in a protected site when it is covalently bound to IgG (C3b-IgG). Such C3b displays a reduced affinity for factor H, with consequent enhanced survival in the presence of factors H and I and increased capacity for promoting alternative pathway consumption of C3. Because erythrocyte CR1 may be a major co-factor for factor I-mediated inactivation of immune complex-borne C3b in blood, we have examined the effect of covalently bound IgG on the C3b-CR1 interaction. Binding of monomeric C3b and C3b-IgG to human erythrocyte CR1 demonstrates identical ionic strength dependence for both species. Identical numbers of binding sites with indistinguishable affinities are detected by both ligands. Cleavage of the alpha'-chain of C3b and the alpha'-heavy chain of C3b-IgG proceeds at the same rate when erythrocyte CR1 serves as co-factor for factor I. Unlike factor H, CR1 supports a second cleavage of fluid-phase iC3b alpha'1 chain (free or bound to IgG) that generates C3c and a 33,000 m.w. fragment, which bears antigenic markers characteristic of C3g. Inactivation of C3b and C3b-IgG by CR1 and factor I also occurs at physiologic ionic strength, but proceeds very slowly relative to rates attainable with sub-physiologic inputs of factor H. CR1 does not recognize IgG-bound C3b as being in a protected site but, because of low binding affinity at physiologic ionic strength, is probably highly dependent on multivalent ligand-receptor interactions to efficiently exert its co-factor functions. Thus, inactivation of C3b-IgG heterodimers or small immune complexes bearing limited numbers of C3b residues may remain largely factor H-dependent in vivo, with resultant enhanced C3b survival.

Antigen-Antibody Complex↗
PubMed1982

Characterization of the lymphocyte membrane receptor for factor H (beta 1H-globulin) with an antibody to anti-factor H idiotype.

Antibody to the binding site (idiotype) of anti-factor H was shown to have specificity for both B lymphocyte membrane H receptors and C3b. Goat F(ab')(2) anti-human H was purified by absorption and elution from H agarose and used for rabbit immunization to produce anti-anti-H (aaH). After absorption with nonimmune goat IgG, (125)I-labeled aaH bound to B lymphocytes and to sheep erythrocytes coated with C3b (EC3b) but did not bind to T lymphocytes or to EC3d. All B cell- and C3b-specific activities of the aaH were removed and subsequently recovered by absorption and elution of the antibody from either C3-agarose or goat-anti-H-agarose. This indicated that the aaH probably recognized a single common antigenic structure that was shared by anti-H, C3b, and the membranes of B cells. Affinity-purified aaH resembled H in that it bound to B cells, blocked the uptake of H onto B cell H receptors, and triggered B cells to release endogenous factor I (C3b inactivator). In addition, aaH functioned with factor I as either a cofactor for cleavage of fluid-phase C3b or a potentiator for cleavage of bound C3b. This same spectrum of C3 binding functions could not be demonstrated with either sheep anti-C3b or rabbit-anti-C3c. Analysis by sodium dodecyl sulfate- polyacrylamide get electrophoresis of the [(3)H]leucine intrinsically labeled B cell proteins reactive with the purified aaH revealed proteins of 100,000 M(r) and 50,000 M(r) without reduction, and after complete reduction of disulfide bonds, a single protein band of 50,000 M(r). This same protein molecular weight profile was also demonstrated with labeled B cell proteins that were absorbed and eluted from H-agarose. Thus, aaH is apparently specific for both B cell H receptors and C3b. However, because parallel analysis of C3b confirmed its known 115,000- and 75,000-M(r) polypeptide chain structure, the H receptor is probably not C3b and shares only the structure of the H binding site with C3b.

Animals↗
PubMed1990

Class II antigen expression in diabetic preretinal membranes.

Using immunofluorescence and immunoperoxidase procedures, we found large amounts of IgG, IgA, IgM, and IgE, as well as C1q, C3c, and C3d, in the connective stroma and within the vascular walls on eight specimens of preretinal membranes obtained from diabetic patients with proliferative retinopathy. The membranes contained many isolated human leukocyte antigen (HLA) DR- and DQ-expressing cells, and vascular endothelial cells strongly expressed class II determinants. Monoclonal antibodies to immunocompetent cells disclosed only rare B lymphocytes or suppressor/cytotoxic T cells and few monocytes. These findings confirm previous evidence of immune reactions in the pars plana of patients with proliferative diabetic retinopathy, and suggest that an autoimmune reaction is a factor in this complication. It is yet not possible to determine whether this reaction is a nonspecific consequence of the vasoproliferative processes or if it plays a direct role in the development and extension of preretinal membranes.

Adult↗
PubMed2000

The effect of testosterone replacement treatment on immunological features of patients with Klinefelter's syndrome.

Although the effects of androgen deficiency in the immune system have long been appreciated, little is known about the immunological features of patients with Klinefelter's syndrome (KS). On the other hand, interest in androgens as a possible treatment for some autoimmune diseases is growing. In the present study, some immunological parameters were evaluated in 26 patients with KS prior to androgen replacement treatment (ART) and the results were compared with those in 19 healthy control subjects. Patients were then treated with testosterone for 6 months and the pre- and post-treatment findings were compared. Serum levels of IgG, IgA, IgM, C3c and C4 were measured by nephelometry and lymphocyte subsets and CD4+/CD8+ ratios were examined by flow cytometry. IL-2 and IL-4 levels were measured by ELISA. Pretreatment levels of the serum IgA, IgG, IgM, IL-2 and IL-4 of the patients were higher than those of the controls and were all decreased significantly following ART. The pretreatment absolute numbers and percentages of CD3+, CD4+, CD19+ cells and CD4+/CD8+ ratios of patients with KS were higher than those of the controls and were all decreased with ART. Percentages of CD8+ cells were increased significantly, while C3 and C4 levels were both significantly decreased after ART. It is concluded that the lack of testosterone in patients with KS enhances cellular and humoral immunity and that ART may suppress this.

Adult↗
PubMed1989

Enzyme immunoassay detection of circulating immune complexes by monoclonal antibodies to C3 neoepitopes with special reference to IgG concentration and to interfering anti-immunoglobulin antibodies.

A sensitive solid-phase anti-C3 enzyme immunoassay for detection of circulating immune complexes (CIC) is described. A mixture of the monoclonal antibodies (MoAbs) bH6 and Clone 9 specific for neoepitopes on C3 activation products was used as capture reagent. MoAb bH6 recognized C3b, iC3b and C3c, and Clone 9 recognized iC3b and C3dg. Detection antibody was a polyclonal peroxidase-conjugated rabbit anti-human Ig antiserum. A quantitative assay was constructed using serum incubated with heat aggregated IgG (HAG) as standard. The lower detection limit was 5 micrograms/ml of HAG. Interassay and intra-assay coefficient of variation was 15% and 5%, respectively. Anti-animal immunoglobulin antibodies were detected both in normal and pathological sera. This activity was efficiently absorbed by nonimmune immunoglobulins added to the samples. The present assay was compared with a polyethylene glycol precipitation assay for CIC determination. The latter assay was strongly influenced by the IgG concentration (rs = 0.78; P = 0.006), whereas no such correlation was seen for the anti-C3 immune complex assay (rs = -0.30; P = 0.20).

Animals↗
PubMed1990

Demonstration in human plasma of a form of C3 that has the conformation of "C3b-like C3".

Disruption of the thioester in native C3 yields a C3 molecule that functionally resembles C3b. It has been proposed that this C3 molecule (iC3) plays a key role in initiation of the alternative pathway of the C system. However, its presence in plasma has never been demonstrated. We investigated the presence of iC3 in plasma, using mAb that recognize iC3 as well as C3 activation products but not native C3. One of these mAb, anti-C3-5, which binds iC3 via its C3a moiety, was used together with polyclonal 125I-anti-C3c to develop a RIA for iC3. Plasma incubated with methylamine yielded a strong response in this RIA, whereas neither fresh plasma nor serum in which the C system had been activated by incubation with aggregated IgG, did show this strong response. The specificity of this RIA was further demonstrated by additional experiments including experiments with purified preparations of the various forms of C3. Mean level of iC3 in freshly obtained plasma samples from 10 normal donors was 27 nmol/liter, which is 0.49% of total C3. Analysis by SDS-PAGE of C3 species that had been immunoprecipitated by mAb antiC3-5, revealed that some iC3 consisted of C3 molecules with an intact alpha-chain whereas another part consisted of iC3 molecules with an alpha-chain that had been cleaved by factor I. Thus, this study shows that fresh human plasma contains a C3 species with the conformation of "C3b-like C3" (iC3).

Antibodies, Monoclonal↗
PubMed1993

Activated C3 (C3b) in the nephritic glomerulus.

An autoradiographic technique was developed to assess in the nephritic glomerulus the relative amount of C3 which is in the activated form, C3b, compared with the inactivated form, iC3b. Frozen renal biopsy sections from children and young adults with glomerulonephritis were assessed for the C3b fraction of total C3, using a radiolabeled monoclonal anti-C3c. Grain counts with this antibody, before and after reacting the section with 0.0002% trypsin, gave the relative amounts of total C3 and C3b, respectively. C3b was found in all diseases studied. To explain its presence, glomerular C3b acceptors which would restrict C3b inactivation were sought by immunofluorescence studies. C3b acceptor candidates were: IgG in aggregated form, IgA as found in the IgA nephropathies and the C3/C5 convertase, C4b,2a,3b. In acute post-streptococcal glomerulonephritis and membranoproliferative glomerulonephritis type III, diseases in which these acceptors were lacking, it is postulated that the nephritis strain-associated protein and absence of membrane cofactor protein, respectively, may be responsible for C3b deposition. The phlogistic effect of C3b is mediated largely by one of its products, C5b-9. However, the C3b: total C3 ratio failed to correlate with indices of glomerular inflammation, probably in part because the ratio is not a measure of total glomerular C3b.

Adolescent↗