Abstract: The alternative or properdin pathway of complement is primarily controlled by the endopeptidase C3b inactivator (C3bINA) and the nonproteolytic glycoprotein β1H. The molecular mechanisms of control were investigated by performing binding studies of radiolabeled complement proteins to C3b bearing sheep erythrocytes (ESC3b). C3b was found to have distinct binding sites for β1H, C3bINA, Factor B, and properdin. β1H binding increased C3bINA binding 30-fold, while Factor B binding prevented C3bINA action on C3b and was competitive with β1H binding. Properdin binding, which facilitates Factor B interaction with C3b, had no effect on the β1H and C3bINA sites. Activators such as rabbit erythrocytes (ER) have previously been shown to interfere with the effectiveness of the control by C3bINA and β1H, thereby allowing unrestricted formation of C3 convertase. Such restriction of control does not occur on the surface of ES, a nonactivator of the alternative pathway. On the basis of comparative binding studies, restriction of control is explained entirely by reduced binding of β1H to ERC3b relative to ESC3b. Access of properdin, Factor B, C3bINA, and the Fab fragment of anti-C3 to the two cell types was unrestricted. Restriction of β1H control could be generated on the surface of ES by removal of cell-surface sialic acid with neuraminidase (acylneuraminyl hydrolase; EC 3.2.1.18). This enzymatic treatment converted ES from a nonactivator to an activator of the alternative pathway.

Abstract: Eosinophils from the peritoneal cavity of normal rats, in the presence of fresh normal rat serum (NRS), adhered to schistosomula of Schistosoma mansoni in vitro and killed the majority of parasites within 18 h. The reaction differed from the previously described antibody-mediated eosinophil adherence to schistosomula which occurs in heat-inactivated immune rat serum (IRS) and where adherence is mediated through Fc receptors. Adherence of eosinophils in fresh NRS was shown to be due to the activation of complement at the schistosomular surface by the alternative pathway, and it was effected through C3 receptors. The ability of eosinophils to kill in Fc-mediated adherence. This enhancement of killer activity may be due to the generation by complement activation of eosinophil chemotactic factors which increase the concentration of cells at the target surface. It is suggested that eosinophil adherence mediated through complement activation could be the principla mechanism of destroying schistosomula in the host.

Abstract: We evaluated 20 patients with primary biliary cirrhosis and seven controls with extrahepatic biliary obstruction for presence of circulating immune complexes, having found serologic evidence of alternate complement-pathway activation in eight of the 20. Immune complexes were isolated by cryoprecipitation from serum and measured directly by the sensitive Raji-cell radioimmunoassay. Cryoproteins, found in high concentrations in 90 per cent of the patients with cirrhosis but undetectable in the controls, were composed of IgM (60 per cent), IgG-IgM (25 per cent) and IgA-IgM (5 per cent) and were capable of activating the complement system in vitro. Immune complexes detected by the Raji assay were found in 95 per cent of the patients with cirrhosis and circulated in exceedingly high concentrations (474 μg per milliliter; range, 16.2 to 2192) but were absent in the controls. Furthermore, the alternate complement pathway was activated in eight cirrhotic patients. These complement-fixing immune complexes...

Abstract: The results of these studies demonstrate that cell wall teichoic acid, rather than peptidoglycan, is responsible for pneumococcal activation of the alternative pathway.

Abstract: Activation of the alternate complement pathway in human serum by several bacterial components was compared. Peptidoglycan from group A streptococcal cell walls was the most active material, on a weight basis, followed by cell walls, protoplast membranes, and whole cells. The group-specific carbohydrate was inactive. Treatment of peptidoglycan with low concentrations of lysozyme or short periods of sonic treatment enhanced complement activation. High concentrations of lysozyme or extended sonic treatment of peptidoglycan destroyed or greatly reduced the capacity to activate complement. Lysozyme treatment of group A streptococcal cell walls or lipopolysaccharide had no measurable effect. Activation of the alternate complement pathway by group D streptococcal cell walls was destroyed by lysozyme. Activity of peptidoglycan was not inhibited by N-acetyl glucosamine, N-acetyl muramic acid, or D-alanine-D-alanine. Conversion of C3 and factored B by peptidoglycan was shown to occur by immunoelectrophoresis and crossed immunoelectrophoresis.

Abstract: Moloney leukemia virus activated both the classical and alternative pathways of human complement. About 500,000 virions were required to detect activation of the classical pathway whereas 5,000 times as many virions were necessary to initiate the alternative pathway, indicating that in this system only the former is of biological significance. Disruption of the virus with Triton X-100 destroyed its ability to initiate the alternative pathway without affecting its ability to activate the classical pathway. After ultracentrifugation of disrupted virus the active component could be recovered in the supernate and was isolated by isoelectric focusing in granulated gels. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic and analysis and cyanogen bromide digestion studies revealed that the activity resided in a methionine-containing protein having a pI of 7.5 and a molecular weight of approximately equal to 15,000 daltons. The purified protein interacts strongly with Clq and efficiently activates Cl. RNase and lipolytic enzymes had no effect on the isolated protein but incubation with trypsin resulted in loss of activity. Enzymatic digestion studies of surface-labeled virus indicate that the active protein is a viral membrane protein. On the basis of these results it is concluded that the complement receptor of Moloney leukemia virus is the surface protein p15E.

Abstract: Activators of the alternative C pathway were ingested by human monocytes in a synthetic medium devoid of serum or proteins of the alternative C pathway. The rank order of the percentage of human monocytes ingesting rabbit, mouse C57B, and mouse CBA erythrocytes was the same as that observed for lysis of these erythrocytes by the alternative C pathway in C2-deficient human serum diluted in GVB-Mg-EGTA. Sheep and guinea pig erythrocytes, which were not ingested by human monocytes, were not lysed by the alternative C pathway. Although the capacity of human monocytes to ingest rabbit erythrocytes was consistent for a particular donor, there was a bimodal distribution among donors in terms of the percentage of monocytes involved. Zymosan ingestion, on the other hand, was observed with a high proportion of monocytes from all donors. The capacity of human monocytes to ingest zymosan particles and rabbit erythrocytes was largely diminished when the monocytes were pretreated with quantities of affinity-purified trypsin, which had little or no effect on their ability to phagocytose EA or even to bind E(IgM)C43b intermediates. Latex particles that did not activate the alternative pathway of C were ingested by human monocytes after treatment with a concentration of trypsin 10 times that required to reduce substantially ingestion of zymosan. The trypsin-sensitive mechanism of monocytes involved in the ingestion of zymosan and rabbit erythrocytes was regenerated over 48 hr during in vitro culture and is a result compatible with the restoration of function of a membrane protein. The capacity of human monocytes to ingest particulate activators of the alternative C pathway in the absence of surface-associated opsonizing proteins such as IgG or C3b by a trypsin-sensitive step represents a monocyte recognition mechanism specific for activating surfaces of the alternative C pathway.

Abstract: In this paper we examine the role of the classical pathway in the complement-mediated solubilization of immune precipitates (CRA). Serum reagents were depleted of the alternative pathway components properdin and factor D. Both depleted reagents lack CRA although they have almost intact hemolytic activity. Also, immune complexes were not solubilized when incubated with high concentrations of the classical pathway components (C1, C4, C2, and C3. We conclude that CRA is not mediated by the classical pathway alone. Activation of the classical pathway by the immune aggregates greatly enhances CRA. The effect of the classical pathway is to deposit C3b on the antigen-antibody lattice and promote the assembly of a lattice-associated, properdin-dependent C3-convertase. Although C3, C4, and properdin were detected on complexes solubilized by serum in the presence of Ca++ and Mg++, only C3 and properdin were found on the complexes when Ca++ had been chelated by ethylene glycol-bis-(beta-aminoethyl ether), N,N'-tetraacetic acid. In both situations the aggregates were capable of converting C5 in the fluid phase. However, no C5 was found on the solubilized complexes. These findings suggest that in contrast to nascent C3b and C4b, nascent C5-9 lacks binding affinity for immune aggregates.

Abstract: Activation of the alternative complement pathway by human B cell lymphoma lines is correlated with the presence of Epstein Barr virus (EBV) in the cell genome. EBV-negative B cell lymphoma lines produce little activation of the alternative pathway as measured either by C3 deposition on the cell surface or C3 conversion and consumption of alternative pathway activity in the supernatant serum. By contrast, EBV-positive sublines derived by in vitro EBV conversion of EBV-negative parental lines produce considerable activation of the alternative pathway. This membrane-associated complement-activating mechanism reflects an EBV-induced membrane change in these cells and may provide a mechanism whereby EBV-transformed cells are controlled in vivo.

Abstract: Rabbit erythrocytes (RaRBC) can serve as both the initiator and target of alternative pathway activity. We describe here a turbidimetric assay of RaRBC hemolysis that depends on the observation that, within the limits applied, absorbance at 700 nm of an erythrocyte suspension is directly proportional to cell concentration. Absorbance of multiple samples can be measured at frequent short intervals, which permits kinetic analysis. Sera to be studied are chelated with 0.008 M EGTA in the presence of 0.002 M Mg++, which prevents lysis of antibody-coated sheep erythrocytes through the classical pathway, even by concentrated serum. The time required to lyse one-half the RaRBC (t½) is directly proportional to the reciprocal of the serum dilution (1/S). RaRBC lysis by human serum appears to require factor B in that sera heated to 50°C for 30 min to inactivate this protein lost all activity in the assay, and activity could be restored in a dose-response fashion with purified factor B. Sera from two humans with homozygous C2 deficiency and approximately half-normal serum levels of factor B were mildly deficient in the assay; this abnormality could be removed by the addition of purified factor B but not C2. C4-deficient serum functioned normally. Serum from guinea pigs, mice, and chickens also demonstrated alternative pathway activity by this technique. The relationship between t½ and serum dilution can be expressed by a time-dependent von Krogh equation for complement-mediated lysis, which extends the usefulness of the assay in analyzing rate-limiting components of alternative pathway activation. The maximum rate of hemolysis reported for antibody-coated sheep erythrocytes is about 50-fold greater than that calculated by this equation for RaRBC lysis, in agreement with observations in other systems that biologic activity is generated more slowly through the alternative pathway than through the classical pathway.

Abstract: Eighty per cent of blood culture isolates of K-1 Escherichia coli are “resistant” to in vitro opsonization and phagocytosis by normal human leukocytes and serum. To investigate why most of bacteremic K-1 E. coli are poorly opsonized by normal human serum, we undertook this study to assess the serum opsonic requirements for K-1 isolates particularly in regard to the role of the alternative pathway of complement activation. To ascertain the role of complement in opsonization, we used an in vitro opsonic and bactericidal killing system and a method that determines complement activation by measuring consumption of hemolytic complement. For both assays, MgEGTA was used to specifically block the classic pathway while leaving the alternative intact. Additionally, we used the bactericidal killing assay to determine the possible participation of specific opsonins, like antibody, in the opsonization of these E. coli by using a serum source that was either previously absorbed with bacteria to remove antibody or by using purified K-1 capsular antigen to block specific K-1 antibody. Data demonstrate that E. coli K-1 are opsonized only by the classic pathway of complement via antibody that is not specific for the capsular K-1 antigen. Furthermore, the hemolytic complement consumption method was shown to be far less sensitive in determining which complement pathway is activated during opsonization of E. coli K-1. The restriction of these bacteremic K-1 E. coli isolates to the classic pathway of complement activation and the requirements for specific antibody for effective opsonization may be important factors underlying the pathogenicity of these bacteria.

Abstract: A requirement for the classic complement pathway in opsonization of group B streptococci was observed by using both a chemiluminescence and a radiolabeled bacterial uptake technique. The classic pathway increased levels of opsonization for types Ia and II stock and wild strains and for some type III wild strains. In contrast, other type III wild strains and the type III stock strain had accelerated kinetics of uptake in the presence of an intact classic pathway, but the level of opsonization was unchanged from that with antibody alone. We could not demonstrate a significant role for the alternative pathway in opsonizing stock or wild strains of group B streptococci. Futhermore, electrophoretic and complement consumption analysis by hemolytic titration failed to reveal alternative pathway activation by the majority of strains of this group. Therapy aimed at supplying opsonins for these organisms will require the presence of type-specific antibody.

Abstract: The role of complement in experimental disseminated candidiasis was studied in normal guinea pigs, animals congenitally deficient in the fourth component of complement (C4), and animals depleted of alternative pathway activity by cobra venom factor (CVF). Animals pretreated with CVF and challenged with Candida albicans had a high rate of mortality. Results of quantitative organ cultures corroborated prior reports that the kidney was the major target organ of infection. Infection of the kidney was markedly enhanced by CVF-induced depletion of the alternative pathway but not by classical pathway deficiency (deficiency in C4). There were differences among organs (kidney, liver, and spleen) in their requirement for complement to mount an effective host defense response. Ultimately, the integrity of the alternative pathway and late components of complement appears necessary for the limitation of and survival from sepsis due to C. albicans in nonimmune animals.

Abstract: CONGENITAL deficiency of the second component of complement (C2) has been reported frequently in association with collagen-vascular diseases.1 , 2 Most patients with this disorder have not had serious infections.1 2 3 In the cases in which infections have been described, their pattern has been like that sometimes seen in persons without a demonstrable defect in the immune system,3 4 5 6 7 8 making it difficult to relate with certainty the C2 deficiency and the infection. In the two unrelated C2-deficient children described below, each having three episodes of septicemia, we found an associated partial deficiency of alternative pathway factor B and a functional abnormality of the alternative . . .

Abstract: The yeast phase of P. brasiliensis is poorly phagocytosed by “stimulated” mouse peritoneal macrophages in culture. However, either heterologous (sera from patients with South American Blastomycosis) or homologous antibodies (sera from mice chronically infected with P. brasiliensis ) greatly enhanced the phagocytosis of P. brasiliensis organisms by macrophages. The same effect was observed when guinea-pig, mouse, or human sera were added to the fungus-macrophage cultures. Heating of the sera at 56°C or the addition of EDTA to the system completely abolished this effect. Conversely, EGTA had no effect. These data indicate that the alternative pathway of complement activation is involved in the expression of this phenomenon. This conclusion is further supported by the fact that Cobra venom Factor (CoF) treated serum, properdin, or factor B depleted serum had no opsonic properties. When yeast cells of P. brasiliensis were incubated with fresh normal serum, the third component of the complement system changed its electophoretic mobility of β1C to β1A.

Abstract: Peripheral leucocytes and platelets from twenty healthy volunteers were incubated in vitro with radiographic contrast media (diatrizoate-Hypaque, iothalamate-Conray and iodipamide-Cholografin) under varying conditions. All radiographic contrast media (RCM) were able to induce histamine release from peripheral leucocytes and the release reaction was dose-dependent. There were individual differences in the sensitivity of leucocytes to different RCM. The highest values (up to 80% histamine release) were found with high concentrations (0.07-0.3 M) of diatrizoate. The addition of normal human serum (NHS) to the reaction mixture led to a further increase in histamine release (P is less than 0.01), probably due to complement activation. The mechanism seems to be mediated by proteins of the alternative pathway, because serum depleted of complement components (factor B, factor D, properdin) did not show this synergistic effect. IgG-depleted serum, however, was able to show the augmented release reaction. Washed platelets incubated with RCM released serotonin in a dose- and time-dependent reaction. The most powerful serotonin-releasing RCM was found to be iodipamide, which produced a release reaction in all people investigated at concentrations of 0.04-0.09 M, while diatrizoate and iothalamate were effective only in half of the tested individuals at high concentrations (0.3 and 0.2 M respectively). The addition of plasma proteins to the reaction mixture inhibited the serotonin release quantitatively. There was no difference in inhibitory potency between autologous and heterologous plasma or serum; sera depleted of various complement components showed similar effects as NHS. The serotonin release was not due to platelet lysis, as determined by the concentration of lactic dehydrogenase present in the supernatant during serotonin release. Incubation of the leucocytes with RCM produced ultrastructural changes, including degranulation of basophils, aggregation of platelets and infiltration of the aggregates by polymorphonuclear leucocytes. The most prominent changes were observed when complement was present in the reaction mixture.

Abstract: The known cytolytic function of the alternative pathway in serum was quantitatively reproduced by combining 11 isolated plasma proteins at their respective serum concentrations. These proteins are: C3, Factor B, Factor D, C3b inactivator, beta1H, native properdin, C5, C6, C7, C8, and C9. In absence of activators of the alternative pathway, this mixture was stable at 37 degrees C as evidenced by lack of consumption of Factor B, C3, and C5. Upon addition of either rabbit erythrocytes or neuraminidase-treated sheep erythrocytes, cell lysis ensued and the extent of lysis was dependent on dose of the component mixture. The dose-response curves obtained with the isolated component mixture and with C4-depleted serum were virtually indistinguishable. Nonactivator erythrocytes (untreated sheep erythrocytes) were not lysed by the component mixture. Deletion of properdin resulted only in a twofold diminution of the hemolytic activity of the component mixture. No immunoglobulin requirement was apparent. These results indicate that the cytolytic systems studied are internally sufficient and capable of coupling the initiation and amplification sequence with the cytolytic membrane attack sequence.

Abstract: The efficiency of some anti-tumour polysaccharides, such as lentinan, pachyman, pachymaran, carboxymethylpachymaran and hydroxyethylpachyman to trigger the alternative pathway of complement activation (APC) was investigated in detail, in order to clarify whether solid phase activation of APC by these polysaccharides will occur or not. From the eight polysaccharides tested, all were found to be potent activators of the alternative pathway except for carboxymethylpachymaran, regardless of their potency of inhibition of sarcoma 180 (S 180) transplanted in mice. This activation was observed both for the insoluble as well as the soluble part of the same polysaccharide. The turnover of C3, C5 and factor B showed no difference among these seven active polysaccharides. The polysaccharide particles (PX), isolated after treatment with C4d GPS showed prominent C3 consuming activity, which disappeared during prolonged incubation at 37 degrees. The activity of the decayed enzyme could be regenerated by treatment with the purified factor B. The stability of the particulate enzyme PX was comparable to the zymosan-complex (ZX) previously described.

Abstract: Culture supernatants from monolayers prepared with guinea-pig peritoneal macrophages were found to contain functional D and C3 activity. Factor D was detected by consumption of C3 in the presence of culture supernatant, factor B and insoluble C3b. Preincubation of culture supernatant with anti-D IgG totally inhibited C3 consumption in the D assay which identified factor D as the B activating enzyme. The synthesis of D and C3 by macrophages was proven by the fact that cycloheximide in the culture medium strongly reduced the amount of detectable D and C3 and also incorporation experiments with 14C-labelled aminoacids resulted in the production by macrophages of radio-labelled D and C3. In addition radiolabelled B and P were also detected. The majority of the B protein appeared in its cleaved (Bb) form and, therefore, factor B escaped detection in the functional assay. For unknown reasons functional activity of P was not detectable. The enzyme responsible for B cleavage in the culture supernatants was identified as factor D. Activation of B by the D enzyme in culture supernatants probably occurred through the C3b-dependent feed back cycle of the alternative pathway. This is concluded from our observation that C3 was consumed on incubation of the culture supernatant at 37°, although at a low rate.

Abstract: Effects of various aminoacids and their derivatives on the classical pathway and alternative pathway of the complement were studied. Leupeptin, acetyl-leucyl-leucyl-arginal, inhibited CH50 and Cl-esterase, but did not inhibit the alternative pathway. When aminoacids of carbon chains of the order of seven were used, arginine and lysine had stronger effects than trans-aminomethyl cyclohexane carboxylic acid (t-AMCHA), cis-aminomethyl cyclohexane carboxylic acid (cis-AMCHA) and epsilon aminocaproic acid (EACA). SH-compounds, cysteine, homocysteine and glutathione, had the strongest inhibitory effects among these aminoacids on both classical and alternative pathways. When effects on Cl esterase were compared, arginine, lysine, t-AMCHA, cis-AMCHA and EACA had weak inhibition while SH-compounds showed strong inhibition. Poly-L-lysine, which had extremely strong inhibition of CH50, had no inhibition of Cl esterase. The inhibitory effects of antifibrinolytic agents, EACA and t-AMCHA, were weak but when effects on early parts of the classical pathway, C(1,4,2)H50 were tested, some inhibitory activities were recognized. Thus inhibitory effects of these agents were due to their activities on the early parts of the classical pathway.

Abstract: By using a kinetic assay, we have examined the role of antibody in the lysis of rabbit erythrocytes (RaRBC) through the alternative complement (C) pathway. Sera from some hypogammaglobulinemic (Hγ) humans and all agammaglobulinemic chickens tested had subnormal activity in the assay. Heated normal human or chicken sera, but not heated Hγ sera, restored activity to deficient Hγ serum and initiated hemolysis in the presence of rabbit serum as C source. Absorption of heated normal human serum with RaRBC, but not with sheep erythrocytes or zymosan, removed its ability to reconstitute deficient Hγ serum. Normal hemolytic activity could be restored to Hγ serum with human IgM, IgG, or colostral IgA, with goat anti-RaRBC IgG, or with an eluate from serum-sensitized RaRBC, but not with myeloma IgA. Restoration of hemolytic activity to Hγ serum could be achieved in a dose-dependent fashion with the F(ab′) 2 fragment of IgG. These results suggest that antibody exerts a significant rate-limiting effect on alternative pathway activity in the RaRBC lytic system. This raises the possibility that antibody may be required for efficient alternative pathway activity in vivo and that the pyogenic infections that occur in Hγ individuals are due to inefficient activation and fixation of C3 through either the classical or alternative pathway.

Abstract: The reaction mechanisms of β 1H were studied. The generation of alternative pathway C3 and C5 convertases on the cell surface as well as in the fluid phase was inhibited by

Abstract: A cytopathogenic effect was observed when Entamoeba histolytica was exposed to human sera from individuals with no clinical history or laboratory evidence of amoebiasis. Absorption studies showed that the effect was not due to natural antibodies. Studies performed using ethylenediamine tetracetic acid (EDTA), cobra venom factor (CoF) and heat-inactivation at 56 degrees C, indicated that the cytopathogenic effect was complement dependent. Furthermore, by using ethylene glycol tetracetic acid (EGTA) and Mg++, zymosan, heat-inactivation at 50 degrees C to destroy the activity of factor B of the alternative pathway, as well as electrophoretic studies with anti-human factor B, it was possible to determine that E. histolytica activated the properdin pathway. Finally, complement determinations indicated that incubation of E. histolytica with normal human serum consumed complement. The diminution in CH50 correlated with a consumption of C3 but not of C1, C4 and C2. It was concluded from these results that trophozoites of E. histolytica activate the alternative pathway of the human complement system.

Abstract: Monolayers of human peripheral blood monocytes in the absence of exogenous proteins ingest a variety of natural particulate activators of the human alternative complement pathway. Sheep erythrocytes, which do not ordinarily activate the human alternative complement pathway or initiate a direct monocyte phagocytic response, can be modified to exhibit both functions by the deletion or alteration of membrane sialic acid residues. Enzymatic removal of the sialic acid residues with sialidase or their conversion to heptulosonic acid derivatives by limited oxidation with NaIO4 and reduction with BH4- have equivalent dose-response effects on the capacity of the altered sheep erythrocytes to initiate the phagocytic response by human monocytes or to activate the alternative pathway in human serum. The deposition of C3b on native sheep erythrocytes had little effect on their ingestion by human monocytes, whereas the fixation of C3b on desialated sheep erythrocytes had a synergistic effect on the percentage of monocytes ingesting such a particle. The monocyte receptor essential for ingestion of desialated sheep erythrocytes or desialated sheep erythrocytes bearing C3b was inactivated by concentrations of trypsin that also prevented the monocytes from ingesting natural activators of the human alternative complement pathway, but did not alter the receptors for C3b or the Fc portion of IgG. The capacity of the nonimmune host to respond to desialated particles by initiating the monocyte ingestive process and by activating the alternative complement pathway to provide the synergy afforded by C3b deposition on that particle represents a primitive biochemical basis for differentiation of nonself from self.

Abstract: The opposing actions of component and regulatory proteins of the alternative pathway of C on activation of the sequence were demonstrated in C2-deficient serum utilizing two initiating particles and incremental additions of proteins purified to homogeneity. Activation of the alternative pathway in C2-deficient serum by zymosan was assessed by measurement of the inactivation of C3 and B, while activation by rabbit erythrocytes (ER) was measured by their lysis. Participation of the classical C-component sequence was excluded by the presence of Mg-EGTA and by the failure of reconstitution of C2 to have a measurable effect. Incremental additions of either purified C3bINA or β1H, which increased the serum concentrations of the proteins by 43% to 175% or 15% to 30%, respectively, produced dose-related suppression of zymosan-induced B and C3 inactivation throughout the 45 min period of observation. Additions of either C3 or B to C2-deficient serum so as to increase their concentrations by 35% reversed the combined 50%-inhibitory effect of added C3bINA and β1H on the zymosan-induced inactivation of C3 and B. Dose-dependent inhibition of the rate and extent of lysis of ER in C2-deficient serum was obtained with increases of C3bINA concentrations of 25 to 100% and of β1H concentrations of 12.5 to 50%. As little as a 25% increase in the concentration of B increased the rate of lysis of ER in C2-deficient serum and partially reversed the inhibitory effect of the combined addition of C3bINA and β1H. Thus, the disequilibrium between component and control proteins of the alternative pathway created by the surface of an activating particle, that permits transition to the amplification phase of C3 cleavage, can be modulated by alterations in the concentrations of these proteins. Modest increases in the concentrations of C3bINA and β1H served to maintain balance, whereas comparable augmentations in the concentrations of C3 and B facilitated activation by the initiating particle.