Banana lectin (BanLec) induces non-specific activation of basophils and mast cells in atopic subjects.

Summary: Dietary lectins play a major role in the activation of mast cells / basophils by bridging cell surface IgE glycans to release histamine and other mediators. In the present study, the effect of mannose / glucose-specific banana lectin (BanLec) on the activation of mast cells / basophils from non-atopic and atopic subjects has been investigated. BanLec was purified from banana pulp in a yield of 7 mg/kg. Leukocytes isolated from heparinized blood of non-atopic / atopic subjects were used for quantitation of the released histamine. Approximately 28.2% of the atopics (n = 117) was positive by skin prick test (SPT) to purified BanLec (100 µg/mL concentration), and all the non-atopics (n = 20) were negative. Maximal release of histamine was seen at 2 µg of BanLec. In percent histamine release, an increase of 35-40% is observed in case of atopics (n = 7) compared to non-atopics (n = 5), and the histamine release from atopic and non-atopic subjects correlates fairly well with the total serum IgE levels (R2 = 0.817). BanLec also induces release of histamine (26.7%) from mast cells present in rat peritoneal exudate cells. BanLec can significantly activate and degranulate mast cells and basophils by cross-linking the trimannosidic core mannose of IgE glycans in atopic population as compared to non-atopic population; the activation is marginal in the case of non-atopics.

Skin prick test analysis reveals cross-sensitization to tomato profilin and grass pollen in nasobronchialallergic patients with history of tomato food allergy.

Summary: The association between grass pollen sensitization and food allergy to tomato is of great interest. We report here, the first such study in Indian population. We investigated 246 allergic rhinitis / asthma patients by diagnostic case history and skin prick test (SPT); grass pollen mix, tomato extract and purified tomato profilin were used for SPT. Tomato profilin was purified by affinity chromatography, and analyzed by HPLC (95% purity) and SDS-PAGE (14 kDa). We observed that 38% of the patients had sensitization to both grass pollen and tomato fruit, of which 92% were sensitized to tomato profilin. Among patients with a history of food allergy to tomato fruit, the association was more pronounced (66%). Tomato profilin appears to be an important cross-sensitizing panallergen in respiratory allergic patients in the Indian subcontinent.

Structural and functional characterization of a novel immunomodulatory glycoprotein isolated from ajowan (Trachyspermum ammi L.).

Ajowan (Trachyspermum ammi L.) spice has been used in food preparations and also as a traditional medicine in Ayurveda. Although a number of pharmacological activities have been attributed to ajowan, its role in immunomodulation is not known. The main objective of the present study is to examine the macromolecular immunomodulatory components. Macrophage activation was studied by nitric oxide (NO) release, phagocytosis and secretion of pro-inflammatory cytokines as the markers. Ethanol precipitate (fractional) of ajowan aqueous extract was subjected to conventional chromatography (Q Sepharose followed by Bio-Gel P-100). One of the proteins (30.7 kDa; ajowan glycoprotein or Agp) showed effective mitogenic activity towards splenocytes. Agp is a O-linked glycoprotein with the glycans contributing to one-third of the molecular mass. It has a high content of glutamic acid, serine, aspartic acid and proline whereas galactose (45.7%), arabinose (34.5%), glucose (7%), mannose (5%) and xylose (4%) are the constituent sugars. Secondary structure analysis indicated that Agp contains 79% α-helices and 21% random coil. Internal sequencing of the tryptic peptides did not show homology with the existing proteins in the database (BLAST). Agp at 1 µg/mL induced proliferation of B-cell enriched murine splenocytes and activated macrophages in releasing NO and promoted phagocytosis (p < 0.01). RAW 264.7 cells produced pro-inflammatory cytokines (IL-12, TNF-α and IFN-γ) at 1 µg/mL Agp (p < 0.01). Deproteinized Agp (dpAgp) failed to elicit activation of murine immune cells, whereas deglycosylated Agp (20 kDa; dgAgp) showed compromised efficiency. This is the first report of an immunomodulatory protein from ajowan.

An overview of fruit allergy and the causative allergens.

Plant allergens, being one of the most widespread allergenic substances, are hard to avoid. Hence, their identification and characterization are of prime importance for the diagnosis and treatment of food allergy. The reported allergies to fruits mainly evoke oral allergy syndrome caused by the presence of cross-reactive IgE to certain pollens and thus, allergy to fruits has also been linked to particular pollens. Many fruit allergies are being studied for their causative allergens, and are being characterized. Some tropical or exotic fruits are responsible for region-specific allergies for which only limited information is available, and generally lack allergen characterization. From a survey of the literature on fruit allergy, it is clear that some common fruits (apple, peach, musk melon, kiwi fruit, cherry, grape, strawberry, banana, custard apple, mango and pomegranate) and their allergens appear to be at the center of current research on food allergy. The present review focuses on common fruits reported as allergenic and their identified allergens; a brief description of allergens from six rare/tropical fruits is also covered.

Structural analyses and immunomodulatory properties of fructo-oligosaccharides from onion (Allium cepa).

Onion (Allium cepa) is an immune-boosting food rich in fructans. The major aim of this study is to characterize and investigate the immunomodulatory properties of onion fructo-oligosaccharides (FOS). FOS was isolated from onion bulbs by hot 80% ethanol extraction (yield: ∼4.5 g/100 g fw) followed by gel permeation chromatography. NMR of onion FOS revealed unusual ß-D-Glc terminal residue at the non-reducing end. TLC and ESI-MS analyses showed that onion FOS ranged from trisaccharides to hexasaccharides. Onion FOS (50 µg/mL) significantly increased (∼3-fold) the proliferation of mouse splenocytes/thymocytes vs. control. Further, onion FOS enhanced (∼2.5-fold) the production of nitric oxide by peritoneal exudates cells (PECs) from Wistar rats; intracellular free radicals production and phagocytic activity of isolated murine PECs were also augmented. Our structural and in vitro results indicate that onion FOS comprising of tri- to hexasaccharide units belongs to inulin-type fructans, and possess immunostimulatory activities towards murine lymphocytes and macrophages.

Identification of the protein components displaying immunomodulatory activity in aged garlic extract.

ETHNOPHARMACOLOGICAL RELEVANCE: Traditionally, garlic (Allium sativum L.; Alliaceae) has been known to boost the immune system. Aged garlic has more potent immunomodulatory effects than raw garlic. These effects have been attributed to the transformed organosulfur compounds; the identity of the immunomodulatory proteins in aged garlic extract (AGE) is not known. AIM OF THE STUDY: The major aims are to examine the changes occurring in the protein fraction during ageing of garlic and to identify the immunomodulatory proteins. MATERIALS AND METHODS: Changes occurring in garlic during ageing have been examined by protein quantitation and gel electrophoresis. Purification and identification of the immunomodulatory proteins have been achieved by Q-Sepharose chromatography and mitogenic activity. RESULTS: Only two major proteins (12-14 kDa range by SDS-PAGE) are observed in AGE. The purified protein components QA-1, QA-2, and QA-3 display immunomodulatory and mannose-binding activity; QA-2 shows the highest mitogenic activity. The identity of QA-2 and QA-1 proteins with the garlic lectins ASA I and ASA II, respectively, has been confirmed by hemagglutination analysis. QA-3 exhibits mitogenic activity, but no hemagglutination activity. CONCLUSIONS: The immunomodulatory activity of AGE is also contributed by immunomodulatory proteins. The major immunomodulatory proteins have been identified as the well-known garlic lectins.

A cross-sectional study on the prevalence of food allergy to eggplant (Solanum melongena L.) reveals female predominance.

BACKGROUND: Only a few case reports of allergy to eggplant (Solanum melongena) have been reported. A relatively large number of individuals appear to experience food-related symptoms to eggplant in India. OBJECTIVE: The major aims of this study are to assess the prevalence of food allergy to eggplant and analyse the age and gender distribution. METHODS: Seven hundred and forty-one subjects (age range: 5-60 years) randomly selected from rural and urban areas of Mysore city were analysed for the prevalence of eggplant allergy based on case history, skin prick test (SPT) with eggplant extracts and allergen-specific IgE. The age and gender distribution for the prevalence of eggplant allergy and its association with other atopic conditions were assessed. RESULTS: Sixty-eight (9.2%) subjects reported adverse reactions to ingestion of eggplant, of which 32 (4.3%) subjects had positive history/positive SPT and 36 (4.9%) had positive history/negative SPT. Sixteen (2.2%) subjects had negative history/positive SPT. Ten subjects (1.4%) experienced allergic symptoms in <2 h. Sensitization to eggplant by SPT was more in atopic (16.7%) compared with non-atopic subjects (3.8%). All the SPT-positive subjects (n=48) underwent evaluation for eggplant allergen-specific IgE, which was detected in 6 subjects (0.8%). Majority of the subjects sensitized to eggplant were in the age groups 16-45 years, and females were twice as likely to be sensitized as males. Female predominance (4 : 1) is more in the 16-30 year group. CONCLUSIONS: Many subjects experience adverse reactions to the ingestion of eggplant, possibly due to the pharmacologic action of histamine and other non-protein components, rather than to specific protein allergen(s). The prevalence of IgE-mediated eggplant allergy is estimated at approximately 0.8%, with higher rates of sensitization in females.

Allergy to eggplant (Solanum melongena) caused by a putative secondary metabolite.

We describe a case of allergy caused by ingestion of eggplant in an atopic subject. Symptoms included urticaria, itching of the throat, and hoarseness. Skin prick test (SPT) was positive with 4 varieties of eggplant; however, allergen-specific immunoglobulin E was not detected. SPT with fractions of green long eggplant extract obtained by dialysis and ultrafiltration suggested the allergen to be less than 10 kd. SPT following acetone precipitation of eggplant extract revealed that the allergen was present in the supernatant portion. Further analysis by size-exclusion chromatography of the 10 kd filtrate of eggplant extract on Sephadex G-25 followed by SPT of fractions revealed that the causative allergen was a low molecular weight nonprotein secondary metabolite of less than 1 kd. To our knowledge, this is the first report of allergy to the ingestion of eggplant in which a nonprotein secondary metabolite has been detected as an allergen.

Anaphylaxis following ingestion of mango fruit.

Allergic reactions to fresh fruits and nuts have become increasingly common. Mango (Mangifera indica) is a popular fruit eaten all over the world. We report the case of a 43-year-old woman who experienced oropharyngeal itching, swelling of the face and other parts of the body, and difficulty breathing within a few minutes of eating ripe mango fruit. The woman had no history of pollen or latex allergy. However, she reported instances of milder food allergic reactions to Indian dill and cashew apple. Skin prick tests using mango fruit pulp, Indian dill, and cashew apple extracts were positive. Prick tests with a panel of common grass and weed pollen extracts were negative. Enzyme-linked immunosorbent assay for mango-specific serum immunoglobulin (Ig) E was positive. A specific protein allergen in mango could not be detected by immunoblotting. Based on the strongly positive clinical history and results of allergy testing, it was concluded that the woman had IgE-mediated anaphylactic reactions to mango fruit.

Potato lectin activates basophils and mast cells of atopic subjects by its interaction with core chitobiose of cell-bound non-specific immunoglobulin E.

A major factor in non-allergic food hypersensitivity could be the interaction of dietary lectins with mast cells and basophils. Because immunoglobulin E (IgE) contains 10-12% carbohydrates, lectins can activate and degranulate these cells by cross-linking the glycans of cell-bound IgE. The present objective focuses on the effect of potato lectin (Solanum tuberosum agglutinin; STA) for its ability to release histamine from basophils in vitro and mast cells in vivo from non-atopic and atopic subjects. In this study, subjects were selected randomly based on case history and skin prick test responses with food, pollen and house dust mite extracts. Skin prick test (SPT) was performed with STA at 100 microg/ml concentration. Histamine release was performed using leucocytes from non-atopic and atopic subjects and rat peritoneal exudate cells. SPT on 110 atopic subjects using STA showed 39 subjects positive (35%); however, none showed STA-specific IgE; among 20 non-atopic subjects, none were positive by SPT. Maximal histamine release was found to be 65% in atopic subjects (n = 7) compared to 28% in non-atopic subjects (n = 5); the release was inhibited specifically by oligomers of N-acetylglucosamine and correlates well with serum total IgE levels (R(2) = 0.923). Binding of STA to N-linked glycoproteins (horseradish peroxidase, avidin and IgG) was positive by dot blot and binding assay. As potato lectin activates and degranulates both mast cells and basophils by interacting with the chitobiose core of IgE glycans, higher intake of potato may increase the clinical symptoms as a result of non-allergic food hypersensitivity in atopic subjects.

Generation of an antibody specific to erythritol, a non-immunogenic food additive.

Erythritol, a simple sugar alcohol, is widely used as a food and drug additive owing to its chemical inertness, sweetness and non-toxicity. Adverse reactions to erythritol are rare and only three cases of allergic reactions to foods containing erythritol have been reported. Being inert, erythritol cannot produce an immunological response. In order to explain the mechanism of immunogenicity of erythritol, a method to obtain erythritol epitopes on a carrier protein, which can serve as an immunogen to develop antibodies against erythritol, is described. D-Erythrose was conjugated to bovine serum albumin at pH 8 by reductive amination. The reduction product of the Schiff base of D-erythrose-bovine serum albumin conjugate creates erythritoyl groups. Rabbits immunized with erythritol-bovine serum albumin conjugate (29 haptens/molecule) showed good antibody response (detection of 1 microg antigen, erythritol-keyhole limpet haemocyanin conjugate possessing 50% modified amino groups, at 1 : 50,000 dilution). Anti-erythritol immunoglobulin-G antibodies were purified from the immune serum using hapten-affinity chromatography on an erythritol-keyhole limpet haemocyanin-Sepharose CL-6B affinity matrix. The yield of erythritol-specific antibody was approximately 40 microg ml-1 of rabbit antiserum. Enzyme-linked immunobsorbant assay inhibition studies using sugars, sugar alcohols and L-lysine showed minimal cross-reactivity (approximately 4%) when compared with erythritol; only dithioerythritol showed a cross-reactivity of approximately 33%. D-Threitol and L-threitol (isomers of erythritol) had cross-reactivities of 15 and 11%, respectively. The inhibition studies confirmed the haptenic nature of erythritol and indicated that the erythritoyl group is a single epitope. The reaction scheme outlined here for the generation of erythritol epitopes appears to provide a basis for the immunogenicity of erythritol.

Anaphylaxis to excipient mannitol: evidence for an immunoglobulin E-mediated mechanism.

BACKGROUND: Anaphylaxis to mannitol present naturally in pomegranate and cultivated mushroom in a sensitized subject has been described recently, and an IgE-mediated mechanism to this sugar alcohol has been proposed. The same subject also experienced severe allergic reactions to a chewable pharmaceutical (cisapride drug). OBJECTIVE: The purpose of the study was to identify allergenic component in the pharmaceutical preparation, and also, to understand the mechanism of immediate hypersensitivity to mannitol. METHODS: Methodology involved skin prick tests (SPTs), high-performance liquid chromatographic (HPLC) analysis of pharmaceutical preparations, separation of mannitol by Ca++-ion-moderated cation-exchange chromatography, preparation of alditol-protein conjugates by reductive amination, SPT using the conjugates, hapten affinity purification of the allergic serum on D-mannitol-keyhole limpet haemocyanin (KLH)-Sepharose CL-6B, and detection of serum mannitol-specific IgE by ELISA. RESULTS: Component testing by SPT, and HPLC analysis of various pharmaceuticals indicated that the excipient mannitol is the causative allergen. Mannitol separated from Cisapid MPS showed allergenic activity by SPT. Among the several conjugates tested by SPT, D-mannitol-bovine serum albumin and D-mannitol-KLH showed positive weal/flare reaction, demonstrating the presence of cell-bound mannitol-specific IgE in vivo. Negative results with D-glucitol, D-galactitol, meso-erythritol, and L-mannitol protein conjugates clearly showed that the mannitol-specific human IgE is very specific to the D-isomer of mannitol. ELISA using the hapten affinity-purified allergic serum was positive, demonstrating the presence of mannitol-specific serum IgE in the allergic subject. CONCLUSION: Mannitol, which is widely used as a food and drug additive (excipient), can rarely cause IgE-mediated anaphylaxis. This study is the first one to demonstrate the presence of mannitol-specific human IgE in a sensitized allergic subject to validate an IgE-mediated hypersensitivity mechanism for mannitol.

Galactose-induced dimerization of blocked ricin at acidic pH: evidence for a third galactose-binding site in ricin B-chain.

Blocked ricin is a glycoconjugate formed by covalent modification of each of the two galactose-binding sites of ricin with affinity ligands derived by modification of glycopeptides containing galactose-terminated, triantennary, N-linked oligosaccharides. Blocked ricin undergoes a pH-dependent reversible self-association, being predominantly dimeric at neutral pH and monomeric at acidic pH. The shift in the monomer-dimer equilibrium towards the monomeric form at acidic pH (pH 4) is inhibited by lactose, as shown by size-exclusion chromatography. This behavior of blocked ricin can be reproduced in studies with isolated blocked B-chain. The effect, which is dependent on the concentration of the sugar, is specific for sugars having terminal galactose moieties, or sugars having the same orientation of hydroxyl groups at C2 and C4 as galactose. These results are interpreted as providing further support for the notion that ricin B-chain has a third galactose-binding site, which may be important for the intracellular trafficking of ricin during intoxication of cells.

Activation of human cytolytic cells through CD2/T11. Comparison of the requirements for the induction and direction of lysis of tumor targets by T cells and NK cells.

We studied the mechanisms whereby human T cells and NK cells are activated and directed to lyse tumor targets through the CD2 (T11/E-rosette) Ag. Using two cloned NK lines, we showed that these cells, as had previously been shown for T cells, could be directed to lyse an "NK-resistant" tumor target in the presence of antibody heterodimers. These heterodimers consisted of a (mAb) to CD2 (anti-T11(2) or anti-T11(3] linked to a mAb recognizing the tumor cell (J5, anti-CALLA). However, distinct differences between NK cells and T cells were observed with regard to the requirements for such directed lysis: first, only one epitope of CD2 on NK cells (either T11(2) or T11(3] needed to be recognized by the antibody heterodimer in order for directed lysis to occur, whereas for T cells both T11(2) and T11(3) epitopes had to be recognized. Second, in confirmation of previous data with monomeric anti-T11(2) or anti-T11(3) antibody, heterodimers constructed with these reagents enhanced conjugate formation between NK cells and tumor targets, whereas no such enhancement was seen with T cells. All types of heterodimer directed lysis were dependent on the adhesion molecule LFA-1, as an anti-LFA-1 antibody-blocked lysis. Third, whereas in T cells lysis mediated through CD2 appeared to be regulated by CD3 but not vice versa, all types of lysis by NK cells appeared to be regulated through CD2. Finally we showed that F(ab')2 fragments of the anti-T11(2) and anti-T11(3) antibodies could activate NK cells, but were unable to activate T cells either as cloned cytolytic lines, or in populations of PBL. The implications of our findings with regard to the role of CD2 in the activation of cytolytic cells is discussed.

The esterase-like activity of covalently bound human third complement protein.

The acyl ester bond between the third complement protein, C3, and a variety of molecules is hydrolyzed spontaneously at neutral pH (Venkatesh et al., 1984). Modification of the free, single sulfhydryl group of bound C3 by thiol reagents suggested that a functional group other than the -SH acts as a "catalytic" group in this intramolecular hydrolytic reaction. Complete inhibition of the esterase-like activity is observed with stoichiometric amounts of mercuric chloride, palladium chloride, and the bifunctional organic mercurial, 3,6-bis-(acetoxymercuri)-o-toluidine [BAMT]. Since alkyl and aryl mercuric ions do not inhibit the esterase-like activity of C3-[3H]glycerol, it is conjectured that divalent mercury, palladium, and BAMT will form a complex with the -SH group and an atom of the "catalytic" group X having a lone pair of electrons. The structural features of C3 that are essential for the esterase-like activity remain intact after subjecting C3-[3H]glycerol to covalent chromatography on organomercurial agarose. Based on the observed effects of chemical reagents and the kinetic deuterium solvent isotope effect on the esterase-like activity, a general-base mechanism is proposed for the intramolecular hydrolysis of the acyl ester bond in covalently bound C3. The "catalytic" group X is located in the C3d region (residues 317-632 of the alpha chain), since C3d-[3H]glycerol also has esterase-like activity. A general-base mechanism mediated by the same "catalytic" group X may also apply to the formation of acyl ester bonds following the hydrolysis of the internal thiolester bond in native C3.

Influence of deamidation(s) in the 67-74 region of ribonuclease on its refolding.

The influence of chemical mutation featuring the selective conversion of asparagine or glutamine to aspartic or glutamic acid, respectively, on the kinetics of refolding of reduced RNase has been studied. The monodeamidated derivatives of RNase A, viz. RNase Aa1a, Aa1b, and Aa1c having their deamidations in the region 67-74, were found to regain nearly their original enzymatic activity. However, a marked difference in the kinetics of refolding is seen, the order of regain of enzymic activity being RNase A greater than Aa1c congruent to Aa1a greater than Aa1b. The similarities in the distinct elution positions on Amberlite XE-64, gel electrophoretic mobilities, and u.v. spectra of reoxidized and native derivatives indicated that the native structures are formed. The slower rate of reappearance of enzymic activity in the case of the monodeamidated derivatives appears to result from altered interactions in the early stages of refolding. The roles of some amino acid residues of the 67-74 region in the pathway of refolding of RNase A are discussed.

Isolation and characterization of monodeamidated derivatives of bovine pancreatic ribonuclease A.

The isolation and characterization of the initial intermediates formed during the irreversible acid denaturation of enzyme Ribonuclease A are described. The products obtained when RNase A is maintained in 0.5 M HCl at 30 degrees for periods up to 20 h have been analyzed by ion-exchange chromatography on Amberlite XE-64. Four distinct components were found to elute earlier to RNase A; these have been designated RNase Aa2, Aa1c, Aa1b, and Aa1a in order of their elution. With the exception of RNase Aa2, the other components are nearly as active as RNase A. Polyacrylamide gel electrophoresis at near-neutral pH indicated that RNase Aa1a, Aa1b, and Aa1c are monodeamidated derivatives of RNase A; RNase Aa1c contains, in addition, a small amount of a dideamidated component. RNase Aa2, which has 75% enzymic activity as compared to RNase A, consists of dideamidated and higher deamidated derivatives of RNase A. Except for differences in the proteolytic susceptibilities at an elevated temperature or acidic pH, the monodeamidated derivatives were found to have very nearly the same enzymic activity and the compact folded structure as the native enzyme. Fingerprint analyses of the tryptic peptides of monodeamidated derivatives have shown that the deamidations are restricted to an amide cluster in the region 67-74 of the polypeptide chain. The initial acid-catalyzed deamidation occurs in and around the 65-72 disulfide loop giving rise to at least three distinct monodeamidated derivatives of RNase A without an appreciable change in the catalytic activity and conformation of the ribonuclease molecule. Significance of this specific deamidation occurring in highly acidic conditions, and the biological implications of the physiological deamidation reactions of proteins are discussed.

Natural release of covalently bound C3b from cell surfaces and the study of this phenomenon in the fluid-phase system.

Covalently bound C3b is released from cell surfaces (EAC1423 and zymosan-C3b) on incubation under physiologic conditions. The release of C3b from cell surfaces occurs by the cleavage of the covalent bond. Sodium dodecyl sulfate (SDS) abolishes the release, thereby indicating the requirement of the native structure of C3b in this process. The phenomenon of release of C3b from cell surfaces has also been observed in the fluid-phase system by using C3b-[3H]glycerol. The kinetics of the release of [3H]glycerol from C3b-[3H]glycerol were studied at 37 degrees C in 0.15 M phosphate buffer, pH 7.4. The first-order rate constant was found to be 0.028 +/- 0.003 hr-1. The release does not take place in either 8 M urea or 6 M guanidine hydrochloride, at pH 7.4. Under alkaline conditions, the rate of release is unaffected in the presence of SDS, indicating that the release in this pH range is not dependent on the native structure of the protein. From the Arrhenius plot in the temperature range 18 to 37 degrees C, an apparent activation energy for the hydrolysis reaction of 21.2 kcal/mol was calculated. The release phenomenon is exclusive for ester-linked complexes, as inferred by the absence of release of [3H]threonine from C3b-[3H]threonine, wherein the linkage is of the amide type. The presence or absence of the C3a portion of the molecule has no effect on the rate of release. The modification of the -SH group of C3i-/C3b-[3H]glycerol alters the rate of hydrolysis of the ester bond between C3i/C3b and [3H]glycerol. Protease inhibitors (PMSF, benzamidine HCl, and DFP) do not alter the rate of release, indicating that the hydrolysis reaction is not due to trace amounts of contaminating proteases. Thus, it appears that some chemical group(s) of C3i/C3b is (are) involved in the intramolecular hydrolysis of the ester bond between C3i/C3b and small molecules. This phenomenon may play an important role in the release of C3b from receptive surfaces once the biologic functions that require covalently bound C3b have been mediated.