The immunological activity of some of the chymotryptic peptides of sperm-whale myoglobin

1. Sperm-whale apomyoglobin was digested with chymotrypsin in a dialysis sac. The ultrafiltrate contained incompletely hydrolysed fragments which partially inhibited the precipitation of metmyoglobin and apomyoglobin by some antisera produced against metmyoglobin. The inhibitory activity was stable to heating at 100 degrees and depended on the peptide structure. 2. The fragments were fractionated according to molecular size and were purified by ion-exchange chromatography. Six pure peptides and two peptides which contained a minor impurity were isolated. Their amino acid compositions and N-terminal amino acid sequences were determined and their entire amino acid sequences deduced from the known amino acid sequence of sperm-whale myoglobin. 3. The peptides formed no detectable precipitates with the antisera. Five of the eight peptides partially inhibited the precipitation of apomyoglobin and/or metmyoglobin by one antiserum. Six of the peptides inhibited the precipitation of apomyoglobin by one or other of two antisera; at least two of these peptides inhibited both antisera. One peptide failed to inhibit the precipitation of either antigen by either antiserum. Two of the peptides possessed the same serological specificity. 4. The molar ratios of inhibitors to antigen for 50% of the maximum inhibition decreased as the molecular size of the inhibitor increased. With one antiserum and with apomyoglobin as the antigen, molar ratios 12 and 80 were obtained for peptides with molecular weights 2051 and 793 respectively. 5. The size and structure of an antigenic site is discussed in relation to the known steric configuration of myoglobin.     

Serological Comparison of the Three Morphological Phases of Coccidioides immitis by the Agar Gel Diffusion Method

Hyperimmune sera against spherules and against arthrospores of Coccidioides immitis were prepared by inoculation of rabbits. The antibody content of these sera was studied by the agar gel diffusion method. It was observed that antispherule pooled sera formed multiple precipitin bands with extracts of spherules and of arthrospores. The antiarthrospore pooled serum, however, failed to precipitate with the spherule extract, and formed a single band in the presence of an arthrospore solution. When the spherule and the arthrospore extracts were tested with a variety of different antisera, it was observed that the spherule preparation formed bands only in combination with anti-purified spherule pooled serum, whereas the arthrospore extract precipitated with anti-purified spherule, antiarthrospore, and anti-Histoplasma capsulatum pooled sera. It was also observed that a spherule culture supernatant solution formed five precipitin bands in combination with anti-spherule pooled sera, formed one band with pooled antiserum from rabbits with coccidioidomycosis, and did not precipitate in the presence of antiarthrospore pooled serum. Coccidioidin, however, formed two bands in the presence of any of these antisera. It was therefore concluded that extracts from the spherule phase of C. immitis differed from solutions obtained from the arthrospore and mycelial phases.     

Periodate oxidation of sperm-whale myoglobin and the role of the methionine residues in the antigen–antibody reaction

1. Oxidation of sperm-whale metmyoglobin and its apoprotein with periodate has been investigated under various conditions of pH and temperature to find those under which the reagent acted with specificity. 2. At pH6.8 and 22 degrees consumption of periodate ceased in 3(1/2)hr. at 43 moles of periodate/mole of myoglobin. The two methionine residues, the two tryptophan residues, the three tyrosine residues and two histidine residues were oxidized; serine increased in the hydrolysates from 6 to 9 residues/mol. 3. At pH5.0 and 22 degrees , consumption levelled off in 4(1/2)hr. at 26 moles of periodate/mole of myoglobin and resulted in the modification of the two methionine residues, the two tryptophan residues, the three tyrosine residues and two histidine residues; serine increased from 6 to 7 residues/mol. and, also, ferrihaem suffered considerable oxidation. 4. Oxidation at pH5.0 and 0 degrees resulted at completion (4hr.) in the consumption of 22 moles of periodate/mole of myoglobin and in the modification of the methionine, tyrosine and tryptophan residues. Spectral studies indicated oxidation of the haem group. This derivative reacted very poorly with rabbit antisera to MbX (the major component no. 10 obtained by CM-cellulose chromatography; Atassi, 1964). 5. Oxidation of apomyoglobin at pH5.0 and 0 degrees was complete in 4hr. with the consumption of 7.23 moles of periodate/mole of apoprotein. The rate of oxidation in decreasing order was: methionine; tryptophan; tyrosine; and after 7hr. of reaction the following residues/mol. were oxidized: methionine, 2.0; tryptophan, 1.6; tyrosine, 0.99. No peptide bonds were cleaved. Metmyoglobin prepared from the 7hr.-oxidized apoprotein showed that the reactivity with antisera to MbX had diminished considerably. 6. Milder oxidation of apoprotein (2 molar excess of periodate, pH5.0, 0 degrees , 2hr.) resulted in the modification of 1.66 residues of methionine/mol. Metmyoglobin prepared from this apoprotein was identical with native MbX spectrally, electrophoretically and immunochemically. It was concluded that the methionine residues at positions 55 and 131 were not essential parts of the antigenic sites of metmyoglobin.     

Conformation-dependent antigenic determinants in the toxic lectin ricin.

The major part of the ricin-precipitable antibodies in sera produced by immunizing rabbits with formaldehyde-treated ricin is precipitated also by the isolated ricin A and B chains. In contrast, in antisera produced by immunizing with formaldehyde-treated ricinus agglutinin only a small part of the antibodies cross-reacting with ricin can be precipitated by the isolated A and B chains, or bound to immunoabsorbents containing the isolated ricin chains. In immunodiffusion studies with anti-ricinus agglutinin sera, a star-shaped precipitate was formed when isolated A and B chains recombined to form intact ricin. Both anti-ricin and anti-ricinus agglutinin sera neutralized effectively the ability of ricin to inhibit protein synthesis in HeLa cells. Anti-ricin serum also neutralized the inhibitory effect of the isolated A chain on protein synthesis in a cell-free system and the ability of the isolated B chain to induce indirect hemagglutination. In contrast, antiricinus agglutinin serum did not neutralize the biologic activities of the isolated ricin A and B chains. Anti-ricinus agglutinin serum formed a precipitate with the hybrid ricin A chain/abrin B chain, and protected against the toxic effect on HeLa cells of this hybrid, indicating conformational changes of ricin A chain upon binding to the B chain. It is concluded that the anti-ricinus agglutinin serum contains antibodies directed against conformational determinants present on intact ricin, but not present or exposed in the isolated A and B chains. At least part of these conformational determinants appears to be carried by the A chain.     

Antigenic relationship betweenHaemophilus influenzae andHaemophilus isolated from porcine pleuropneumonia

Comparative double immunodiffusion techniques were used to study capsular and O antigenic relationships betweenHaemophilus influenzae types a-fandH. pleuropneumoniae types 1–5 and a strain (202) closely related toH. pleuropneumoniae. Culture fluids or culture supernatants were used as antigens and rabbit antisera were produced against cell suspensions of the strains tested. A reaction of identity was obtained between the capsular precipitate ofH. influenzae c and a precipitate formed by strain 202, when developed with anti-H. influenzae c serum or the serum produced against strain 202. Mutual cross-absorption of capsular antibodies was also demonstrable. No other capsular or O antigenic cross-reactivity was demonstrable between the strains tested.     

At least two loci encode polymorphic class I MHC antigens in the horse

Summary. Six monoclonal antibodies and ten alloantisera were used to precipitate cell surface molecules of approximately 44kDa (class I MHC antigens) from radiolabelled equine peripheral blood lymphocytes. All ten antisera were raised against antigens of a single donor horse (horse 0834, ELA-A2,-A2). Four methods of producing antisera were compared: one or two pregnancies, skin allografting, and skin grafting followed by pregnancy. Immunization by pregnancy appeared to produce antibodies against class I products only, while skin grafting raised antibodies to class II antigens as well. Nine of the antisera were raised across an entire MHC haplotype barrier, while one recipient carried the ELA-A2 antigen of the donor. The pregnancy antiserum raised across this barrier probably identifies a second polymorphic class I locus in the horse. Sequential immunoprecipitation using this antiserum in the first stage and an anti-MHC haplotype antiserum or monoclonal antibody reagent in the second stage supported this hypothesis. Gene products of this second ELA class I locus are immunogenic in pregnancy.     

The mouse antibody response to Trichinella spiralis defines a single, immunodominant epitope shared by multiple antigens.

Immunoblot analysis was used to characterize the Trichinella spiralis L1 larval Ag recognized by antisera from T. spiralis-infected AKR/J mice. Antisera were analyzed for reactivity with crude worm extract, excretory/secretory proteins and cuticle proteins from L1 larvae. The response was biphasic; antibodies against one set of Ag were detected 13 days after infection (group I Ag), and antibodies against a different set of Ag were detected 35 days after infection (group II Ag). Excretory/secretory and cuticle proteins were recognized only by antibodies produced late in infection. The predominant isotype in day 42 antiserum was IgG1, and 80% of these IgG1 antibodies reacted with a stage-specific determinant shared by virtually all group II Ag. A mAb reactive with the shared determinant was used to purify the group II Ag from L1 larval extract. Such affinity-purified Ag were protective when used to immunize mice against subsequent infection, and T cells from infected mice proliferated when cultured with these Ag in vitro. Other mouse strains also made a strong serum antibody response to the group II Ag. We hypothesize that immune responses to the shared epitope play an important role in determining the outcome of the host-parasite interaction during infection.     

Detection of structural differences between nuclear and mitochondrial glutamate dehydrogenases by the use of immunoadsorbents.

Structural differences between crystalline mitochondrial and nuclear glutamate dehydrogenases from ox liver have been detected by immunological techniques. Antisera prepared against each enzyme precipitate both glutamate dehydrogenases; upon immunodiffusion, the antiserum against the nuclear enzyme gives a line of incomplete identity with the two antigens, whereas the antiserum against the mitochondrial enzyme gives a line of complete identity. Fractionation of the antibodies contained in each antiserum by means of an immunoadsorbent, to which the nuclear or the mitochondrial enzyme has been covalently linked, shows that nuclear glutamate dehydrogenase (GDH) contains specific antigenic determinants as well as determinants common to the mitochondrial enzyme, whereas the latter appears to have no antigenic portions which are not present in the nuclear antigen, in accord with the results of immunodiffusion. The antibodies against determinants common to both enzymes precipitate and inhibit them, whereas the specific anti-nuclear GDH antibodies precipitate but do not inhibit the nuclear antigen.     

Improved antiserum agar method for the serogroup differentiation of Neisseria meningitidis Y and W135

Rabbit antisera prepared to meningococcal serogroups Y and W135 strains were compared with horse antisera using the antiserum agar method (ASA) for the serogroup identification of Neisseria meningitidis. Thirty-seven group Y strains formed immunoprecipitates with the Y rabbit serum only, whereas the same Y strains formed immunoprecipitates with both the Y and W135 horse antisera. Forty-seven W135 strains formed specific immunoprecipitates with both the rabbit and horse W135 antisera by ASA. None of the 166 meningococcal isolates, representative of other meningococcal serogroups, formed immunoprecipitates with the groups Y and W135 rabbit or horse antisera. Use of specific Y and W135 rabbit antisera in ASA provides an improved technique for the serogroup differentiation of groups Y and W135 meningococci.     

Specific and shared idiotypes found on hybridoma anti-DNA autoantibodies derived from rheumatoid arthritis and systemic lupus erythematosus patients

The idiotype determinants found on hybridoma anti-DNA autoantibodies produced from the fusion of peripheral blood lymphocytes from 13 systemic lupus erythematosus (SLE) and five rheumatoid arthritis (RA) patients with the GM 4672 human lymphoblastoid line were analyzed. A total of 47 SLE and 21 RA hybridomas were studied, of which 26 SLE and 10 RA produced anti-DNA autoantibodies. Rabbit antisera, raised to six of the SLE hybridoma anti-DNA IgM antibodies, were rendered idiotype specific by multiple absorptions on human IgM and IgG immunoabsorbent columns. In direct binding radioimmunoassays, all six anti-idiotype antisera reacted specifically with the anti-DNA antibody used as immunogen. In competition studies, five anti-idiotype antisera were able to inhibit the binding of their homologous idiotype to DNA-coated tubes. In addition, DNA and polynucleotides inhibited the binding of the five idiotypes to anti-idiotype-coated tubes, suggesting that these anti-idiotypes react with idiotype determinants located within the antigen-combining sites of the anti-DNA antibody molecules. Shared idiotypes were detected among the 68 hybridoma antibodies by direct binding studies on anti-idiotype-coated tubes. Our results revealed that 58% (21/36) of the anti-DNA antibodies and 16% (5/32) of the non-DNA-binding antibodies reacted with at least one anti-idiotype serum. Five anti-idiotype antisera reacted only with hybridoma anti-DNA antibodies from SLE patients. The other anti-idiotype antiserum reacted with both SLE- and RA-derived hybridoma anti-DNA and non-DNA-binding antibodies. These studies indicate that some anti-idiotype antisera may detect specific idiotypes found only on SLE-derived anti-DNA auto-antibodies, whereas other antisera detect shared idiotypes found on both RA and SLE DNA-binding and non-DNA-binding antibodies.     

The identification of mammalian centrosomal antigens using human autoimmune anticentrosome antisera.

Human autoimmune sera were screened for the presence of anticentrosome autoantibodies. Two high titer sera were identified that reacted with HeLa, CHO, and PtK2 centrosomes by immunofluorescence, although the fluorescent patterns that were obtained using the two antisera were separate and distinct. Serum obtained from patient IJ contained antibodies that reacted with epitopes present only in mitotic centrosomes; staining of interphase centrosomes was never detected uing IJ antiserum. Immunoblot analysis demonstrated that antibodies present in IJ antiserum reacted with a 190 kD spindle pole antigen. Immunofluorescent staining of cultured mammalian cells demonstrated that antibodies present in serum obtained from patient SPJ reacted with both interphase and mitotic centrosomes. Characterization of SPJ antiserum by immunoblotting demonstrated that antibodies present in the SPJ serum recognized proteins of Mrs of 39, 185, and 220 kD, although the possibility that the 185 kD polypeptide was a proteolytic breakdown product of the 220 kD protein has not been eliminated. Neither antiserum was able to inhibit microtubule nucleation from centrosomes in a lysed cell system in which pure 6S tubulin was added to permeabilized cells following pretreatment of the cells with either SPJ or IJ antiserum. These antisera should be useful probes for studying the biochemistry of the mammalian centrosome.     

Syngeneic anti-idiotypic antisera to murine monoclonal antibodies to monomorphic and polymorphic determinants of HLA class I antigens

BALB/c mice were immunized with syngeneic anti-HLA class I monoclonal antibodies. The latter included the anti-HLA-A2, A28 monoclonal antibody (MoAb) CR11-351, the MoAb Q6/64 to a determinant restricted to HLA-B antigens and the MoAb CR10-215 and CR11-115 to the same (or spatially close) monomorphic determinant. Anti-idiotypic antibodies could be detected in bleedings obtained 3 days after the first booster, increased in titer in bleedings obtained after the second booster, and persisted at high levels in subsequent bleedings. The four anti-HLA class I MoAb did not differ in their ability to elicit syngeneic anti-idiotypic antibodies. Cross-blocking studies with a panel of anti-HLA class I, anti-HLA class II, and anti-human melanoma-associated antigen (MAA) MoAb showed that the anti-MoAb CR10-215 and anti-MoAb CR11-115 antisera contain only antibodies to private idiotopes, whereas the anti-HLA MoAb CR11-351 and anti-MoAb Q6/64 antisera also contain antibodies to public idiotopes. The latter are expressed by the anti-HLA class I MoAb CR11-351, Q1/28, Q6/64, and 6/31, and by the anti-HLA class II MoAb Q5/6, Q5/13, 127, and 441. Public idiotopes were not detected on the nine anti-MAA MoAb tested. Public idiotopes do not interfere with the binding of anti-HLA MoAb with the corresponding antigenic determinants. On the other hand private idiotopes are located within the antigen-combining site, because anti-idiotypic antisera specifically inhibit the binding of the corresponding immunizing anti-HLA class I MoAb to cultured human lymphoid cells in a dose-dependent manner. Analysis by isoelectric focusing of the anti-HLA class I MoAb antisera showed that the spectrotype of the anti-MoAb CR11-351 antiserum comprises four components that focus in the pH 6.9 to 6.2 range, the spectrotype of anti-MoAb Q6/64 antiserum comprises three components that focus in the pH 6.5 to 6.1 range, the spectrotype of the anti-MoAb CR10-215 antiserum comprises three components that focus in the pH 6.4 to 6.1 range, and the spectrotype of the anti-MoAb CR11-115 antiserum comprises three components that focus in the pH 6.6 to 6.4 range.     

Ligand-apomyoglobin interactions. Configurational adaptability of the haem-binding site.

1. The interaction of the haem-binding region of apomyoglobin with different ligands was examined by ultrafiltration, equilibrium dialysis and spectrophotometry, to study unspecific features of protein-ligand interactions such as they occur in, for example, serum albumin binding. 2. Apomyoglobin, in contrast with metmyoglobin, binds at pH 7, with a high affinity, one molecule of Bromophenol Blue, bilirubin and protoporphyrin IX, two molecules of n-dodecanoate and n-decyl sulphate and four molecules of n-dodecyl sulphate and n-tetradecyl sulphate. 3. The number of high-affinity sites and/or association constants for the alkyl sulphates are enhanced by an increase of hydrocarbon length, indicating hydrophobic interactions with the protein. 4. Measurements of the temperature-dependence of the association constants of the high-affinity sites imply that the binding processes are largely entropy-driven. 5. Binding studies in the presence of two ligands show that bilirubin plus Bromophenol Blue and dodecanoate plus Bromophenol Blue can be simultaneously bound by apomyoglobin, but with decreased affinities. By contrast, the apomyoglobin-protoporphyrin IX complex does not react with Bromophenol Blue. 6. Optical-rotatory-dispersion measurements show that the laevorotation of apomyoglobin is increased towards that of metmyglobin in the presence of haemin and protoporphyrin IX. Small changes in the optical-rotatory-dispersion spectrum of apomyoglobin are observed in the presence of the other ligands. 7. It is concluded that the binding sites on apomyoglobin probably do not pre-exist but appear to be moulded from predominantly non-polar amino acid residues by reaction with hydrophobic ligands. 8. Comparison with data in the literature indicates that apomyoglobin on a weight basis has a larger hydrophobic area avaialble for binding of ligands than has human serum albumin. On the other hand, the association constants of serum for the ligands used in this study are generally somewhat larger than those of apomyoglobin.     

Determination of monoclonal antibody specificity by mixed precipitation ingel

The enzyme-immunodiffusion technique is advanced which permits testing monoclonal antibodies included in the precipitate line formed in gel by polyclonal antibodies with the corresponding antigen. Rat or mouse monoclonal antibodies were mixed with polyclonal rabbit antiserum to the antigen at issue. The precipitate formed by immunogen and rabbit polyclonal antibodies included monoclonals.     

Radioimmunoassay for medroxyprogesterone acetate (Provera) using the 11alpha-hydroxy succinyl conjugate

The development of a radioimmunoassay for medroxyprogesterone acetate (MPA) employing rabbit antibodies made against a bovine serum albumin (BSA) conjugate of the hemisuccinate ester of 11alpha-hydroxy MPA are described. The 11alpa-hydroxyl group was introduced into MPA by a series of chemical and microbiological transformations. The acid succinate MPA was coupled to BSA by reacting it with N,N'-carbonyldiimidazole. Cross-reactivities of several MPA analogs were compared suing rabbit antisera developed against the C-11 conjugate of MPA and goat antiserum developed against a C-3 conjugate of MPA. Antibodies formed against the C-11 conjugate showed increased specificity to steroid analogs which changes in the C-21 side chain. The profiles of serum MPA levels vs time, measured with both the C-3 and C-11 antisera, after oral drug administration to the monkey were similar. After intramuscular drug administration to dogs, serum MPA levels measured with the C-3 antisera were consistently greater as compared with those with C-11 antisera. Factors affecting precipitation of the antibody-antigen complex, assay precision, and assay sensitivity were evaluated.     

Immunoprecipitation of phytochrome from green Avena by rabbit antisera to phytochrome from etiolated Avena

Thirty-nine antiserum preparations from eight rabbits were screened for their ability to precipitate the immunochemically distinct phytochrome that is obtained from green oat (Avena sativa L.) shoots. The antisera were obtained from rabbits immunized with either proteolytically degraded, but still photoreversible, 60-kDa (kilodalton) phytochrome, or approx. 120-kDa phytochrome, both of which were purified from etiolated oat shoots. The ability of these antisera to precipitate phytochrome from green oats was independent of the size of phytochrome used for immunization. While crude antisera immunoprecipitated as much as 80% of the phytochrome isolated from green oat shoots, antibodies immunopurified from these sera with a column of highly purified, approx. 120-kDa phytochrome from etiolated oats precipitated no more than about 5–10%.Abbreviations kDa kilodalton - mU milliunit     

Secondary structural changes of metmyoglobin and apomyoglobin in anionic and cationic surfactant solutions: Effect of the hydrophobic chain length of the surfactants on the structural changes

Secondary structural changes of metmyoglobin and apomyoglobin were examined in solutions of sodium alkylsulfates with hydrocarbon numbers of 8 and 12, and alkyltrimethylammonium bromides with hydrocarbon numbers of 10, 12, 14, and 16. The relative proportion ofa-helical structure was estimated by the curve-fitting method of circular dichroic spectrum. The helical proportions of metmyoglobin and apomyoglobin were 82 and 63%, respectively. The shorter the hydrocarbon chain the surfactant had, the higher the concentration necessary to disrupt the secondary structures of these proteins. However, the helical proportion had a tendency to decrease down to lower values in solutions of the cationic surfactants with short hydrophobic groups. On the other hand, thea-helical structure of apomyoglobin was disrupted in lower concentrations of each cationic surfactant than that of metmyoglobin, although the disruptions of the same structures in both the proteins occurred in the same concentration range of each anionic surfactant. It appeared likely that the removal of the heme group unstabilized the myoglobin conformation only in the cationic surfactant solutions.     

Immunological Properties and Constituent Amino Acids of Three Xylanases Produced Inductively from Streptomyces sp.

Three xylanases produced inductively by methyl β-xyloside from Streptomyces sp. No. 3137 were purified to homogeneity. Rabbit antisera against two xylanases, X-I and X-II-B, were prepared.

In double diffusion experiments, antiserum to X-I (anti-I serum) and antiserum to X-II-B (anti-II-B serum) formed a precipitation band with X-I, and X-II-B, respectively. No immunoprecipitate, however, was observed when xylanase X-II-A or X-II-B was tested against anti-I serum, and when X-I was done against anti-II-B serum. Anti-II-B serum formed a line of identity between X-II-A and X-II-B. The data indicate that X-II-A and X-II-B are closely related in immunology, while X-I is distinct.

The amino terminals were an alanine residue for X-I and threonine residues for X-II-A and X-II-B by the dansyl chloride method. The carboxyl terminals were an aspartic acid residue for X-I and an alanine residue for X-II-B by carboxypeptidase Y method.

Amino acid analysis showed that X-II-A and X-II-B had a large amount of glutamic acid and alanine residues, and a small amount of tyrosine, threonine, and serine residues compared with X-I.     

Preparation and specificity of avian anti-GM2(NeuGc) ganglioside antiserum

Antibodies to N-glycolyl neuraminic acid-containing GM2 ganglioside, GM2(NeuGc), were prepared by immunizing chickens. The specificity of the antibodies was examined by the double immunodiffusion test and solid-phase radioimmunoassay (RIA). One(C-4) of two antisera produced did not cross-react with GM3(NeuGc) but the other(C-3) did as assessed by the double immunodiffusion test. In RIA, the antibody activity of C-4 antiserum was detected only in the IgG fraction. Specificity of the serum was examined using authentic glycolipids which were structurally related to GM2(NeuGc). The antiserum showed a high specificity for the homologous ganglioside by either an RIA or an inhibition assay. This antiserum is a useful tool for the detection of GM2(NeuGc) in human and animal tissues under normal and/or disease condition.     

Immunochemical analysis of the types Ia and Ib group B streptococcal polysaccharides

The types Ia and Ib group B streptococcal type-specific polysaccharides have remarkable immunologic differences despite a great deal of structural similarity. Although these two complex polysaccharides differ only by a single glycosidic linkage, they are antigenically distinct. Furthermore, terminal sialic acid residues appear to be critical to the immunodeterminant on the type Ia polysaccharide, whereas the antigenicity of the type Ib polysaccharide does not show this dependence on sialic acid. In the current investigation we defined better the immunodeterminant of these polysaccharides. With homologous rabbit antiserum, the type Ia native and core polysaccharides demonstrated partial serologic identity, whereas the type Ib native and core polysaccharides demonstrated complete serologic identity. Surprisingly, the type I degalactosylated polysaccharide, degraded structure, was capable of reacting with a population of antibodies present in type Ia antiserum similar to the complete type Ia native polysaccharide, although demonstrating a reduced level of immunodeterminant expression. Unlike the reactions of the type Ia polysaccharides with homologous rabbit antiserum, the Ib native and core polysaccharides were able to react with identical populations of antibodies in type Ib-specific antiserum. A minor population of antibodies was demonstrated in the type Ib antiserum, which was reactive with the degalactosylated polysaccharide. That a population of antibodies reactive toward the degalactosylated polysaccharide is present in both type Ia and type Ib antisera suggests that the Iabc cross-reacting determinant is due to the presence of serum antibodies reactive with this trisaccharide repeating unit, which is shared by both the type Ia and the type Ib native and core polysaccharides.