IgG Anti-tetanus Toxoid Antibody Research Articles

Oral Immunization with Recombinant Salmonella Induces Mucosal IgA Antibodies in CD4-Deficient Mice

It is well established that CD4+ T cells are major contributors to the induction and regulation of antigen-specific antibody responses in both mucosal and systemic compartments. However, our previous studies have demonstrated that oral immunization of CD4-/- mice with soluble protein plus cholera toxin as adjuvant elicited antigen-specific IgA antibody responses in the gastrointestinal tract. In this study, we investigated whether oral delivery of live bacteria could elicit mucosal immunity in CD4-/- mice. Oral immunization with recombinant Salmonella expressing fragment C of tetanus toxin resulted in serum IgG and IgA anti-tetanus toxoid antibody responses. Furthermore, CD4-/- mice are capable of eliciting secretory IgA antibodies in the intestinal tract. Antibody-forming cell analysis confirmed the antibody titers by revealing significant numbers of tetanus toxoid-specific IgG- and IgA-forming cells in the spleen and IgA-forming cells in intestinal lamina propria. In addition, significant delayed-type hypersensitivity responses were detected in CD4-/- mice given rSalmonella-Tox C. These results suggest that antigen-specific mucosal as well as systemic antibody responses can be induced in the absence of CD4+ T cells after oral immunization with Salmonella.

Evaluation of the immunogenicity and safety of an indigenously developed DTwP-Hib tetravalent combination vaccine (Shan 4) with EasyFourTM in Indian infants administered per EPI schedule

The study was planned to assess and compared immunogenicity and safety of an indigenous DTPw-Hib combination vaccine (Shan 4) with EasyFour, the available DTwP-Hib vaccine in India. Overall 210 healthy infants, six to eight weeks of age were randomized to receive three doses of either Shan 4 or EasyFour at 6, 10 and 14 weeks of age. Antibodies were analyzed prior to and four to six weeks post third vaccine dose. Solicited and unsolicited local and systemic events in the follow up period after each dose were recorded. Post vaccination 100% of the infants in Shan 4 and EasyFour groups had seroprotective concentrations of Anti PRP-T IgG antibodies, IgG anti-diphtheria toxoid antibodies and IgG anti-tetanus toxoid antibodies. Following third dose of vaccination 86.99% subjects in the Shan 4 group and 73.85% subjects in the EasyFour group seroconverted for anti-pertussis antibody titres. Two Serious Adverse Events (SAEs) were reported during the course of the study, all unrelated to the respective vaccine administered. Most commonly reported adverse events in both the groups were pain at injection site, mild fever (< 103° F) and minor swelling at injection site. The study proved that Shan 4 was safe and immunogenic compared to the available licensed vaccine.

T and B cell responses following immunization with tetanus toxoid in IgA nephropathy.

The B and T cell responses were investigated in IgA nephropathy before and after immunization with tetanus toxoid (TT). Both IgA and IgG anti-tetanus toxoid antibodies were elicited, but the IgA antibodies were significantly greater in patients (92.6 +/- 11.7 ELISA units) than in the controls (49.2 +/- 7.5 ELISA units). This was associated with a significantly greater proportion of IgA+ B cells in patients than controls before immunization. However, a significant increase in the proportion of IgA1 binding CD4 and CD8 cells was also found. The proportion of CD3 cells with gamma delta T cell receptors (CD3+TCR gamma delta +), was significantly greater before immunization in the IgA nephropathy patients (37.0% +/- 2.4), compared with controls (10.0% +/- 2.3; P less than 0.001). Immunization with TT further enhanced the CD3+TCR gamma delta + cells in patients to 45.8% +/- 7.2 compared with controls (16.3% +/- 4.5), with a corresponding decrease in CD3+TCR alpha beta + cells in the patients (P less than 0.001). CD3+TCR gamma delta + cells are upregulated by common microbial antigens and clinical exacerbations of IgA nephropathy are frequently associated with mucosal infections and a rise in serum IgA concentration. The increased TCR gamma delta expression may be responsible for the enhanced IgA antibody response in IgA nephropathy. The increase in IgA antibodies may than exert a controlling effect by binding to augmenting T cells and thereby inhibiting their function.

Coexistent yellow nail syndrome and selective antibody deficiency

Yellow nail syndrome (YNS) is a rare, often underdiagnosed condition of unknown origin. The clinical features of the syndrome include yellow nails, chronic sinusitis, bronchiectasis, pleural effusion, and lymphoedema. Despite the frequent occurrence of upper and lower respiratory tract infections in patients with YNS, comprehensive analysis of their humoral immunity has not been previously reported. To present the case of a patient with YNS whose recurrent upper and lower respiratory tract infections may have been caused by an underlying selective antibody deficiency that manifests as impaired IgG antibody response to polysaccharide antigens. The patient underwent cultures of purulent sputum for Streptococcus pneumoniae and Haemophilus influenzae, bronchial washings for H. influenzae, and nail scrapings for fungi. Her serum levels of IgG, IgA, IgM, IgG subclasses, and serum titers of IgG antitetanus toxoid, anti-H. influenzae, and anti-S. pneumoniae antibodies were measured. Cultures of purulent sputum were positive on multiple occasions for S. pneumoniae and H. influenzae and bronchial washings were positive for H. influenzae. Nail scrapings were consistently negative for fungi. She had no reductions in serum levels of IgG, IgA, IgM, or IgG subclasses and had normal serum titers of IgG antitetanus toxoid antibodies. However, she demonstrated impaired IgG antibody responses following immunization with Pneumovax and an H. influenza B vaccine. This case report describes the first comprehensive analysis of humoral immune function in a patient with YNS. The finding of a selective antibody deficiency in our patient provides a potential explanation for the occurrence of respiratory infections in YNS. Accordingly, we recommend that functional antibody determinations and quantitative serum immunoglobulins be evaluated in patients diagnosed as having this unusual, enigmatic syndrome.

A typical bacterial ornithine-containing lipid Nalpha-(D)-[3-(hexadecanoyloxy)hexadecanoyl]-ornithine is a strong stimulant for macrophages and a useful adjuvant.

Nalpha-[3-(Hexadecanoyloxy)hexadecanoyl]-ornithine is a typical bacterial ornithine-containing lipid (OL). The configuration of the 3-hydroxy fatty acids in the OL was proved to be D by using HPLC with chiral column. For this analysis, Nalpha-(D or L)-[3-(hexadecanoyloxy)hexadecanoyl]-L-ornithine were synthesized and used as standards. The typical bacterial OL, as well as the synthesized one, exhibited strong interleukin-1- and prostaglandin E2-inducing activities, and further, it induced the production of high IgG anti-tetanus toxoid antibodies in mice. The typical OL is expected to be utilized as a nontoxic, potent adjuvant.

A typical bacterial ornithine-containing lipid N α-( d)-[3-(hexadecanoyloxy)hexadecanoyl]-ornithine is a strong stimulant for macrophages and a useful adjuvant

N α -[3-(Hexadecanoyloxy)hexadecanoyl]-ornithine is a typical bacterial ornithine-containing lipid (OL). The configuration of the 3-hydroxy fatty acids in the OL was proved to be d by using HPLC with chiral column. For this analysis, N α -( d or l)-[3-(hexadecanoyloxy)hexadecanoyl]- l-ornithine were synthesized and used as standards. The typical bacterial OL, as well as the synthesized one, exhibited strong interleukin-1- and prostaglandin E 2-inducing activities, and further, it induced the production of high IgG anti-tetanus toxoid antibodies in mice. The typical OL is expected to be utilized as a nontoxic, potent adjuvant.

IgG anti-tetanus toxoid antibody synthesis by human bone marrow. I. Two distinct populations of marrow B cells and functional differences between marrow and peripheral blood B cells

This investigation uses a system for inducing and detecting anti-tetanus toxoid antibody (anti-TT) synthesis to study specific antibody (Ab) synthesis by bone marrow mononuclear cells (MC). We measured the amounts of anti-TT secreted and the number of B cells secreting antibody (Ab). The ELISA plaque detects single B cells secreting specific Ab. The results show that (1) spontaneous anti-TT secretion by MC is higher than spontaneous anti-TT secretion by peripheral blood lymphocytes (PBL) using an ELISA plaque (P less than 0.01); (2) spontaneous anti-TT production by MC correlated with the serum anti-TT titers as measured by an ELISA (r = 0.75, P = 0.005); (3) two types of marrow B cells were identified--one that spontaneously secretes anti-TT and another that produces anti-TT after TT-stimulation; (4) the frequency of anti-TT-secreting B cells is higher in MC than in PBL; (5) the amount of Ab secreted per marrow B cell is not different from that secreted by a peripheral B cell; and (6) marrow B cells could be induced to produce anti-TT in vitro up to 10 months without added cytokines. These results show that bone marrow is a major repository for differentiated B cells that spontaneously produce Abs to maintain circulating Abs titers and for memory B cells that can be induced to produce specific Ab.

Immunological responses to chronic heat exposure and food restriction in rats

Immunological variables were studied in rats chronically exposed to high environmental temperature (35 degrees C). Responses were compared with those of rats at 25 degrees C both fed ad libitum and pair fed to the decreased intake found in heat-exposed rats. Heat-exposed rats showed slower delayed-type hypersensitivity responses to keyhole limpet hemocyanin. They showed lower counts of peripheral blood total T cells (OX19+) as well as helper T cells (W3/25+) and smaller numbers of splenic T cells. The thymus was decreased in size. Increased levels of serum IgG antitetanus toxoid antibodies were found in heat-exposed rats. [3H]-thymidine incorporation into Concanavalin A (ConA)-stimulated splenic lymphocytes was decreased in pair-fed rats but not significantly altered in heat-exposed rats compared with controls. Heat exposure alters some aspects of both cellular and humoral immune function in a manner different from that induced by comparable food restriction without heat exposure.

The effects of aggregated human IgG and IgG-immune complexes on the agglutination of bacteria mediated by non-immunoglobulin salivary agglutinins

Non-immunoglobulin salivary agglutinins (SBA) for bacteria which bind to Streptococcus milleri TJ7 were isolated from parotid saliva and their interactions with human IgG studied. Purified SBA showed a single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis with a molecular weight of approx. 500,000. Heat aggregated human IgG (63 °C, 30 min), but not native IgG, interacted with SBA and thereby interfered with the ability of the SBA to agglutinate Strep. milleri. Immune complexes prepared from tetanus toxoid and isolated human IgG anti-tetanus toxoid antibody also inhibited salivary bacterial agglutination by SBA; antigen (tetanus toxoid) alone or antibody (anti-tetanus toxoid antibody) alone did not have this effect. Direct-binding studies with immobilized SBA on nitrocellulose paper showed that aggregated IgG bound to immobilized SBA and that this binding was inhibited by EDTA. Thus it appears that heat or specific antigen is able to induce an aggregation of IgG which results in the binding of the aggregated form of IgG to SBA.

The transfer of antigen-specific humoral immunity from marrow donors to marrow recipients

This study shows that specific humoral immunity could be transferred from marrow donors to marrow recipients. Peripheral blood lymphocytes from long-term human marrow recipients produced IgG anti-tetanus toxoid antibody after in vitro tetanus toxoid stimulation. Antitetanus toxoid antibody biosynthesis was induced using a new tetanus toxoid-specific system employing high lymphocyte numbers, many replicate microcultures, and antigen washout. Anti-tetanus toxoid antibody in 12-day culture supernatants was detected using an enzyme-linked immunosorbent assay. Of 14 marrow recipients, 6 (5 with and 1 without chronic graft-vs-host disease) had lymphocytes that produced anti-tetanus toxoid antibody. Culturing additional numbers of marrow recipient lymphocytes increased in vitro biosynthesis of anti-tetanus toxoid antibody. The presence of circulating serum antibodies to tetanus toxoid and the production of specific anti-tetanus toxoid antibody by peripheral blood lymphocytes from marrow recipients show that engrafted donor lymphocytes can produce in vitro specific antibodies to recall antigens without postgrafting reimmunization.

IgG anti-tetanus toxoid antibody production induced by Epstein-Barr virus from B cells of human marrow transplant recipients

This investigation shows that Epstein-Barr virus (EBV)-activated human B cells from marrow transplant recipients can produce in vitro IgG anti-tetanus toxoid antibody (anti-TT) without booster immunizations with tetanus toxoid (TT). Purified B cells (E-rosette negative) from 8 normal subjects, 6 healthy long-term marrow graft recipients, and 15 long-term marrow graft recipients with chronic graft-vs-host disease (GVHD), were stimulated for 12 days with EBV to induce anti-TT production in culture supernatants. The amount of anti-TT in culture supernatants was quantitated using a enzyme-linked immunosorbent assay. B cells from all 8 normal controls produced in vitro IgG anti-TT after EBV stimulation. Five of 6 healthy recipients had B cells that produced anti-TT after EBV stimulation. Four of 15 recipients with chronic GVHD had B cells capable of producing anti-TT after EBV stimulation. The number of cultures making anti-TT responses was less in those with chronic GVHD than in those without chronic GVHD or normal individuals ( P < 0.001). B cells from patients with chronic GVHD had fewer responses exceeding the overall median of 0.7 ng/ml when compared with the other two groups ( P < 0.03). These data show that B cells of donor origin can produce in vitro IgG anti-TT antibody to tetanus toxoid antigen in a T-independent fashion.

Antigen-specific antibody responses of lymphocytes to tetanus toxoid after human marrow transplantation.

In vitro IgG anti-tetanus toxoid (IgG anti-TT) antibody produced by peripheral blood lymphocytes (PBL) from 16 normal subjects (9 marrow donors and 7 random healthy subjects) and 17 marrow graft recipients from 45-1058 days postgrafting was measured with an enzyme-linked immunosorbent assay (ELISA). PBL from 11 of 13 seropositive (anti-TT greater than or equal to 1:1024) normal subjects produced IgG anti-TT in vitro, whereas the PBL from the 3 seronegative (IgG anti-TT less than 1:1024) normal subjects did not. In our normal subjects, there was a high correlation between seropositivity and in vitro IgG anti-TT production (P = .0048, chi 2, two-tailed). PBL from only one of 13 seropositive marrow graft recipients produced in vitro IgG anti-TT antibody. B and T cell functions of 8 marrow graft recipients were assessed by coculturing their T and B cells with those from their HLA-identical marrow donors. One short-term patient and 7 long-term patients (4 with and 3 without chronic graft-versus-host disease) were studied. Recipient B cells failed to produce antibody in the presence of donor T cells in 7 of these 8 cases. However, T cells from long-term survivors provided helper activity to immune donor B cells in 7 of 9 evaluable cases. TT-specific helper T cell activity was present in most seropositive long-term recipients, and B cells from marrow recipients failed to produce specific antibody in the presence of normal donor TT-specific helper T cells. These results, TT-specific T cell helper activity, and normal circulating serum IgG anti-TT antibody levels in marrow graft recipients without postgrafting TT boosters suggest that specific immunity had been transferred from the marrow donor to the marrow recipient.

The induction and suppression of in vitro IgG anti-tetanus toxoid antibody synthesis by human lymphocytes stimulated with tetanus toxoid in the absence of in vivo booster immunizations.

This report presents a new approach that by-passes booster immunizations with tetanus toxoid (TT) before in vitro studies of antibody (Ab) production. The methodology for optimal TT-induced synthesis of specific IgG anti-tetanus toxoid Ab (IgG anti-TT) by peripheral blood mononuclear cells (PBMC) from randomly selected TT immune individuals without recent booster immunizations is described. PBMC from most normal immune subjects could be repeatedly induced to produce in vitro IgG anti-TT; PBMC from subjects with high TT titers are not required for this new approach. This approach uses high cell concentrations in multiple replicate microcultures and TT washout to obtain optimal IgG anti-TT synthesis. Washed cultures produced more Ab than nonwashed cultures (p less than or equal to 0.005). The readdition of TT (2.5 to 250 ng/ml) to the culture media after washout of TT on day 4 suppressed specific Ab formation, whereas diphtheria toxoid added at comparable doses did not inhibit specific Ab formation. Suppression of antibody synthesis mediated by T cells could be induced by TT per se, and was not due to binding of synthesized Ab to TT in the latter 8 days of culture. In addition, suppression could not be induced in the first 4 days of culture by IgG anti-TT, IgG, or IgM. This approach permits the analysis of antigen-specific regulatory circuits in the steady and activated immune states, and the evaluation of in vivo and in vitro effects of biologic response modifiers on specific Ab production.

T suppressor cells are required for the maintenance of the antigen-induced B-cell unresponsive state in humans

Tetanus toxoid immunization of humans generates circulating B cells which secrete IgG anti-tetanus toxoid antibodies (IgG-Tet) when stimulated in vitro with T cells and pokeweed mitogen (PWM). A unique property of these cells is the inhibition of maturation into antibody-secreting plasma cells following a 1-hr in vitro pulse with tetanus toxoid. Studies were undertaken to determine if different T-cell subsets could modulate the in vitro generated B-cell unresponsive state. The addition of OKT4+/OKT8- cells to antigen-treated B cells resulted in a partial reversal of the antigen-induced inhibition of IgG-Tet synthesis. The addition of OKT4-/OKT8+ cells to the treated B cells caused a suppression of IgG-Tet synthesis comparable to that seen in cultures containing unfractionated T cells. These results indicate that (1) the B-cell unresponsive state generated by antigen treatment is not absolute, (2) the degree of B-cell unresponsiveness results from a balance of suppressor and helper signals, and (3) T-suppressor cells need to be present to induce and maintain the B-cell unresponsive state.

Age-related decrease in frequencies of B-cell precursors and specific helper T cells involved in the IgG anti-tetanus toxoid antibody production in humans

B cells from each of 12 aged subjects (72 to 83 years old) and 12 young adults (24 to 32 years old) were cocultured with pokeweed mitogen (PWM) and an equal number of T cells isolated from a single tetanus toxoid-boosted donor for 7 days. The quantities of IgG antibody produced were significantly different in the two groups, with geometric means of 142 and 181 ng/ml in young adults, and 14 and 33 ng/ml in aged donors at 2 and 4 weeks after booster immunization, respectively ( P < 0.01 at 2 weeks, P = 0.02 at 4 weeks). There was no evidence indicating increased specific suppressor activity in the aged. Precursor frequency of B cells active in IgG antibody production was approximately 5.2-fold less in the aged than in young adults ( P = 0.01). Tetanus toxoid-specific helper T-cell frequency also decreased by 3.5-fold in the aged as compared with young adults ( P = 0.05).

Immunoglobulin-bearing cells are a target for the antigen-induced inhibition of pokeweed mitogen-stimulated antibody production.

In vivo immunization with tetanus toxoid causes the appearance of B cells in the circulation that can produce IgG-antitetanus toxoid antibody in vitro after stimulation by T cells and pokeweed mitogen. Addition of soluble tetanus toxoid to these cultures, however, causes a profound inhibition of the in vitro IgG, but not IgM, antitetanus toxoid antibody production. Treatment of the lymphocytes with 10 microgram of tetanus toxoid for as little as 1 hr at 37 degrees C was sufficient to promote this antigen-specific inhibition. The antigen could be added, however, as late as 3 days after the culture initiation for the inhibition to be observed. Cell separation studies indicated that the inhibitory activity could be found in both the E-rosette positive and E-rosette negative populations. Further studies on the non-E-rosetting cells showed that: 1) inhibition induced in antigen-treated cells could not be transferred to untreated cells; 2) adherent cells were not responsible for the inhibition; 3) treatment of the surface Ig+ cells with antigen was sufficient for the inhibitory effects; and 4) in vivo generated lymphoblastoid cells that produce IgG-antitetanus toxoid antibody in vitro but are surface Ig- were not inhibited by soluble antigen. The results in this study thus indicate that cells in both the E-rosette positive and negative subsets can be responsible for the antigen-induced inhibition of in vitro antibody production observed in the majority of immunized donors.

Expression of surface membrane IgG on pokeweed mitogen-reactive anti-tetanus toxoid antibody-producing cells.

Anti-tetanus toxoid antibody-producing cells, differentially expressing surface membrane IgM, were analyzed for the additional expression of surface membrane IgG. micron+ and micron- cells were rosetted with anti-gamma-ox red blood cells and separated by density centrifugation into fractions enriched or depleted of gamma + cells. These B-cell subsets were assayed for the production of IgM and IgG anti-tetanus toxoid antibody and total IgM and IgG. The results indicated that the majority of anti-tetanus toxoid antibody synthesis in the micron- fraction was by gamma + cells. In the microm+ fraction, however, both IgM and IgG anti-tetanus toxoid antibody production was detected in the micron+ gamma- and micron+ gamma+ fraction. The inclusion of isotype-specific antisera during the first 2 days of culture further established that gamma was expressed on the surface of the majority of the precursors for IgG anti-tetanus antibody production in vitro. Studies performed to determine the culture requirements of micron- and micron+ cells revealed that production of IgG anti-tetanus toxoid antibody by both cell subsets was dependent on T cells and pokeweed mitogen. However, some micron- cells could produce IgG in the presence of T cells alone.

Antigen-induced suppression of human in vitro pokeweed mitogen-stimulated antibody production

Combinations of T and B lymphocytes from normal individuals booster immunized 14–30 days previously with a combination of diphtheria and tetanus toxoids, synthesized IgG antitetanus toxoid, and IgG antidiphtheria toxoid antibodies when stimulated by pokeweed mitogen in vitro. The addition of 5 μg of soluble tetanus toxoid to the cultures during the first 2 days incubation resulted in greater than 90% suppression of the subsequent production of IgG antitetanus toxoid antibodies. The synthesis of IgM antitetanus toxoid antibodies, total IgG, total IgM, and IgG antidiphtheria toxoid antibodies were unaffected. Similarly, the addition of 5 μg of soluble diphtheria toxoid suppressed the synthesis of IgG antidiphtheria toxoid antibodies with no effect on the synthesis of IgG antitetanus antibodies. Allogeneic combinations of B and T lymphocytes were capable of mediating the suppression, and irradiation of the T cells caused only a partial and variable reversal of the suppression. The antigen-induced specific suppression of antibody synthesis could not be demonstrated in cultures stimulated with soluble T-cell-derived helper factors.

Inability of patients with common variable hypogammaglobulinemia to generate lymphoblastoid B cells following booster immunization

Twelve patients with common variable hypogammaglobulinemia were compared with normals for the ability of their lymphocytes to produce IgG and IgM antitetanus toxoid antibody in vitro following in vivo booster immunization with soluble tetanus toxoid. Five days after booster immunization, 0.4 × 10 5 non-E-rosette-forming lymphocytes from normal individuals produced an average of 202 ng of IgG antitetanus toxoid antibody during in vitro culture in the absence of T cells or mitogen stimulation. This spontaneous antibody-producing B-cell population (lymphoblastoid B cells) was not concurrently detected in the circulation of any of the patients following booster immunization. Additionally, cultures of patients' lymphocytes initiated either before or after the peak normal lymphoblastoid cell response also failed to produce antibody. Neither the patients nor normals had circulating lymphoblastoid cells capable of spontaneously synthesizing IgM antitetanus toxoid antibody in vitro. In the presence of pokeweed mitogen and irradiated T cells, the non-E-rosetting cells from recently booster-immunized normal individuals synthesized an average of 115 ng IgM antitetanus toxoid antibody during an 8-day in vitro culture. Three of the twelve immune-deficient individuals produced normal or near normal levels of IgM antitetanus toxoid antibody in vitro in the presence of irradiated T cells. Two of these individuals produced significant levels of total IgM in culture whereas in one individual, the in vitro production of IgM could be totally accounted for by the synthesis of IgM antitetanus toxoid antibody. All patients also showed highly depressed in vitro pokeweed mitogen-stimulated synthesis of total IgG as well as IgG antitetanus toxoid antibody.

Antibody potential of human peripheral blood lymphocytes differentially expressing surface membrane IgM.

The appearance of circulating pokeweed mitogen-reactive B lymphocyte subpopulations responsible for the synthesis of IgM and IgG anti-tetanus toxoid antibody after in vivo booster immunization has been analyzed for differential expression of surface membrane IgM. B cells were rosetted with human mu-chain specific antiglobulin-coupled ox red blood cells and separated by density centrifugation into mu+ and mu- populations. After 8 days in vitro culture with irradiated T cells and pokeweed mitogen, antibody synthesis was assessed by quantitative radioimmunoassay. Mu+ cells synthesized both IgG and IgM anti-tetanus toxoid antibodies, whereas mu- cells predominantly synthesized IgG anti-tetanus toxoid antibodies. Limiting dilution analysis revealed a greater number of IgG anti-tetanus toxoid precursors in the mu+ population at all times after immunization compared with mu- cells. However, the activity, as measured by the amount of antibody produced per IgG anti-tetanus toxoid antibody precursor, was 2 to 4 times greater in the mu- population than that in the mu+ population. These results indicate that functionally distinct human circulating B lymphocytes can be identified by their differential expression of surface membrane IgM.