Immunocompetent cells of the upper airway: functions in normal and diseased mucosa

Mechanism of Lower Airway Hyperresponsiveness Induced by Allergic Rhinitis

Allergic rhinitis is a global illness that puzzles many researchers. Most patients with allergic rhinitis also have lower airway hyperresponsiveness, and an allergic rhinitis attack can increase lower airway hyperresponsiveness. However, the mechanism of the effect of allergic rhinitis on the lower airways is still unclear. In this paper, the effects of allergic rhinitis on the lower airways are studied in terms of epidemiology, anatomy, pathophysiology, nasal function loss, inflammation drainage, nasobronchial reflex, and whole-body circulatory flow to determine the mechanism involved and provide ideas for future diagnosis, treatment, and experiments.

COVID-19 in the tonsillectomised population

Objective

Interactions between SARS-CoV-2 and pharyngeal associated lymphoid tissue are thought to influence the manifestations of COVID-19. We aimed to determine whether a previous history of tonsillectomy, as a surrogate indicator of a dysfunctional pharyngeal associated lymphoid tissue, could predict the presentation and course of COVID-19.

Methods

Multicentric cross-sectional observational study involving seven hospitals in Northern and Central Italy. Data on the clinical course and signs and symptoms of the infection were collected from 779 adults who tested positive for SARS-CoV-2, and analysed in relation to previous tonsillectomy, together with demographic and anamnestic data.

Results

Patients with previous tonsillectomy showed a greater risk of fever, temperature higher than 39°C, chills and malaise. No significant differences in hospital admissions were found.

Conclusions

A previous history of tonsillectomy, as a surrogate indicator of immunological dysfunction of the pharyngeal associated lymphoid tissue, could predict a more intense systemic manifestation of COVID-19. These results could provide a simple clinical marker to discriminate suspected carriers and to delineate more precise prognostic models.

Increased expression of HLA-DR and CD86 in nasal epithelial cells in allergic rhinitics: antigen presentation to T cells and up-regulation by diesel exhaust particles

BACKGROUND

A proportion of nasal epithelial cells (NEC) in patients with allergic rhinitis (AR) are known to express the major histocompatibility complex Class II molecule (HLA-DR).

OBJECTIVE

We hypothesized that NEC may play a role in antigen presentation to T cells. To elucidate the possible role of NEC in antigen presentation, we examined the expression of HLA-DR, CD80 and CD86 in NEC, their regulation by cytokines and the capacity of NEC to induce antigen-specific proliferation of T cells.

METHODS

We examined the expression of HLA-DR, CD80 and CD86 in nasal epithelial scrapings of patients with seasonal allergic rhinitis (SAR) to Japanese cedar pollen pre-season and in-season, by immunohistochemistry. Next, we examined the effect of IL-1beta, TNF-alpha, (IFN-gamma), IL-4 alpha, IL-13 and diesel exhaust particles (DEP) on the HLA-DR, CD80 and CD86 expression in cultured nasal epithelial cells (CNEC), by flow cytometry. Further, we analysed the capacity of mite antigen (Der f II)-pulsed mitomycin-C-treated CNEC to induce proliferation of autologous T cells from patients with perennial allergic rhinitis.

RESULTS

NEC constitutively expressed HLA-DR and CD86, but not CD80. The expression of HLA-DR and CD86 in NEC was significantly increased in-season, in patients with SAR as compared with that of pre-season. While IFN-gamma up-regulated the expression of HLA-DR, IL-1beta and TNF-alpha up-regulated the expression of CD86 in CNEC. Furthermore, in the presence of mite antigen, CNEC induced the proliferation of autologous peripheral blood T lymphocytes. Anti-CD86 and anti-HLA-DR monoclonal antibody but not anti-CD80 inhibited the epithelial cell-induced T cell proliferation. Stimulation with a combination of DEP and mite antigen significantly up-regulated HLA-DR and CD86 expression in CNEC.

CONCLUSIONS

These studies suggest that NEC in patients with AR may play a role in antigen presentation through the enhanced expression of HLA-DR and CD86. Furthermore, these results suggest the possibility that DEP may enhance the antigen-presenting function of CNEC.

Immune response to Mycoplasma pulmonis in nasal mucosa is modulated by the normal microbiota

The impact of commensal bacteria on lymphocyte responses in the upper airways was studied in rat nasal mucosa after infection with the pathogen Mycoplasma pulmonis. Phenotyping was performed in situ by paired immunofluorescence staining in germ-free (GF) and conventional (CV) rats before and 3 wk after the monoinfection. Intraepithelial lymphocytes had expanded significantly in GF (P = 0.02) but not in CV rats. Furthermore, a striking proportional increase of T-cell receptor (TCR)alphabeta(+)CD4(+) cells was observed both in the lamina propria and epithelium of GF (P < 0.01) but not of CV rats. Notably, in contrast to the pre-infection state, both mucosal compartments showed a percentage of TCRalphabeta(+)CD4(+) cells that was significantly higher in GF (P = 0.03-P < 0.01) than in CV rats after the monoinfection. In parallel, both compartments displayed a percentage of TCRalphabeta(+) CD8(+) cells that was decreased in GF (P < 0.01) but not in CV rats. The small fraction of TCRgammadelta(+) T cells observed (< 5%) did not change quantitatively or phenotypically after infection. The size of organized nose-associated lymphoid tissue was, on average, increased 5.2-fold in GF rats versus 2.6-fold in CV rats. Collectively, our results demonstrated that the normal microbiota modulated markedly the nasal immune response elicited by monoinfection with M. pulmonis.

Soluble adhesion molecules and cytokines in children affected by recurrent infections of the upper respiratory tract

The objective of this study was to compare plasma levels of soluble adhesion molecules and Th1-Th2 type cytokines in 44 children with frequently recurrent respiratory infections (FRRI) of upper airways, defined as having nine or more episodes per year, and in 34 children without recurrence; all subjects were followed-up for 12 mo. The viral etiology was determined by cultures from nasal, pharyngeal, and ear secretions, using PCR and immunofluorescence. Plasma levels of five soluble adhesion molecules (E-selectin, P-selectin, L-selectin, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1) and interferon (IFN)-gamma, IL-12, IL-4, and IL-10 were measured in patients and in 15 healthy controls using sandwich ELISA. During acute phase, all patients showed significant increase in plasma levels of soluble adhesion molecules; during the follow-up, the levels were greater in children with FRRI. A difference of cytokine profile was demonstrated between the patients with and without FRRI: an increased IL-4 and IL-10 release with decreased levels of IFN-gamma and IL-12 suggested a skewing into Th2-type response, in patients with FRRI. This pattern persisted during the follow-up. In patients without recurrence, an increased IFN-gamma and IL-12 release, together with decreased levels of IL-4 and IL-10, showed a skewing into Th1-type responses; in the follow-up these cytokines reached normal values. In conclusion, the abnormal levels of all examined parameters in children with FRRI may reflect the persistence of an inflammatory microenvironment in the airways and an activation of the immune system that may contribute to the frequently recurrence of the respiratory disease.

Noticeable differences in bacterial defence on tonsillar surfaces between bacteria-induced and virus-induced acute tonsillitis

OBJECTIVE

Oral and pharyngeal cavities harbor a commensal bacterial flora which is kept in check by several innate and acquired agents. In this study, we focused on the proportions in which some antibacterial moderators (lysozyme, lactoferrin, IgG and S-IgA) coat the tonsillar surface bacteria in healthy individuals, in patients with acute tonsillitis (AT) culture-positive for Streptococcus pyogenes, and in patients with infectious mononucleosis (IM) caused by Epstein-Barr virus (EBV).

METHODS

Bacterial samples were collected for aerobic culturing and immunocytochemical evaluation from the tonsillar surfaces of eight healthy individuals (four males, four females; age range 16-22 years), eight patients with current AT (two males, six females; age range 16-29 years) and seven patients with IM (four males, three females; age range 15-21 years). The immunocytochemical assay was based on gold-labeled antiserum to human lysozyme, lactoferrin, IgG and S-IgA followed by gold particle tracing in the transmission electron microscope.

RESULTS

During AT, a significant increase in lysozyme coating (P<0.05) and lactoferrin coating (P<0.0005) of the bacteria was noted, whereas the S-IgA coating was significantly reduced (P<0.0005). During IM infection, a significant increase in lactoferrin coating was noted (P<0.0005) whereas immunoglobulin coating was significantly reduced (IgG P<0.025; S-IgA P<0.0005) compared with healthy controls. During IM, all antibacterial moderators evaluated were significantly reduced compared with the situation during AT.

CONCLUSIONS

Noticeable changes in the local innate and acquired bacterial defence system were observed during tonsillar infections, particularly during IM.

Causes for massive bacterial colonization on mucosal membranes during infectious mononucleosis: implications for acute otitis media

OBJECTIVE

A common complication of virus-induced upper respiratory tract infections is acute otitis media caused by bacterial pathogens. Simultaneously, increased bacterial colonization in the nasopharynx occurs. Our intention in this study was to identify the causes of this increased colonization of bacteria by evaluating their coating with the antibacterial substances lysozyme, lactoferrin and immunoglobulins IgG, S-IgA and IgM and their ability to penetrate epithelial cells during infectious mononucleosis (IM) caused by Epstein-Barr virus.

METHODS

Cellular samples were collected from the oropharynx of 21 patients (16 males, five females; age range 10-21 years) with current IM. An immunocytochemical assay using gold-labelled antiserum to human lysozyme, lactoferrin, IgG, S-IgA and IgM followed by gold particle and epithelial cell tracing in the transmission electron microscope.

RESULTS

A significant reduction in bacterial coating with IgG (P<0.05) and S-IgA (P<0.01) was noted, whereas there was a significant increase in coating with lactoferrin (P<0.01) and IgM (P<0.01). No significant change in lysozyme coating of the bacteria was noted, compared with healthy controls. Bacterial penetration into epithelial cells was seen particularly in patients culture-positive for beta-haemolytic streptococci.

CONCLUSIONS

Reduced bacterial coating with IgG and S-IgA immunoglobulins, combined with bacterial penetration into epithelial cells, may exacerbate the bacterial colonization on oropharyngeal mucosal membranes observed during IM.

The nose and paranasal sinuses physiology and anatomy

The paranasal sinuses and nose are much more than two cavities behind a projection on the centre of the face. They humidify, filter, warm, and sense what we breathe. The anatomy and physiology interact forming a dynamic system. The anatomy, airflow, nasal resistance, its turbulence, the nasal cycle - a process by which the turbinates or cushions lining the nose alternatively swell and congest from side to side, can all potentially influence the nasal delivery of drugs. Along with these factors mucus rheology and mucociliary clearance influence the removal of substances delivered to the nose. The health of the nose and its immunological response to what is inhaled, be it pollutants, allergens, drugs or vaccines, all need to be considered. It is a fascinating sensor for the body, not only detecting the potentially harmful substances such as smoke, but its psychosexual aspects have far reaching implications and the olfactory pathway has potential as a pathway for the delivery of drugs.

The presence of a local immune system in the upper blind and lower part of the human nasolacrimal duct

The nasolacrimal duct is exposed to exogenous agents, including potentially harmful microorganisms, coming from the eye surface by the lacrimal sac, and from the nasal cavity by the inferior meatus of the nose. The upper blind and lower part of the human nasolacrimal duct were examined immunohistochemically to ascertain the presence and localization of immunoglobulin-producing cells and the epithelial expression of IgA, IgM, and IgG in order to verify the possible antimicrobial properties of this duct. IgA-, IgM-, and IgG-positive immunocompetent cells were recognizable in the lamina propria of the upper blind and lower part of the human nasolacrimal duct, while an evident immunoreactivity for sIgA, IgM, and IgG was demonstrated in the cytoplasm of the apical epithelial cells. The results suggest that all the effector components of the mucosal immune system are present in that area of the human nasal mucosa next to the opening of the nasolacrimal duct as well as in the human lacrimal sac.

Involvement of inferior turbinate mucosa in chronic sinusitis--localization of T-cell subset

BACKGROUND

In chronic sinusitis (CS), different subsets of leukocytes are involved in development of persistent inflammation of the nasal mucosa. The localization and differentiation of these infiltrating lymphocytes may help us to understand the inflammatory interactions in the epithelium, lamina propria, and seromucous glands of the nasal mucosa in CS.

METHODS

We examined frozen sections of inferior turbinates from 14 patients with nonallergic CS and 10 normal nonallergic controls. We used the avidin-biotin-peroxidase (ABC) technique with monoclonal antibodies against CD3 (total T cells), CD4 (T-helper/inducer cells), CD8 (T-suppressor/cytotoxic cells), CD22 (B cells), CD56 (natural killer cells), elastase (neutrophil granulocytes), eosinophil cationic protein (eosinophil granulocytes), and CD68 (macrophages).

RESULTS

We found significant increases (P < 0.05) of CD3, CD4, and CD8 T cells and B cells in the nasal mucosa of patients with CS. The number of CD68 cells and eosinophils showed no significant rise.

CONCLUSIONS

The different types of leukocytes play a key role in the defense of the respiratory tract. The analysis of the distribution of cells in the epithelium, mucosa, and glands of the inferior turbinate confirmed that nonallergic CS is, in fact, chronic, bacterial rhinosinusitis involving the inferior turbinates, and that the pathomechanism is therefore different from that of nasal polyposis.

Increased occurrence of IgE+ and FcepsilonRI+ cells in adenoids from atopic children

BACKGROUND

To examine the influence of atopy on the different cell populations in adenoids, we investigated the presence of IgE+ cells, cells expressing the high-affinity receptor for IgE (Fc(epsilon)RI), and various other cell populations in adenoid tissue, in atopic and nonatopic children with otitis media with effusion (OME) or adenoid hyperplasia (AH).

METHODS

Cryostat sections of adenoids from 14 atopic and 16 nonatopic children suffering from long-lasting OME (n=15) or obstructive AH (n=15) were investigated with immunohistochemical markers for T-cell subsets, mast cells, eosinophils, plasma cells, CD25, CD1a, IgE, and Fc(epsilon)RI.

RESULTS

Sensitization to allergens was correlated to an increase of IgE+ cells in the epithelium (P<0.01), the extrafollicular area (P<0.0001), and the follicles (P<0.001) of the adenoids and an increase of Fc(epsilon)RI+ cells in the extrafollicular area (P<0.01). A minority of the IgE+ cells were plasma cells. No significant differences in cells stained for IgE, Fc(epsilon)RI, or the other markers were observed between patients with OME and AH.

CONCLUSIONS

Atopy is associated with increased numbers of IgE+ and Fc(epsilon)RI+ cells in adenoids irrespective of whether the child has OME or AH.

Immunoglobulin- and complement-coated bacteria in pus from peritonsillar abscesses

Fifty-five samples of pus were collected from 51 acute, non-perforated, two spontaneously ruptured and two recurrent peritonsillar abscesses (35 males and 18 females; median age 18 years) and analysed regarding (i) aerobic and anaerobic bacteria (standard culturing), (ii) morphology of bacteria and inflammatory cells (direct microscopy of acridine orange-stained material), and (iii) the percentage of bacteria coated with immunoglobulins IgG, secretory IgA (SIgA) and IgM and complement cleavage product C3b (immunofluorescence assay). Seventy-one per cent of the abscesses harboured a mixed bacterial flora of various aerobes and anaerobes. In none of the cases with a single bacterial species (27 per cent) could immunoglobulin- or complement-coated bacteria be found. In abscesses with a mixed flora, 18 per cent harboured IgG-coated, 15 per cent SIgA-coated, five per cent IgM-coated and five per cent C3b-coated bacteria, respectively. All pus samples contained inflammatory cells in abundance but they were mostly deformed and only occasionally could intracellular bacteria be recognized. Insufficient immunoglobulin-coating of bacteria might be an important aetiopathogenic factor in the development of a peritonsillar abscess. Bactericide in the abscesses is accomplished chiefly by protective mechanisms not dependent on antigen recognition by antibodies.

Phenotypic distribution of T cells in human nasal mucosa differs from that in the gut

Phenotypic and functional studies are required to understand the immunoregulatory role of mucosal T cells. Information about T cells in the human upper respiratory tract is limited and conflicting. Therefore, we phenotyped T cells in nasal mucosa by means of multicolor in situ immunofluorescence. In normal mucosa, most CD3+ intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs) (> 90%) expressed T-cell receptor (TCR)alpha/beta, and only approximately 5% expressed TCRgamma/delta. Although most IELs in the surface epithelium were CD8+ (64%), many expressed CD4 (30%) and the CD4 phenotype dominated (55%) only slightly in the lamina propria. This result was strikingly different from that obtained for comparable compartments in histologically normal jejunal mucosa, where IELs consisted of 83% CD8+ and LPLs of 73% CD4(+) T cells. Nasal CD3+ IELs and LPLs were mainly CD45RO+CD45RA- and usually expressed CD7. The integrin alphaEbeta7 was, as expected, more common on IELs than on LPLs (78 versus 20%). In conclusion, nasal T cells show several similarities to those of the normal jejunum but some notable differences exist, especially a relative increase in CD4+ T cells in the epithelium and a decrease in the lamina propria. It should be explored whether this disparity, together with an increased expression of epithelial adhesion molecules, might contribute to local immunological overstimulation and partly explain the relatively high frequency of airway allergy.

The surface epithelium of recurrent infected palatine tonsils is rich in gammadelta T cells

Using a large panel of MoAbs in quantitative morphometric analysis of immunohistochemically stained tissue sections, we compared the frequency and distribution of immune cells in palatine tonsils from patients with recurrent tonsillitis (RT) and patients with idiopathic tonsillar hypertrophy (ITH). We found that differences between the two patient groups in leucocyte populations were limited to the surface epithelium, whereas the cellular composition of interfollicular and follicular areas was similar. Most intraepithelial lymphocytes were CD8+ T cells in both groups. However, the number of intraepithelial T cells was significantly higher in RT compared with ITH. This was due to a selective increase in the number of intraepithelial CD8+ gammadelta T cells utilizing Vdelta1 and Vgamma9. In both patient groups the majority of the intraepithelial gammadelta T cells expressed Vdelta1 and Vgamma9. Subepithelially, gammadelta T cells utilizing Vgamma9 dominated over cells utilizing Vgamma8, while equal proportions expressed Vdelta1 and Vdelta2. These results suggest that cells utilizing the otherwise rare combination Vdelta1/Vgamma9 in their T cell receptors (TCR) may constitute a major gammadelta T cell population in palatine tonsils and are probably reactive to antigens specific to the tonsillar milieu. Furthermore, they indicate that preferentially this gammadelta T cell subpopulation is involved in immune reactions within the surface epithelium in RT. We speculate that gammadelta T cells are involved in clearing infectious bacteria at the tonsillar surface and in limiting inflammatory responses in the tonsils. Both local expansion and infiltration of blood cells probably contribute to the high numbers of gammadelta T cells in RT patients.

Mouse lung immune response after acute exposure to flour dust

To approach the physiopathology of the mucosal immune response in flour-mill and bakery workers, the authors developed a mouse model, which allowed them to study immune responses induced in the deep lung by acute inhalation of flour dust. In situ quantification of T lymphocytes (Thy1-2) and T-cell subsets (CD4 and CD8), macrophages, B lymphocytes, and immunoglobulin A plasma cells in the lungs of all animals revealed significant modifications of these cell compartments as early as 3 d after exposure; within 10 d of exposure, levels returned to baseline. The data suggest that the lung could be a sensitive target for inhaled xenobiotics, which might generate rapid immune local modifications that result in the maintenance of homeostasis.

Anticarrier immunity suppresses the antibody response to polysaccharide antigens after intranasal immunization with the polysaccharide-protein conjugate

We have conjugated cholera toxin (CT) B subunit (CTB) to dextran and studied the effect in mice of previous immunization with CT and CTB on the response to dextran after intranasal immunizations with conjugate. Preexisting immunity to CTB was found to inhibit both the lung mucosal response and serum antibody response to dextran, but this effect could be overcome by using a higher dose of conjugate and delaying the conjugate immunization until the CTB antibody titers had declined. The role of anti-CTB antibodies on the mucosal surface was probably to prevent uptake of the conjugate through a mechanism of immune exclusion. Passively transferred serum antibodies against CTB, on the other hand, suppressed both the serum response and the local antibody response against CTB but did not affect the response to dextran after intranasal immunization with conjugate.

Age dependency of the composition of immunocompetent cells and the expression of adhesion molecules in rat laryngeal mucosa

Clinical evidence shows that laryngeal infections in infants differ significantly from those in adults. Therefore, the composition of the mucosal immune system (granulocytes, macrophages, dendritic cells, natural killer cells, and T and B lymphocytes) and the epithelial expression of class II-MHC molecules and adhesion molecules ICAM-1, VCAM-1, and E-selectin were studied in the larynx of newborn, 5-week-old, and 3-year-old rats. With the exception of macrophages, the immunocompetent cells began to immigrate into the laryngeal mucosa after birth, indicating that the laryngeal mucosa in newborn rats is immature. In contrast, ICAM-1 was already expressed. The number of immunocompetent cells and the expression of epithelial class II-MHC and ICAM-1 increased with age. Immunocompetent cells and epithelial class II-MHC and ICAM-1 expression were mainly detected in the subglottic region, but were almost absent in the vocal fold region.

Salivary, nasal, genital, and systemic antibody responses in monkeys immunized intranasally with a bacterial protein antigen and the Cholera toxin B subunit

Previous attempts to induce mucosal antibodies in rhesus monkeys by enteric immunization have resulted in only modest and short-lived responses, dominated by immunoglobulin M (IgM) antibodies in the plasma. In this study, two groups of rhesus monkeys were immunized intranasally three times at 2-week intervals with a bacterial protein antigen (AgI/II) either chemically coupled to or mixed with the B subunit of cholera toxin (CT), a known potent mucosal immunogen and carrier for other immunogens. Cells secreting antibodies, predominantly of the IgA isotype, to AgI/II and to CT were detected in the peripheral blood 1 week after each immunization, indicating the dissemination of IgA-secreting precursor cells through the mucosal immune system. IgG and, to a lesser extent, IgA antibodies to both proteins were induced in the plasma commencing after the second immunization. Plasma IgE concentrations and IgE antibody levels were not consistently raised during the immunization period. IgA antibodies were found in nasal and vaginal washes. Nasal IgG but not IgA antibodies showed a significant positive correlation with plasma IgG antibody levels, suggesting that they were largely derived by transudation from the circulation. Analysis of the molecular form of vaginal IgA indicated that both secretory and monomeric forms of IgA were present in various proportions. Furthermore, neither IgG nor IgA antibodies in vaginal washes were correlated with plasma antibody responses, suggesting the contribution of locally synthesized antibodies of both isotypes. Comparison of the responses between the two groups of animals showed only sporadic significant differences, indicating that intranasal immunization with AgI/II either coupled to or mixed with the B subunit of CT was equally effective at inducing generalized IgA antibody responses in the mucosal immune system and predominantly IgG antibodies in the plasma.

Immune functions and immunopathology of the mucosa of the upper respiratory pathways

The specific defence of airway mucosae depends primarily on secretory immunity. The B cells involved are initially stimulated in organized mucosa-associated lymphoid tissue, apparently including the tonsils and adenoid. From these inductive sites, memory cells migrate to secretory effector sites where they differentiate terminally to immunoglobulin (Ig)-producing plasma cells. Locally produced Ig consists mainly of J chain-containing dimers and larger polymers of IgA (pIgA) that are selectively transported through glandular cells by an epithelial receptor called secretory component or the pIg receptor. IgG can participate in immune exclusion because it reaches the secretions by passive diffusion. However, its proinflammatory properties render IgG antibodies of local immunopathological importance when elimination of penetrating antigens is unsuccessful. T helper (Th) cells activated in this process may by a Th2 cytokine profile promote persistent inflammation with extravasation and priming of eosinophils. This development appears to be part of the late-phase allergic reaction, perhaps initially driven by interleukin-4 (IL-4) released from mast cells that are subjected to IgE-mediated activation, and subsequently also by Th2 cell activation. Eosinophils are potentially tissue-damaging, particularly after priming with IL-5. Various cytokines up-regulate adhesion molecules on endothelial and epithelial cells, thereby enhancing migration of eosinophils into the mucosa, and perhaps in addition causing aberrant immune regulation within the epithelium. Soluble antigens bombarding the epithelial surfaces normally seem to induce several immunosuppressive mechanisms, but mucosal homeostasis appears less patent in the airways than oral tolerance to dietary antigens operating in the gut.

History of oral tolerance and mucosal immunity

Mucosal immunity depends on antigen stimulation in specialized lymphoepithelial structures such as the Peyer's patches. Although these inductive compartments were discovered more than 300 years ago, their functional role has become clear only over the last few decades. Research on homing of primed lymphoid cells to the intestinal mucosa began with animal experimentation in the 1960s and 1970s and has recently been brought to the molecular level. The major effector substance of mucosal immunity is secretory IgA (SIgA). The first evidence for its local antibody activity was obtained in humans in 1922, but its unique properties were not defined until the mid-1960s. Several models were subsequently proposed for selective external transport of IgA involving the secretory component (SC). In the early 1970s SC was suggested to act as a transmembrane polymeric Ig receptor common for dimeric IgA and pentameric IgM; this transport mechanism has now been confirmed by detailed studies at the level of cellular/molecular biology. Although SIgA antibodies performing immune exclusion are the main goal for exploitation of the mucosal immune system by oral vaccination, little is known about the precise mechanisms for induction of mucosal immunity against soluble proteins and chemicals. A peripheral immunosuppressive effect of oral immunization with such substances was apparently exploited by ancient people, and "oral tolerance" has since 1910 been subjected to numerous feeding experiments in rodents. The basis for the whole phenomenon appears to be intact epithelial barrier. Mucosal induction of suppression may in the future be exploited not only to modulate autoimmune diseases through the gut but also to prevent the development of IgE-mediated allergy and other untoward immune reactions by way of the respiratory tract.