Immunology of the Lower Respiratory Tract

16 Pages·2007·2.59 MB·English

Immunology of the Lower Respiratory Tract
SerialMorphologic Changes in theLungsand
TracheobronchialLymphNodesofDogsAfterIntrapulmonary
Immunization With SheepErythrocytes
David G. Brownstein, DVM, Alan H. Rebar, DVM, PhD,
David E. Bice, PhD, BruceA. Muggenburg, DVM, PhD,
and Joseph 0. Hill, PhD
Sequential histologic changes in immunized andcontralateral lung lobes, and ipsilateral
and contralateral tracheobronchial lymph nodes were evaluated from dogs after intra-
pulmonary immunization with sheep erythrocytes. In addition, changes in the relative
numbers ofunstimulated lymphocytes, activated lymphocytes, andplasma cellswere de-
terminedforimmunizedandcontralateral lunglobesbytheuseofbronchopulmonary la-
vage. Histologic evidence ofan immune response occurred2 days after immunization in
ipsilateral tracheobronchial lymph nodes andon Day5in immunizedlunglobes. Pulmo-
nary lymphoid infiltrates appeared initially aroundpulmonary venules andveins. There
was expansion ofthese infiltrates into alveolar spaces, where mixed mononuclear aggre-
gateswereformedinassociationwithalveolarmacrophages7daysafterimmunization. A
similar but attenuated and delayed response occurred within contralateral control lung
lobes,althoughmononuclearaggregateswerenotfound.Activatedlymphocytesin lavage
samples increased prior to histologic evidence ofpulmonary lymphoid infiltrates. These
results suggest that after intrapulmonary immunization the lungrecruits circulating im-
munocytes produced in ipsilateral lung-associated lymph nodes as the source of specific
antibody-forming cells in bronchoalveolar air spaces. (AmJ Pathol 98:499-514, 1980)
ANTIBODY-MEDIATED IMMUNOLOGIC reactions that occur in
the lower respiratory tract in response to antigenic stimulation have been
the subjects of a number of recent investigations.' We have used
bronchopulmonary lavage to define a humoral immune system in the
lower respiratory tract that is distinct from that in the upper respiratory
tract. Whereas IgA, produced locally or systemically and actively trans-
ported across airway epithelium, is the major immunoglobulin of the up-
per respiratory tract, IgG, produced predominantly by plasma cells
within the bronchoalveolar air spaces, is the major immunoglobulin of the
lower respiratory tract.8-13
Local deposition of sheep erythrocytes in the lower respiratory tract of
dogs results in the appearance ofantigen-specific IgM- and IgG-producing
plasma cells in the bronchoalveolar air spaces,2'4-7 with significantly
From the Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental
Research Institute, Albuquerque, New Mexico.
Accepted for publication September5, 1979.
Address reprint requests to Dr. David Brownstein, Section ofComparative Medicine, 375 Con-
gress Avenue, New Haven, CT06510.
0002-9440/80/0211-0499$01.00 499
C)American Association of Pathologists
500 BROWNSTEIN ETAL AmericanJournal
ofPathology
greater iiumbers accumulating in immunized lung lobes than in unim-
munized lobes.7 This response to locally deposited antigen occurs in the
presence ofa specific immune response in the regional lymph nodes.2'4'5"0
With decreasing doses of antigen, specific-antibody-producing cells dis-
appear from the lung before they disappear from the regional lymph
nodes.5"0
It is evident that afferent and efferent components of a humoral im-
mune mechanism are operative in the lower respiratory tract. What re-
mains to be determined is whether this humoral response occurs totally in
situ, independent of the systemic immune apparatus, or is a local manifes-
tation of immune responses occurring within the systemic immune appa-
ratus with circulating immunocyte recruitment by the immunized
lung.57,1012 The purpose of the present study was to characterize the se-
quential morphologic changes in the lung and tracheobronchial lymph
nodes (TBLNs) of dogs immunized by the intrapulmonary route with
sheep erythrocytes to gain insight into the mechanisms involved in the
generation of humoral immune responses in the lower respiratory tract.
Materials and Methods
Animals and Experimental Design
Twenty-five beagle dogs ofboth sexes, 3 to 9 years ofage, were studied. In Group 1, 8
dogs were immunized by the instillation of 101" washedsheep erythrocytes in 1.0 ml ster-
ile saline (0.9%) into specific airways ofthe right or left apical lunglobe underlight halo-
thane anesthesia. The contralateral apical lobe received 1.0 ml sterile saline. In Group 2
(control), 4 dogs received 101" washed autologous erythrocytes in 1.0 ml sterile saline de-
livered in a manner identical to that used fordogs receiving sheep erythrocytes. The con-
tralateral apical lobe was instilled with 1.0 ml sterile saline. Dogs from Groups 1 and 2
were killedby intravenous barbiturates followedbycardiac exsanguination 2,5, 7, and 21
days after erythrocyte exposure. In Group 3 (control), 2 dogs were administered 1.0 ml
sterile saline in a specific airway of the right or left apical lobe and killed 5 and 7 days
later. In Group 4 (control), 2 dogs were anesthetized, and a bronchoscope was used in a
manner identical to that used with dogs of Groups 1-3, but no airway instillations were
performed. These dogs were killed2and21 days later. In Group 5 (lunglavage cytology),
9 dogs were immunized with 10'" sheep erythrocytes asdescribed above. The immunized
airway andacontralateral control airwaywere lavagedwith sterile saline (0.9%) with the
use ofa fibrotic bronchoscope 3, 5, 7, 10, 12, 14, and 21 days after immunization.
Airway Immunization
A flexible fiberoptic bronchoscope (Model BF3, OlympusCorp. ofAmerica, New Hyde
Park, NY) wasused to locate specific airways, and thesheep erythrocyte suspension or sa-
line wasdelivered throughpolyethylene tubing 1.6mm indiameter. These materials were
deposited in airways approximately 2 mm in diameter.
Vol. 98,No. 2 LOWER RESPIRATORYTRACT 501
February 1980
Histologic Studies
The entire lungblock was removed from each dog in Groups 1-4. The fiberoptic bron-
choscope was reinserted to identify the immunized and saline-treated airways. A 1.6-mm
polyethylene catheter was passed down the bronchoscope, and the bronchoscope was re-
moved, leaving the catheter in place to mark the treated airway. We anchored the cath-
eter by placing a ligature around the mainstem bronchus. The lungs were perfused
through the catheter with neutral-buffered formalin at30cm fixative pressure. After fixa-
tiontheairwayswereopenedandthecourseofthecatheterwasfollowedsothatwe could
identifylungtissue suppliedby treatedairways. Multiplelungsamples, aswell as sections
oftherightandlefttracheobronchial lymphnodes, were embedded in paraffin, cutat5fA,
and stainedwith hematoxylin andeosin (H&E). Selected sections were stained with Prus-
sian blue and methyl green pyronine for ribonucleic acid.'4
Lung LavageCytology
Specific immunizedandcontrol airwaysofdogsin Group5were lavagedwith5washes
of 10 ml saline solution (0.9%) with the use ofa fiberoptic bronchoscope. An average vol-
ume of 42.4 ml lavage fluid was recovered. Duplicate lavage samples (150 ml, 100 ml)
wereplacedinaShandon cytocentrifuge (Shandon Scientific Company, London) and cen-
trifugedat40gfor5minutes. Cytocentrifuge slidepreparations werestainedwitha rapid
Romanowsky stain (Diff-Quick, Harleco Company, Gibbstown, NJ). The relative propor-
tion ofthe three established classesoflymphoidcellsbasedon cytologic features '"'7 was
determinedon the basisofcounts of 100 lymphoid cells. The classesconsistedofunstimu-
lated lymphocytes, activated lymphocytes, and plasma cells.
Results
Tracheobronchial Lymph Nodes: Histology
The anatomic arrangement of the TBLNs is relatively constant in the
dog.'8 The right and left TBLNs are ipsilateral in their drainage ofmateri-
als deposited in the right apical and left apicocardiac lung lobes.'8 The
left TBLN is slightly larger than the right.
Histologically the TBLNs from the untreated and saline-treated control
dogs varied within narrow limits (Figure LA and B). Each node had a thin
cortex, sparsely populated by small lymphocytes. The outer cortex con-
tained scattered germinal centers with thin mantle layers. Primary lym-
phoid follicles were not seen. The paracortex contained a uniform popu-
lation of small lymphocytes. The medullary cords were thin and
composed predominantly of small lymphocytes. Small numbers of me-
dium and large lymphocytes with basophilic, pyroninophilic cytoplasm
populated the medullary cords. Plasma cells, with basophilic, pyronino-
philic cytoplasm plus eccentric nuclei and prominent perinuclear halos,
were also present in small numbers in medullary cords. The subcapsular,
cortical and medullary sinuses were composed of stellate mononuclear
cells and macrophages with a loosely packed lymphoid component similar
502 BROWNSTEIN ET AL AmericanJournal
ofPathology
to that of the medullary cords. Erythrocytes were usually present within
the sinusoids.
Tracheobronchial lymph nodes ipsilateral and contralateral to lung
lobes receiving autologous erythrocytes were indistinguishable from
TBLNs from untreated and saline-treated control dogs at 2, 5, 7, and 21
days after immunization.
Two days after immunization with sheep erythrocytes there were
marked cortical changes in the ipsilateral TBLNs (Figure 1C). There was
irregular cortical thickening due to increased cellularity of the outer cor-
tex and paracortex. Multiple primary follicles filled the outer cortex in re-
gions ofcortical thickening. The paracortex in these regions was expanded
by densely packed small lymphocytes. Medullary cords beneath the thick-
ened cortex were expanded predominantly by neutrophils. The lymphoid
component ofthe medullary cords was more pleomorphic than that ofun-
stimulated nodes due to large numbers of medium and large lymphocytes
with basophilic, pyroninophilic cytoplasm. This lymphoid pleomorphism
was also reflected in the sinuses. TBLNs contralateral to lung lobes receiv-
ing sheep erythrocytes were indistinguishable from those ofuntreated and
saline-treated control dogs at 2 days and throughout the remainder of the
study.
Five days after immunization the ipsilateral TBLNs had diffuse cortical
thickening due to numerous germinal centers with thick mantle layers
(Figure ID). Some germinal centers were forming within thickened med-
ullary cords. Capping was frequent in these nascent germinal centers. The
paracortex was uniformly expanded by small lymphocytes. In addition to
scattered germinal centers, medullary cords were obliterated by medium
and large lymphocytes, many transforming into plasmablasts. Mitoses
were frequent within the medullary cords. Granulocytes, prominent com-
ponents of the medullary cords at 2 days, were not present at 5 days. Sub-
capsular, cortical, and medullary sinuses were packed with medium and
large lymphocytes and plasmablasts.
Seven days after immunization the ipsilateral TBLNs contained ger-
minal centers larger than those at 5 days with narrower mantle layers
(Figure IE). The interfollicular cortex and the paracortex were less cellu-
lar than those at 5 days. Medullary cords were again distinguishable as
thick anastomosing cords distended with large lymphocytes, plasmablasts,
and plasma cells. Although plasma cells were numerous in medullary
cords, they were rare in sinuses, which contained predominantly large
lymphocytes and plasmablasts.
Twenty-one days after immunization the ipsilateral TBLNs contained
poorly circumscribed germinal centers due to depletion of small lympho-
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February 1980
cytes within the mantle layer and interfollicular cortex (Figure IF). The
paracortex was only slightly more cellular than those of the contralateral
TBLNs and ofuntreated and saline-treated control dogs. Many medullary
cords had returned to thin anastomosing cords populated by small lym-
phocytes and a few plasma cells. Subcapsular, cortical, and medullary si-
nuses were indistinguishable from control sinuses relative to cellularity
and cell types.
Lungs: Histology
The intrapulmonary lymphoid apparatus of untreated and saline-
treated control dogs was poorly developed relative to that reported for
other species.'0''9-22 Lymphoid nodules (bronchus associated lymphoid tis-
sues) were lacking along the large and medium-sized bronchi. Lymphoid
infiltrates were sparse within the lamina propria ofairways. "Lymphoepi-
thelial organs," lymphoid aggregates at the bronchoalveolar junctions,'9'20
were rarely present, and those seen contained prominent deposits of bire-
fringent silicates within macrophages associated with lymphoid aggre-
gates. No saline-induced changes were seen.
Dogs exposed to autologous erythrocytes developed a mild, transient
acute inflammatory response detectable on Day 2 but not on Day 5. The
response was characterized by mild neutrophilic infiltrates in alveolar
spaces accompanied by a slight increase in the number ofalveolar macro-
phages. Many erythrocytes were extracellular within alveolar spaces on
Days 2 and 5, with some erythrophagocytosis by alveolar macrophages.
Scattered extracellular erythrocytes were present on Day 7. Many alveo-
lar macrophages contained Prussian-blue-positive-staining cytoplasmic
granules on Day 7, indicating conversion ofheme iron to hemosiderin. By
Day 21, extracellular erythrocytes were not present. Alveolar macro-
phages with Prussian-blue-positive-staining cytoplasmic granules were
numerous by Day 21. No qualitative or quantitative changes were de-
tected in the intrapulmonary lymphoid apparatus during any of the days
on which the animals were killed.
Two days after exposure to sheep erythrocytes there was a similar but
slightly more intense acute inflammatory response in immunized lobes,
compared with that of dogs receiving autologous erythrocytes. Extra-
cellular erythrocytes were not seen, and erythrophagocytosis was more
frequent within alveolar macrophages of dogs receiving sheep erythro-
cytes than within those of dogs receiving autologous erythrocytes. No
qualitative or quantitative changes in the intrapulmonary lymphoid appa-
ratus were detected on Day 2. Unimmunized lobes were not significantly
different from untreated and saline-treated control lobes.
504 BROWNSTEIN ET AL AmericanJournal
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Five days after immunization with sheep erthrocytes the acute inflam-
matory response had resolved. Numerous venules and small veins within
alveolar septums had perivascular interstitial infiltrates of lymphoid cells
(Figures 2A and B). Most of these lymphoid cells had basophilic, pyro-
ninophilic cytoplasm and central nuclei. A number oflymphoid cells had
characteristics ofplasmablasts with additional features ofeccentric nuclei
and prominent perinuclear clear areas. A few of these interstitial lym-
phoid infiltrates were extending into adjacent alveolar spaces. A promi-
nent feature of this alveolar extension was the association of lymphoid
cells with single or multiple adherent large mononuclear cells. (Figure
2C). A spectrum of changes was seen in these large mononuclear cells. In
some cases they had characteristics of activated pulmonary alveolar mac-
rophages consisting ofoval, vesicular nuclei and abundant foamy, slightly
eosinophilic cytoplasm. There were gradual transitions to bizarre megalo-
cytic mononuclear cells measuring up to 120 IL in diameter (Figures 2D
and 2E). These megalocytes contained large round or oval nuclei with
large single nucleoli and reticulated chromatin patterns. On the basis of
intermediate forms, these megalocytes were interpreted as modified mac-
rophages. The earliest interaction of lymphocytes and plasmablasts with
single or multiple adherent modified macrophages consisted of a rim of
lymphoid cells around these large mononuclear cells. (Figure 2F). These
early mixed mononuclear aggregates were continuous with interstitial in-
filtrates extending into alveolar spaces (Figure 2G). No qualitative or
quantitative changes in other components of the intrapulmonary immune
apparatus were seen on Day 5 or on any subsequent day. Unimmunized
lobes did not differ significantly from untreated and saline-treated control
lobes.
Seven days after antigen exposure, mixed mononuclear aggregates
within alveolar spaces were more cellular and complex (Figure 3A). These
aggregates consisted of pyroninophilic lymphocytes, plasmablasts, and
plasma cells surrounding or admixed with modified alveolar macrophages
similar to those described on Day 5 (Figure 3B). The cytoplasmic mem-
branes of many of these aggregating macrophages were not visible, sug-
gesting syncytium formation. Many plasmablasts and plasma cells were
free within alveolar spaces containing mixed mononuclear aggregates.
Approximately half of these aggregates had no demonstrable continuity
with adjacent interstitial lymphoid infiltrates. Unlike dogs receiving autol-
ogous erythrocytes, no cells contained Prussian-blue-positive-staining
cytoplasmic granules on Day 7.
The unimmunized apical lung lobe at 7 days hadperivenular interstitial
lymphoid infiltrates similar to those seen in immunized lobes on Day 5;
Vol.98, No. 2 LOWER RESPIRATORYTRACT 505
February 1980
however, fewer venules and small veins were affected. Although some ex-
tension into alveolar spaces was observed, there was no evidence of mixed
mononuclear aggregates, nor were any modified alveolar macrophages
seen.
Twenty-one days after immunization, perivenular lymphoid infiltrates
and intra-alveolar mixed mononuclear aggregates had largely resolved.
The few that remained were in a much attenuated form. Macrophages
with Prussian-blue-positive-staining cytoplasmic granules were present,
but, unlike intra-alveolar macrophages in dogs receiving autologous
erythrocytes, these macrophages were concentrated at bronchoalveolar
junctions. Unimmunized lobes did not differ significantly from untreated
or saline-treated control lobes.
Lung Lavage Cytology
The relative numbers of unstimulated lymphocytes, activated lympho-
cytes and plasma cells in the lymphoid population recovered in lavage
fluid at various intervals post immunization are shown in Text-figure 1.
Unstimulated lymphocytes were small, round mononuclear cells measur-
ing 9 ,u in diameter (Figure 3C). Nuclei were round with dense, clumped
chromatin. These nuclei were surrounded by a scant rim ofpoorly stained
cytoplasm. Activated lymphocytes were larger than unstimulated lym-
100
80-
60-
TEXT-FIGURE 1-Effect of intra- 40
pulmonary administration of 101) sheep
erythrocytes on the relative proportion 20T
of unstimulated lymphocytes (circles),
activated lymphocytes (squares), and
plasma cells (triangles) constituting the Ioo/-
lymphoid population lavaged from im- i
munized apical (upper panel) and con- 80_
tralateralapical (lowerpanel) lunglobes.
Vertical bars indicate SEM. Where no
errorbarsappear, theerroriswithin the 60
point.
DAY POST-IMMUNIZATION
506 BROWNSTEIN ET AL AmericanJournal
ofPathology
phocytes with more abundant, more intensely stained cytoplasm (Figure
3D). Nuclei were eccentric. Occasional unstained Golgi regions were seen
adjacent to the nuclei of activated lymphocytes. Fully differentiated
plasma cells (Figure 3E) measured 15-20 ,u in diameter. Nuclear chroma-
tin was more clumped in plasma cells than in unstimulated or activated
lymphocytes. The nuclei were eccentric, with distinct unstained peri-
nuclear halos. The cytoplasm of plasma cells stained intensely. Gradual
transitions between these cell types occurred, and cells were classified ac-
cording to the predominant cytologic features.
Unstimulated lymphocytes comprised over 90% of the bronchoalveolar
lymphoid cells of unimmunized lungs. The remainder of the lymphoid
cells were activated lymphocytes. Only rarely were plasma cells identi-
fied. Three days after immunization there was a large increase in the rela-
tive number of activated lymphocytes in immunized lobes. This increase
in activated lymphocytes peaked at Day 7. The relative number of these
cells gradually declined and was normal by Day 21. Plasma cells began to
appear in large numbers 7 days after antigen exposure, maximum num-
bers occurring 10 days after immunization.
Activated lymphocytes in contralateral control lobes did not begin to
increase until Day 5. The peak occurred 10 days after antigen exposure
and gradually returned to normalby Day 21. Plasma cells began to appear
on Day 10 and were present through Day 14. The difference between the
number of activated lymphocytes in unimmunized lobes and immunized
lobes was highly significant (P< 0.001) on Days 3, 5, and 7 and significant
(P < 0.05) on Day 12 (Student t test of paired samples). The number of
plasma cells was significantly higher in immunized versus unimmunized
lobes on Day 7 (P < 0.001) and Day 10 (P < 0.01).
Discussion
Intrapulmonary immunization ofdogs with sheep erythrocytes resulted
in intraseptal, perivenular infiltrates oflymphoid cells in immunized lung
lobes 5 days after immunization and 3 days after morphologic evidence of
an immune response in ipsilateral tracheobronchial lymph nodes. Pre-
vious studies performed at this laboratory and elsewhere have shown that
the intrapulmonary immunization of dogs with sheep erythrocytes results
in the appearance of specific-antibody-forming cells in lung lobes and
tracheobronchial lymph nodes 4 to 5 days after immunization.2'4'7 There-
fore, histologic evidence of an immune response in the tracheobronchial
lymph nodes preceded the appearance of specific-antibody-forming cells
by 2 days, whereas the appearance of lymphoid infiltrates and specific
Vol. 98, No. 2 LOWER RESPIRATORYTRACT 507
February 1980
antibody-forming cells was simultaneous in the immunized lung lobes.
The most likely explanation ofthese findings is that antibody-forming cells
are produced de novo in the lung-associated lymph nodes but are already
immunologically committed when they first appear in the lung. The dif-
ferentiated nature of the lymphoid infiltrates appearing initially in the
lung (cytoplasmic basophilia and pyroninophilia, perinuclear clear re-
gions, and eccentric nuclei) is consistent with this explanation. Such an in-
terpretation supports the recruitment of circulating immunoblasts by the
lung rather than in situ immune responses as the source of specific-anti-
body-producing cells in the bronchoalveolar air spaces ofdogs after intra-
pulmonary immunization. We cannot rule out the possibility that in situ
responses did occur within the lung between Days 2 and 5, since dogs
were not sampled between these intervals. However, perivenular infil-
trates were scant at day 5, and it is unlikely that changes would be recog-
nizable much earlier.
Bronchus-associated lymphoid tissue, diffuse lymphoid infiltrates of
small caliber airways, and lymphoid aggregates at bronchoalveolar junc-
tions have been proposed as in situ sources of immunocytes in bron-
choalveolar air spaces after intrapulmonary immunization.27,0,223 Un-
treated and saline-treated control dogs in the present study had no
identifiable bronchus-associated lymphoid tissue, scant peripheral lym-
phoid infiltrates, and infrequent bronchoalveolar lymphoid aggregates.
Qualitative or quantitative changes in this sparse intrapulmonary lym-
phoid apparatus were not detected after intrapulmonary immunization
with sheep erythrocytes. It has been proposed that lymphocytes ofthe in-
trapulmonary lymphoid apparatus cross adjacent epithelium to enter the
bronchoalveolar air spaces.'0'22 Such a route would require immunocytes
to re-enter the interstitium to explain the initial appearance of lymphoid
cells around venules and small veins of the alveolar septums.
The presence of attenuated lymphoid infiltrates in the unimmunized
lung lobe in the present study and the appearance of specific-antibody-
forming cells in lower numbers in unimmunized than immunized lobes in
a previous study 7 suggest that B-lymphocyte recruitment in the lung is
partially independent of antigen, although antigen exerts a significant ef-
fect on immunocyte localization within the lung. Tissue and antigen
"homing" are believed to be important in the alimentary localization of
IgA-producing cells originating in Peyer's patches or mesenteric lymph
nodes.241' Although specific tissue localization has not been proven for
cells committed to IgG or IgM synthesis, a recent study suggested this
possibility. Hall et al demonstrated that a high percentage of intraven-
508 BROWNSTEIN ET AL AmericanJournal
ofPathology
ously administered '25I-labeled IgG- and IgM-producing immunoblasts,
derived from sheep peripheral somatic lymph nodes, were found in the
lung.26 IgA-producing cells derived from peripheral intestinal lymph were
found in the gut but did not concentrate in the lung, excluding the possi-
bility of nonspecific pulmonary capillary sequestration. The authors con-
cluded that these circulating IgG- and IgM-producing immunocytes were
the sources ofantibody-forming cells that police the alveolar parenchyma.
The anatomic localization of these immunocytes within the lung was not
determined.
While the initial histologic changes in the lung occurring after immuni-
zation coincided with the appearance of specific-antibody-forming cells
in the bronchoalveolar air spaces, changes in lavaged lymphoid cells pre-
ceded both. Three days after immunization there was a significant rise in
the relative numbers of activated lymphocytes among bronchoalveolar
lymphocytes of immunized lobes. The ipsilateral regional lymph nodes
were potential sources of these cells, since nascent follicular development
was evident at 2 days in the ipsilateral TBLNs. The significant increase in
plasma cells at 7 days in immunized and 10 days in unimmunized lobes
followed the appearance of specific-antibody-forming cells in these lobes
by 2 to 3 days. This may reflect differences in the sensitivity of cytology
versus plaque-forming assays in demonstrating immunocompetent cells or
the ability ofcells classified cytologically as activated lymphocytes to pro-
duce immunoglobulin.
Mixed mononuclear cell aggregates began forming within alveolar
spaces of immunized lobes 5 days after immunization and were well devel-
oped by Day 7. The composition and conformation of these aggregates
strongly suggests an interaction between bronchoalveolar immunocytes
and alveolar macrophages. If the immunocytes are immunologically com-
mitted prior to entry into the lung, then this interaction may represent an
event in the efferent limb of the humoral immune response. Antigen pre-
sentation to immunocompetent cells is a proposed function of alveolar
macrophages that would be manifested in the efferent limb.27
One feature ofthe regional lymph node response differed from previous
studies of histologic node changes after immunization.28 These studies re-
ported that unstimulated lymph nodes contain primary lymphoid follicles
that are transformed into germinal centers during antigenic stimulation.
Primary follicles, in the present study, did not appear until 48 hours after
immunization, germinal centers forming during the subsequent 72 hours.
The reasons for this difference are not known, although species variation
may be responsible, since rodents have been the main subjects ofprevious
studies.