MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3b Lymphatic Immune Systems

What this quiz covers

This quiz focuses on 3b Lymphatic Immune Systems, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. B cells are activated only by macrophages in the paracortex, whereas T cells are activated only by follicular dendritic cells in follicles
2. Naive lymphocytes localize using distinct chemokine cues; T cells interact with dendritic cells in T-cell zones, while B cells encounter antigen and Tfh help in follicles (correct answer)
3. T cells require intact antigen binding to surface immunoglobulin, whereas B cells require peptide–MHC I presentation
4. Both B and T cells proliferate in the subcapsular sinus because it is the primary site of clonal expansion

Explanation: This question tests lymph node compartmentalization and lymphocyte activation in the lymphatic and immune systems. Naive B and T cells home to distinct lymph node regions via chemokine gradients: T cells to paracortex for dendritic cell interactions, B cells to follicles for antigen encounter and T follicular helper support. In this experiment, antigen-specific B-cell proliferation in follicles and T-cell proliferation in paracortex reflect these compartmentalized activations. Thus, choice B correctly explains the response via distinct chemokine cues and cellular interactions. Choice D is incorrect because clonal expansion occurs in specific zones, not uniformly in the subcapsular sinus, addressing a misconception about lymph node architecture. For similar questions, identify if compartmentalization drives differential lymphocyte responses. Verify by mapping cell types to their lymph node zones and activation requirements.

Question 2

1. Compression increases venous capillary filtration, forcing tracer into lymph nodes for antibody production
2. Compression enhances lymph propulsion through one-way valves, increasing return of interstitial fluid and macromolecules to the bloodstream (correct answer)
3. Compression increases arterial pressure, which directly drives lymph through lymphatic vessels
4. Compression causes lymph to flow retrograde from the thoracic duct into peripheral tissues, diluting tracer

Explanation: This question tests lymphatic vessel function in fluid and macromolecule transport within the immune and lymphatic systems. Lymphatic vessels propel lymph through intrinsic contractions and external forces like muscle compression, facilitated by one-way valves, to return interstitial fluid and proteins to circulation. In this study, rhythmic compression on the leg enhances clearance of an interstitial fluorescent tracer, mimicking natural propulsion mechanisms. Therefore, choice B accurately describes how compression boosts lymph flow through valves, increasing clearance. Choice C fails as arterial pressure does not directly drive lymph, a misconception confusing vascular and lymphatic systems. To reason through comparable scenarios, assess if interventions enhance lymphatic propulsion rather than altering blood flow. Additionally, differentiate antegrade lymph flow from potential retrograde disruptions.

Question 3

1. Antibody responses to vaccines are independent of lymph nodes, so titers should be normal
2. Reduced CD4+ T cells increase MHC II expression, causing excessive antibody class switching
3. Low CD4+ T cells selectively enhance T-independent responses, producing high-affinity IgG to protein antigens
4. Reduced CD4+ T-cell help impairs germinal center reactions, limiting class switching and affinity maturation to protein antigens (correct answer)

Explanation: This question tests CD4+ T-cell roles in humoral immunity within the lymphatic and immune systems. CD4+ T cells provide help in germinal centers for B-cell class switching and affinity maturation, essential for robust antibody responses to protein antigens. In HIV with low CD4+ counts, impaired T-cell help results in low titers of class-switched antibodies post-vaccination. Therefore, choice D accurately explains reduced germinal center reactions limiting switched antibodies. Choice C fails as low CD4+ impairs, not enhances, responses, a misconception about T-independent compensation. To assess similar immunodeficiencies, evaluate impacts on T-dependent antibody production. Confirm by checking antibody titers to T-dependent versus T-independent antigens.

Question 4

1. Neutrophils are required for immunologic memory formation, explaining persistent viremia in Group 1
2. B cells primarily mediate early containment within hours, whereas neutrophils mediate late clearance by day 10
3. Adaptive humoral responses are critical for long-term clearance, while early containment can be similar due to innate mechanisms (correct answer)
4. Both groups should clear infection equally because innate immunity alone is sufficient for viral elimination by day 10

Explanation: This question tests the interplay between innate and adaptive immune components in viral clearance through the lymphatic and immune systems. Innate immunity provides early containment via cells like neutrophils, while adaptive humoral responses, mediated by B cells, are crucial for long-term viral elimination through antibody production. In this study, both groups show similar early viral loads due to shared innate mechanisms, but Group 1's lack of B cells leads to persistent viremia, unlike Group 2 which clears the virus. Therefore, choice C logically emphasizes the role of adaptive humoral responses in long-term clearance while innate mechanisms handle early containment. Choice A fails as neutrophils contribute to innate, not memory, responses, reflecting a misconception conflating innate and adaptive roles. To approach similar problems, differentiate timelines of innate versus adaptive contributions to pathogen control. Verify by noting if persistent infection correlates with adaptive deficiencies despite intact early responses.

Question 5

1. Loss of splenic filtration reduces clearance of opsonized blood-borne microbes, increasing susceptibility to encapsulated organisms (correct answer)
2. Loss of splenic afferent lymphatics prevents tissue antigens from reaching lymph nodes
3. Splenectomy increases T-cell priming in Peyer patches, diverting immunity away from the bloodstream
4. Removal of the spleen directly decreases bone marrow hematopoiesis, eliminating neutrophil production

Explanation: This question tests the spleen's role in immune filtration and response to blood-borne pathogens in the lymphatic and immune systems. The spleen filters opsonized microbes from blood, particularly encapsulated bacteria, using macrophages in the marginal zone to initiate clearance. Post-splenectomy, reduced filtration increases susceptibility to sepsis from encapsulated bacteria due to impaired clearance. Thus, choice A correctly links loss of splenic filtration to heightened risk. Choice B is incorrect because the spleen lacks afferent lymphatics, a misconception confusing splenic and nodal functions. For similar risks, check if the organ's absence affects blood filtration over lymphatic drainage. Differentiate splenic handling of systemic antigens from lymph node processing of tissue-derived ones.

Question 6

1. Increased naive T-cell entry into lymph nodes through high endothelial venules, accelerating priming
2. Reduced naive T-cell homing to lymph nodes, decreasing the probability of encountering antigen-presenting dendritic cells (correct answer)
3. Normal priming because naive T cells enter lymph nodes primarily via afferent lymphatics
4. Enhanced antibody class switching because L-selectin blockade selectively activates follicular helper T cells

Explanation: This question tests the mechanisms of lymphocyte homing and entry into lymph nodes via the lymphatic and immune systems. L-selectin (CD62L) on naive T cells mediates rolling and adhesion to high endothelial venules, facilitating entry into lymph nodes for antigen encounter with dendritic cells. In this experiment, blocking L-selectin prevents naive T-cell entry into the draining lymph node despite normal antigen arrival, impairing T-cell priming. Thus, choice B correctly predicts reduced naive T-cell homing, decreasing the chance of antigen-specific activation. Choice A is wrong because L-selectin blockade reduces, not increases, T-cell entry, a misconception ignoring its role in homing. For analogous questions, confirm if interventions target adhesion molecules affecting lymphocyte trafficking to secondary lymphoid organs. Additionally, distinguish entry via high endothelial venules from afferent lymphatic routes used by dendritic cells.

Question 7

1. Naive CD8+ T cells bind intact antigen directly in the subcapsular sinus and immediately secrete IgG
2. Dendritic cells that captured antigen in peripheral tissue migrate via afferent lymphatics to present peptide–MHC to naive T cells in the lymph node (correct answer)
3. Antigen enters venous capillaries first and is filtered by the spleen before reaching the draining lymph node
4. Neutrophils in the lymph node generate antigen-specific memory without requiring antigen presentation

Explanation: This question tests understanding of antigen transport and presentation in the lymphatic and immune systems. Antigens from peripheral tissues are captured by dendritic cells, which then migrate through afferent lymphatic vessels to draining lymph nodes to present processed peptides on MHC molecules to naive T cells. In this murine footpad model, the fluorescently labeled bacterial protein injected subcutaneously appears in the popliteal lymph node, first in the subcapsular sinus and then in T-cell zones, consistent with dendritic cell migration and antigen presentation. Therefore, choice B logically follows as it describes dendritic cells capturing antigen peripherally and migrating via afferent lymphatics to present peptide-MHC to naive T cells. Choice A fails because naive CD8+ T cells do not bind intact antigen directly or secrete IgG, a common misconception confusing T-cell and B-cell roles in antigen recognition and effector functions. To reason through similar tasks, verify if the scenario involves dendritic cell-mediated antigen transport to lymph nodes for T-cell priming rather than direct antigen entry via blood. Additionally, distinguish between lymphatic drainage pathways and splenic filtration for blood-borne antigens.

Question 8

1. Adjuvant decreases antigen processing, forcing B cells to recognize peptide–MHC directly
2. Adjuvant enhances innate activation of antigen-presenting cells, increasing costimulatory molecule expression needed for naive T-cell activation (correct answer)
3. Adjuvant bypasses the lymph node by delivering antigen directly into the bloodstream for spleen-only activation
4. Adjuvant causes T cells to proliferate without antigen specificity, eliminating the need for clonal selection

Explanation: This question tests the function of adjuvants in enhancing immune responses through lymphatic and innate immune activation. Adjuvants stimulate pattern-recognition receptors on antigen-presenting cells, boosting costimulatory signals and cytokine production essential for effective naive T-cell activation in lymph nodes. In this vaccine study, the adjuvant activates innate responses, leading to earlier expansion of antigen-specific T cells in draining lymph nodes compared to antigen alone. Therefore, choice B accurately describes how the adjuvant enhances antigen-presenting cell activation and costimulatory molecule expression for T-cell priming. Choice A fails as adjuvants actually promote antigen processing, not decrease it, addressing a misconception that adjuvants bypass standard presentation pathways. To evaluate similar scenarios, check if the adjuvant's role involves innate immune potentiation for adaptive responses rather than direct lymphocyte stimulation. Also, differentiate lymph node-based priming from spleen-exclusive activation in intramuscular vaccinations.

Question 9

1. Loss of lymphatic drainage reduces return of interstitial fluid to the circulation, increasing interstitial volume and causing lymphedema. (correct answer)
2. Removal of lymph nodes decreases arterial hydrostatic pressure in the arm, reducing capillary filtration and causing swelling.
3. Lymph node removal increases plasma albumin synthesis, raising oncotic pressure and drawing excess fluid into the interstitium.
4. Lymphatic vessels primarily transport erythrocytes, so their loss leads to local blood pooling and edema.

Explanation: This question tests understanding of lymphatic system function in maintaining fluid balance and the pathophysiology of lymphedema. The lymphatic system returns approximately 3 liters of interstitial fluid daily to the circulation, preventing fluid accumulation in tissues. Following lymph node dissection, disrupted lymphatic drainage prevents this return, causing interstitial fluid to accumulate and manifest as lymphedema in the affected limb. Option A correctly identifies this mechanism, explaining how loss of lymphatic drainage increases interstitial volume. Option D incorrectly states that lymphatic vessels transport erythrocytes - they actually transport lymph containing proteins, lipids, and white blood cells, not red blood cells which remain in blood vessels. When evaluating fluid balance problems, remember that lymphatics handle interstitial fluid return while blood vessels handle cellular transport, and that lymphedema specifically results from impaired lymphatic drainage, not vascular issues.

Question 10

1. Type I interferon release and macrophage activation are innate responses, whereas naïve CD8+ T-cell proliferation requires antigen presentation by professional APCs. (correct answer)
2. Naïve CD8+ T cells proliferate in response to viral RNA analogs directly because T-cell receptors recognize pathogen-associated molecular patterns.
3. MHC II upregulation on macrophages is most consistent with direct activation of B-cell class switching in the airway epithelium.
4. The absence of CD8+ proliferation without dendritic cells indicates that innate immunity is slower than adaptive immunity in this model.

Explanation: This question tests understanding of innate versus adaptive immune responses and the requirement for antigen presentation in T-cell activation. Viral RNA analogs trigger innate immunity through pattern recognition receptors, leading to type I interferon production and macrophage activation within hours - these are antigen-independent responses. In contrast, naïve CD8+ T cells require specific antigen presentation via MHC I molecules along with costimulation from professional antigen-presenting cells (APCs) like dendritic cells to proliferate. Option A correctly distinguishes these mechanisms, explaining why CD8+ T cells fail to respond without dendritic cells present. Option B incorrectly suggests T-cell receptors recognize PAMPs directly - TCRs only recognize peptide-MHC complexes, not pathogen patterns. To verify answers about immune cell activation, check whether the described mechanism matches the cell type: innate cells respond to PAMPs directly, while T cells require processed antigen presentation via MHC molecules.

Question 11

1. Low thymic output primarily impairs neutrophil oxidative burst, explaining susceptibility to Candida.
2. Normal B-cell numbers ensure normal responses to new protein vaccines because B cells do not require T-cell help for protein antigens.
3. Reduced thymic production of naïve T cells limits helper T-cell availability, impairing robust responses to novel protein antigens. (correct answer)
4. Weak vaccine responses are expected because the thymus is the main site of antibody class switching in humans.

Explanation: This question tests understanding of thymic function in T-cell development and the requirement for T-cell help in antibody responses to protein antigens. The thymus produces naïve T cells, including CD4+ helper T cells essential for B-cell activation in response to T-dependent antigens like proteins. Reduced thymic output limits the available helper T-cell repertoire, impairing B-cell responses to novel protein vaccines despite normal B-cell numbers, as these cells require T-cell-derived signals for optimal activation, class switching, and affinity maturation. Option C correctly identifies this T-cell deficiency as the cause of weak vaccine responses. Option B incorrectly claims B cells don't need T-cell help for protein antigens - protein antigens are classic T-dependent antigens requiring helper T cells, unlike polysaccharides which can activate B cells independently. To verify immunodeficiency patterns, match the defect location (thymus = T cells) with the observed phenotype (poor protein vaccine responses = T-cell help required).

Question 12

1. A faster rise in neutralizing antibodies is consistent with activation of memory B cells generated during prior antigen exposure. (correct answer)
2. The rapid antibody increase is best explained by immediate differentiation of naïve B cells without prior clonal expansion.
3. Shorter symptom duration indicates that innate immunity has antigen-specific memory that improves with vaccination.
4. Vaccination shortens symptoms primarily by increasing lymphatic fluid hydrostatic pressure, preventing viral spread through tissues.

Explanation: This question tests understanding of immunological memory and the anamnestic response following vaccination or prior exposure. Vaccination generates memory B cells that persist long-term and can rapidly differentiate into antibody-secreting plasma cells upon re-exposure to the same antigen, producing higher affinity antibodies more quickly than during primary responses. Group 2's rapid antibody rise within 3 days and shorter symptoms demonstrate this memory response, as memory B cells bypass the lengthy primary response requirements of naïve B-cell activation, clonal expansion, and affinity maturation. Option A correctly identifies memory B-cell activation as the mechanism. Option C incorrectly attributes memory to innate immunity - immunological memory is a defining feature of adaptive immunity mediated by lymphocytes, not innate cells which lack antigen-specific memory. To verify memory responses, look for faster kinetics (days vs weeks), higher antibody titers, and prior antigen exposure history that would generate memory cells.

Question 13

1. Macrophage debris in the node is most consistent with direct drainage of venous blood into lymphatic sinuses.
2. Lymph node enlargement indicates increased erythrocyte destruction, a primary function of lymph nodes during infection.
3. Negative blood cultures occur because lymph nodes prevent arterial blood from reaching the infected tissue.
4. Regional lymph nodes filter lymph from affected tissues, concentrating antigen and supporting local immune cell activation. (correct answer)

Explanation: This question tests understanding of lymph node function in filtering lymph and concentrating immune responses during localized infections. Regional lymph nodes receive lymphatic drainage from infected tissues, carrying antigens, pathogens, and activated immune cells from the infection site. This drainage system allows lymph nodes to act as immunological filters, concentrating antigens and facilitating interactions between antigen-presenting cells and lymphocytes, resulting in local proliferation and the observed lymphadenopathy. Option D correctly describes this filtering and concentration function that supports local immune activation. Option B incorrectly assigns erythrocyte destruction as a primary lymph node function - this occurs in the spleen's red pulp, not lymph nodes which lack the specialized vasculature for filtering blood. When evaluating lymphadenopathy, remember that enlarged nodes indicate active immune responses to antigens draining from their catchment area, explaining why infections cause regional rather than systemic node enlargement.

Question 14

1. Reduced somatic hypermutation and class switching, leading to less affinity-matured IgG despite preserved early IgM production (correct answer)
2. Increased cytotoxic T-cell killing of extracellular bacteria, compensating for reduced antibody responses
3. Enhanced complement activation because germinal centers normally suppress C3 convertase
4. Normal high-affinity IgG because affinity maturation occurs primarily in the thymus

Explanation: This question tests germinal center functions in antibody maturation within lymph nodes of the lymphatic and immune systems. Germinal centers facilitate somatic hypermutation, affinity maturation, and class switching in B cells, driven by T-cell help, leading to high-affinity IgG production. Preventing germinal center formation reduces high-affinity IgG while preserving modest IgM, as seen in the experiment after protein immunization. Therefore, choice A logically predicts impaired maturation and switching. Choice D fails because affinity maturation occurs in germinal centers, not the thymus, addressing a misconception about B-cell processes. To reason through interventions, assess impacts on germinal center-dependent humoral responses. Confirm by evaluating antibody isotypes and affinities post-challenge.

Question 15

1. Secondary exposure relies exclusively on neutrophil phagocytosis, which is faster than lymphocyte responses
2. Memory B and T cells generated during the primary response expand more rapidly upon re-exposure, producing a larger, faster antibody response (correct answer)
3. Booster antigens bypass lymph nodes and directly stimulate hepatocytes to secrete immunoglobulins
4. Primary responses are faster because naive lymphocytes are already fully activated before antigen exposure

Explanation: This question tests primary versus secondary adaptive immune responses in the lymphatic and immune systems. Primary exposure activates naive lymphocytes, leading to slow clonal expansion and antibody production, while secondary exposure stimulates memory cells for faster, amplified responses. The slow rise in antibody titers after primary antigen exposure contrasts with the rapid, higher peak after booster, indicating memory cell involvement. Therefore, choice B logically attributes this to memory B and T cells expanding more rapidly upon re-exposure. Choice D fails as primary responses are slower due to naive cell activation, a misconception reversing response kinetics. To evaluate analogous patterns, compare timelines and magnitudes of primary and secondary responses. Confirm if boosted responses suggest immunological memory rather than innate mechanisms.

Question 16

1. Dendritic cells present antigen on MHC II to naïve CD4+ T cells, promoting clonal expansion and providing help for B-cell class switching (correct answer)
2. Neutrophils present antigen on MHC I to CD8+ T cells, directly inducing cytotoxic killing of infected tissue cells
3. Complement activation suppresses lymphocyte proliferation by preventing IL-2 secretion in the lymph node
4. Antigen drains into venous capillaries and bypasses lymph nodes, so T-cell activation occurs primarily in the spleen

Explanation: This question tests understanding of antigen presentation pathways and T-cell activation in lymph nodes. Extracellular bacterial antigens are taken up by dendritic cells through endocytosis and processed in endosomal compartments, where peptides are loaded onto MHC class II molecules for presentation to CD4+ T cells. The scenario describes classic CD4+ T-cell activation markers (CD25/IL-2 receptor expression and IL-2 secretion), confirming successful antigen presentation via the MHC II pathway. Choice A correctly describes this process, including the downstream effect of CD4+ T-cell help for B-cell class switching. Choice B incorrectly suggests neutrophils present on MHC I to CD8+ T cells, but neutrophils are not professional antigen-presenting cells and MHC I presents intracellular antigens. To identify the correct pathway, remember that extracellular antigens follow the MHC II pathway to activate CD4+ T cells, while intracellular antigens use MHC I for CD8+ T cells.

Question 17

1. Lymphatic vessels deliver antigen directly into arterioles to enhance local perfusion and washout
2. Lymphatic vessels primarily increase hydrostatic pressure in tissues to drive proteins into venous capillaries
3. Lymph nodes secrete albumin into the interstitium to osmotically pull protein antigen back into the limb
4. Lymphatic capillaries absorb excess interstitial fluid and macromolecules and deliver them to lymph nodes for filtration (correct answer)

Explanation: This question tests understanding of lymphatic system function in fluid and protein transport. The lymphatic system serves as a drainage pathway for interstitial fluid and large molecules that cannot easily enter blood capillaries due to size or pressure gradients. The scenario demonstrates classic lymphatic function: the fluorescent protein moves from interstitial space to lymph node via afferent lymphatics within 30 minutes, and interstitial fluorescence decreases over 2 hours as lymphatic drainage continues. Choice D correctly describes this primary function of absorbing excess fluid and macromolecules for filtration in lymph nodes. Choice B incorrectly suggests lymphatics increase hydrostatic pressure, when they actually reduce it by draining fluid. A key principle is that lymphatics provide a low-resistance pathway for proteins and fluid that accumulate in tissues, preventing edema while enabling immune surveillance.

Question 18

1. The mutation blocks antigen drainage into lymphatics, so antibodies cannot form despite the reported findings
2. The mutation prevents somatic hypermutation in germinal centers, explaining delayed neutrophil recruitment
3. The mutation increases complement activation, which suppresses fever and neutrophil recruitment
4. The mutation primarily impairs innate immune activation early, while adaptive immunity can still develop through antigen-specific lymphocyte responses (correct answer)

Explanation: This question tests understanding of innate versus adaptive immunity timing and components. Pattern recognition receptors (PRRs) are crucial for early innate immune responses, detecting pathogen-associated molecular patterns and triggering inflammation, fever, and neutrophil recruitment within hours. Loss of PRR function impairs these early responses but does not prevent later adaptive immunity, which depends on antigen-specific lymphocyte activation. Choice D correctly identifies that the mutation primarily affects early innate immunity while preserving adaptive antibody responses. Choice B incorrectly links the mutation to somatic hypermutation, which occurs in B cells during antibody affinity maturation, not in early neutrophil recruitment. The key principle is that innate and adaptive immunity operate through distinct mechanisms with different timing - PRRs drive rapid innate responses while lymphocytes mediate slower adaptive responses.

Question 19

1. Vaccine Y more strongly engages innate antiviral pathways early, while both vaccines can generate adaptive humoral immunity over time (correct answer)
2. Vaccine X induces a stronger innate response because only nonreplicating antigens can activate interferon signaling
3. Neutralizing IgG by week 3 indicates that NK cells have differentiated into plasma cells in response to both vaccines
4. Type I interferon is produced only by B cells after class switching, explaining the 24-hour response to Vaccine Y

Explanation: This question tests understanding of innate versus adaptive immune responses to different vaccine formulations. Live-attenuated vaccines contain replicating organisms that trigger pattern recognition receptors (PRRs), leading to rapid type I interferon production and NK cell activation within hours - classic innate immune responses. Both vaccine types eventually stimulate B cells to produce neutralizing antibodies through adaptive immunity, which takes weeks to develop. Choice A correctly identifies that Vaccine Y (live-attenuated) more strongly activates innate antiviral pathways early, while both generate adaptive humoral immunity. Choice C incorrectly suggests NK cells differentiate into plasma cells, but these are distinct lineages - NK cells are innate lymphocytes while plasma cells derive from B cells. To distinguish vaccine responses, remember that live vaccines trigger stronger innate immunity through PRR activation, while all effective vaccines ultimately generate adaptive antibody responses.

Question 20

1. Group 2 will show reduced entry of naïve lymphocytes from blood into the node, limiting adaptive immune activation despite antigen arrival (correct answer)
2. Group 2 will show increased naïve lymphocyte entry because HEVs normally prevent lymphocytes from leaving the bloodstream
3. Group 2 will have normal lymphocyte priming because afferent lymphatics, not blood vessels, deliver naïve T cells to nodes
4. Group 2 will have enhanced antibody production because disrupting HEVs increases antigen filtration by macrophages

Explanation: This question tests understanding of lymphocyte trafficking through lymph nodes. High endothelial venules (HEVs) are specialized blood vessels in lymph nodes that express adhesion molecules allowing naïve lymphocytes to exit the bloodstream and enter lymphoid tissue. While antigen arrives via afferent lymphatics, naïve T and B cells enter nodes primarily through HEVs from blood circulation. Disrupting HEV function prevents this crucial lymphocyte entry, severely limiting the pool of naïve cells available for antigen recognition and activation. Choice A correctly identifies that Group 2 will have reduced naïve lymphocyte entry, limiting adaptive immune activation. Choice C incorrectly claims naïve T cells arrive via afferent lymphatics, but these vessels primarily carry antigen and activated cells from tissues. To understand lymph node function, remember that antigen and lymphocytes arrive through different routes - antigen via lymphatics, naïve cells via HEVs.