Lymphoid System
Function of immune system
- Protection from invasion by foreign organisms
- Protection from abnormal cells (e.g. neoplasia) or substances
Innate immunity
- Inflammatory response
 Occurs at site of infection
 Rapid and nonspecific response
o Responds to certain type of injury
o Responds to features shared by certain type of pathogen
 Mediated by various cell types
o Neutrophils, eosinophils, and macrophages
o Natural killer (NK) and mast cells
Acquired immunity
- Slow, precise response
- Mediated by lymphocytes and lymphoid tissues
 Lymphocytes circulate through blood, lymph, and tissue
o Involved in immunosurveillance
 Monitoring body for the presence of foreign substances, cancers and other
potentially harmful substances (bacteria, viruses, fungi)
o Can circulate for years before activation nucleus to multiply
o Has greater potential than mechanism of innate immunity (leukocytes leave blood for tissue
and remain in tissue for a limited amount of time)
- Primary cells of lymphoid system
 B and T lymphocytes
 Support cells
o Antigen presenting cells (APCs)
o Specialized epithelial cells and stromal cells
 Facilitate immune response to antigen
 Provide appropriate environment in which immune reactions occur
- Specificity
 Each lymphocyte has membrane receptors specific for a specific region (epitope) on a foreign
macromolecule (antigen)
o Antigen
 Molecule that triggers the production of antibody when introduced into body
 Can trigger formation of cells designed to kill foreign/infected cells (tumor cells)
o Epitope
 Composed of a series of amino acids (1-6) and/or a sugar moiety
 Small antigens may have only 1 epitope
 Large antigens may contain many different epitopes
 Although T lymphocytes have receptors designed for a specific epitope, they require assistance
from cells known as antigen presenting cells (APCs)
 Slow response time
o Slow b/c lymphocytes must travel throughout the body to find specific epitopes. Once they
meet, it takes time for the initially-quiescent lymphocytes to become activated and respond
to the threat by proliferating and producing either antibodies or regulatory cytokines
 Reaction takes place in secondary lymphoid organs
o Include lymph nodes, spleen, tonsils and mucosal associated lymphoid tissue (MALT)
- Adaptation
 First interaction with antigen triggers blastogenesis and clonal expansion
o After antigen activation, B and T cells “turn on” nucleus and synthetic mechanisms
 Nucleus becomes larger with increased numbers of polyribosomes and other
organelles associated with protein synthesis and mitosis
 Nuclear chromatin becomes lighter in color (euchromatin)


Many lymphoblasts (large lymphocytes) have prominent nucleolus
In blastogenesis, activated lymphoblasts undergo clonal expansion (proliferation),
producing more lymphocytes with the same specific antigen receptor
o Lymphocytes formed after antigenic stimulation undergo differentiation
 Although lymphocyte nuclei are not active all the time, they have the potential to
be stimulated. When this happens, the nuclear chromatin will change from
heterochromatin to euchromatin. By contrast, granulocytes are terminally
differentiated and do not have the ability to reactivate their nuclei
 Can become memory cells
o Long-lived, small, little cytoplasm, inactive heterochromatin
 Can become effector cells
 Produces plasma cells (antibody factories)
 “Army” of memory cells is primed and ready to react
o Second infection results in faster and stronger response
 This is the basis for vaccination: injecting molecules from an infectious agent
activates lymphocytes and the production of antibodies and memory cells that can
respond quickly to subsequent exposure to that infectious organism. In other
words, animals already will have the machinery for fighting the infection
- Discrimination (self from non-self)
- Memory
Cells of Acquired Immunity
- Lymphocyte development
 Lymphocyte precursors arise from pluripotent stems cells in marrow that commit to becoming
different types of cells, which are not yet immunocompetent
o Commit to becoming B cell, T cell, or NK cell
 Immunocompetence
o Lymphocytes that are able to distinguish self from non-self substances and able to react to
the presence of foreign antigens
- B lymphocytes
 Develop in cloacal bursa (birds)
 Develop in bursa-equivalent organs like bone marrow and GALT of ileum (mammals)
 Involved in humoral immunity
o Production of antibodies (immunoglobulins)
 Antibodies on B cell membrane act as receptors for specific antigen epitopes
 Each naïve B cell has many copies of unique antibody on its surface
- Plasma cells (effector B cells)
 Form from activated B cells
 Primary function is antibody production
 Histological appearance
o Moderate to large amount of cytoplasm
o Eccentric nucleus
o Dark, clumped chromatin
o +/- pale golgi zone
- Antibodies
 Secreted into blood and tissue fluids by plasma cells
 Large, Y-shaped protein
o Binding sites at the two tips of the “Y” specifically recognize and bind to target antigen
 Functions
o Promote attack by phagocytes
o Bind to foreign toxin to promote its clearance from the body
o Binding to pathogen may prevent it from infecting cells via neutralization
o Activation of complement can lead to lysis of pathogen or infected cell
- T lymphocytes
 Develop in the thymus



-
-
-
-
-
~70% of circulating lymphocytes
Long lifespan (years)
Subsets
o Helper T cells
 Activate B cells and promote humoral response
o Suppressor T cells
 Suppress the immune response
o Cytotoxic T cells (effector cells)
 Involved in cell-mediated immunity
 Kill infected or neoplastic cells
 Require antigen to be presented on the surface of APCs
Antigen-presenting cells (APCs)
 Includes several cell types
o B lymphocytes
o Dendritic cells
o Macrophages (e.g. Kupffer cells in liver)
o Epithelioreticular cells (in thymus and cloacal bursa)
 Phagocytose and process antigens
o Break antigens into small peptides, which are returned to the cell surface in such as way
that a specific T lymphocyte will be stimulated
Dendritic cells
 Most efficient APCs
o Only present to T-cells (not B-cells)
 Active in capture, uptake, and processing of antigens
 Generally found in various tissue throughout the body but return to the secondary lymphoid tissue
(e.g. regional lymph node), where they lose the ability to process antigen and gain the capacity to
interact with naïve T lymphocytes
 Not seen in routine H&E stains
Langerhans cells
 Derived from monocyte precursor
 APCs of skin
 Special intra-epidermal dendritic cells
o Interdigitated between keratinocytes
o Mop up invaders that penetrate skin
 Not readily identified in a routine H&E stained histologic section of skin
Macrophages
 Derived from monocyte precursor
 Inefficient at antigen presentation
 Functions
o Lymphocyte activation
 APC function and cytokine secretion
o Inflammation
 Cytokines induce acute phase response, fever, and inflammation
o Tissue reorganization
 Secrete important factors
o Elastases, collagenases, FGF, angiogenesis factors
o Micobicidal activity
o Tumor immunity
 Secrete toxic factors, free radicals, hydrolases, and TNF- a
Stromal cells
 Fixed cells that act as supportive scaffolding for other cells of the immune system
 Reticular cells (mesenchymal, CT)
o Predominant stromal cells of most lymphoid organs (lymph nodes, spleen, MALT)
o Produce type III collagen (reticular fibers)
o Express surface molecules and substances that attract lymphocytes and dendritic cells
Epithelioreticular cells (epithelial)
o Stromal cells of thymus and cloacal bursa
o Have long cytoplasmic processes with desmosomal attachments (stellate appearance)
o No reticular fiber formation
Lymphoid Tissue Organization
- Lymphoid tissues are divided into primary and secondary organs
 Primary lymphoid organs
o Populations of lymphocyte precursors from bone marrow
o Provide specialized environment where lymphocytes proliferate, develop and are educated
 Lymphocytes gain machinery for antigen recognition (unique antigen receptors)
 Self-reactive lymphocytes are eliminated
 T cell education in thymus
 B cell education varies with species
o Bone marrow (primates, rodents)
o Cloacal bursa (birds)
o Peyer’s patches of distal ileum (ruminants, carnivores, pigs)
o Spleen (maybe reptiles)
o Lymphocytes are immunocompetence but naïve (never encountered antigen)
o Organs typically involutes after sexual maturity
 Immunosurveillance
o Antigens are carried to the lymph node by dendritic cells as soluble compounds
o Naïve, immunocompetent lymphocytes “examine” antigens at node
 If do not encounter specific antigen, lymphocytes move on
 If they do encounter antigen, lymphocytes are activated (antigenic stimulation)
o Proliferation of a group of cells with identical antigen receptors (clones)
o Activated B cells will produce plasma cells and memory cells
o Activated T cells can strengthen or suppress B cell response
o Lymphocyte production takes place in lymphoid tissues where antigenic stimulation occurs
 Secondary lymphoid organs
o Populations of immunocompetent lymphocytes that form organs
 Tonsils
 Lymph nodes
 Spleen
 Mucosal associated lymphatic tissue (MALT)
o Peyer’s patches of the ileum (gut-associated lymphoid tissue – GALT),
o Lymphoid tissue of respiratory tract (bronchiolar-associated lymphatic
tissue – BALT)
o Can produce both humoral and cell-mediated responses to antigens
o General architecture
 Diffuse lymphoid tissue portion (interfollicular tissue)
o Unorganized collection of lymphocytes and other free cells
 Lymphoblasts, macrophages, plasma cells
o Primarily T cell zone
o Supported by stroma composed of dendritic cells and CT
o Found in the lamina propria of many organs where it is often transient
o Permanent in lymphatic organs
o After contact with an antigen, activated lymphocytes travel regional lymph
nodes where it undergoes blast transformation and proliferates. Its
progeny return to the lamina propria as effector B and T lymphocytes
 Nodular lymphoid tissue portion (organized)
o Primarily B cell zone
o Always surrounded by diffuse tissue (T cells)

o
o
o
o
o
o
Dispersed dendritic cells
Supported by reticular cells
Primary nodules (follicles)
 Tightly-packed, small lymphocytes
 Primarily naïve B cells
Secondary nodules
 Sign of antigenic stimulation – lymphocytes that recognize an
antigen return to a primary nodule and undergo proliferation
 Germinal center containing lymphoblasts
 Blastogenesis and clonal expansion occur
 Lighter appearance due to euchromatin of blasts
 Mitotic figures common
 Mantle zone (corona)
 Dark, outer ring of naïve lymphocytes that failed to
encounter their antigen
 Usually oriented toward the source of antigen (e.g. the
tonsilar mantle zone is pronounces near the oral cavity)
Encapsulated
 Thymus (primary lymphoid organ)
 Lymph nodes
 Spleen
 Tonsils (partially)
Non-encapsulated
 Other MALT
 GALT
 BALT
Primary Lymphoid Organs
- Thymus
 Primary organ responsible for the education of T lymphocytes
 Architecture
o Epithelioreticular cells form the framework of the organ that is then seeded with
developing lymphocytes from the bone marrow
o Divided into lobes by thin, CT capsule
o Lobes divided into lobules by CT trabeculae
o Capsule and trabeculae contain blood vessels, nerves, and only efferent lymphatics
o Each lobule has 2 parts
 Cortex (outer layer)
o Darkly-stained due to more developing T cells (thymocytes)
 Medulla (inner layer)
o Pale-colored because has fewer thymocytes and more stromal cells
 Vascular supply
o Trabeculae carry blood vessels, nerves, and efferent lymphatics
o Arteries enter at the corticomedullary junction and divide into arterioles that course along
the junction and give rise to a capillary network in the cortex
o Capillary network in cortex forms blood-thymus barrier
 May prevent foreign antigens from entering the thymus in an uncontrolled fashion
o Capillaries drain into postcapillary venules in the medulla
o Postcapillary venules joins veins to carry blood out of the thymus
 T Lymphocyte education
o T cell precursors enter cortex from bloodstream (bone marrow)
o T cells are “tested” twice by epithelioreticular cells in the cortex and medulla
 If pass cortex test, cells migrate from cortex to medulla
 If pass medulla test, cells migrate to 2° lymphoid organs

If fail medulla test, cells apoptose
(~98%) and are phagocytosed by
macrophages
o May fail if bind self-antigens
or are poorly-reactive
 Epithelioreticular cells
o Only found in thymus and cloacal bursa
(presence can help identify organ)
o Sheets of cytoplasm surround and support T
lymphocytes
 Large, light-staining nuclei
o Several types, each with a different function
o Thymic (Hassall’s) corpuscles (type VI cells)
 Closely-packed, flat keratinized cells
Thymic corpuscle
in concentric arrangement
 May produce interleukins that facilitate differentiation and education of T
lymphocytes
 Thymic involution
o After sexual maturity, most of the thymus is replaced with adipose and CT tissue
o Level of involution is species specific
o Residual epithelioreticular cell proliferation can give rise to a thymoma in geriatric adults
 Tumors often contain large numbers of T lymphocytes that are not neoplastic
- Cloacal bursa – “bursa of fabricius” (birds)
 Primary lymphoid organ for maturation and education of B cells in birds
 Blind sac in dorsal wall of cloaca with invaginations of simple, columnar epithelium
 Architecture similar to thymus
o Lobes separated into lobules by surface epithelium (not capsule)
o Lobules separated by trabeculae
 Have both cortex and medulla
 Epithelioreticular cells promote lymphopoiesis (differentiation of B cells)
 Bursa atrophies after about six months
- Peyer’s patches (young ruminants, pigs, and carnivores)
 Large aggregate of submucosal lymphatic nodules in the distal ileum
o Each nodule has cortex and medulla
o Rich in B cells
 Responsible for diversification of pre-immune antigen-receptor repertoire
 Removal of this lymphoid tissue before birth causes immunodeficiency
o Decreased number of mature B cells
 Involutes after sexual maturity
- Bone marrow
 Primary source of B and T cell precursors
 Primary organ for differentiation and education of B cells in primates and rodents
 B-cell precursors start out near endosteum of bone
 Differentiation and selection proceeds as cells migrate toward venous sinuses
Secondary Lymphoid Organs
- Tonsils
 Aggregates of lymphoid nodules at entrance to oropharynx
 Covered various types of epithelium depending on the location
o Palatine (either side of pharynx)
 Non-keratinized stratified squamous
 Surface epithelium is often smooth (in dog)
 Surface epithelium can form deep invaginations (tonsilar crypts) that increase the
SA and maximized antigen-lymphocyte interactions (horses, ruminants, pigs)
 Organ separated by distinct CT capsule
 Lack afferent lymphatics
Lingual (base of tongue)
 Non-keratinized stratified squamous
o Pharyngeal (wall of nasopharynx)
 Pseudostratified ciliated columnar
 Mantle of each nodule is more prominent on side closest to epithelium (on side that faces where the
antigen is coming in. In the case of tonsil, it will face the oral cavity)
 Lymphocytes migrate into epithelium to sample antigen at mucosal surface (dark nuclei here)
Peyer’s patch
 30-50 aggregated nodules along length of intestinal mucosa (digestive tract)
 Most numerous in caudal ileum
 Cause gross elevation in mucosa
 Different function and morphologic appearance compared to the primary organs
o Germinal center of B cells
o Mantle zone facing intestinal mucosa
o Interfollicular area rich in T cells
o Dome area
 Dense region of dendritic cells immediately beneath the epithelium
o Epithelium over dome
 Less well-developed microvilli
 No goblet cells
 Scattered M (microfold) cells
o Modified enterocytes designed for antigen-transport and presentation
 Microfolds rather than microvilli on apical (luminal) surface
 Pinocytose particulates in lumen
 Antigen presented to lymphocytes in pockets on basolateral side
o
-
Primary lymphoid organ
Secondary lymphoid organ
-
Lymph node
 Increased number of lymph nodes found in certain areas of body (e.g. axilla, groin)
 Architecture
o Bean-shaped
o Surrounded by dense CT capsule that sends trabeculae (carries vessels and nn.) into cortex
o Afferent lymphatics pierce capsule
o Efferent lymphatics leave at hilus
o Blood vessels enter and leave at hilus
o Cortex
 Superficial (nodular) cortex (outer)
1° and 2° lymphatic nodules with germinal centers (B cell rich) separated
by diffuse lymphatic tissue (T cell rich)
 Paracortex (deep, inner)
o Diffuse lymphatic tissue (primarily T cells)
o Medulla
 Has distinct cords and sinuses
o Represented by large spaces (will not see this in other lymphoid organs)
Lymph flow
o Afferent lymphatics pierce capsule and drain into subcapsular sinuses, which lead to
trabecular sinuses, which lead to medullary sinuses, which form a network of
anastomosing channels that converge at hilus and open into the efferent lymph vessels
Sinuses
o Lined by flattened endothelial cells that form continuous lining adjacent to CT capsule
o Discontinuous endothelium adjacent to lymphatic parenchyma
 Allows lymphocytes and macrophages to access sinus
 Allows lymph to pass into parenchyma
o Macrophages hunt for antigen
o Reticular cell processes and fibers slow movement of lymph and facilitate interactions
between antigen and cells as well as the phagocytic activities of macrophages
 Metastatic cancer may be trapped in these sinuses and destroyed
 However, large numbers of cancer cells may overwhelm the immune system,
establishing a new metastatic site in the lymph node
Blood and lymph flow
o Arteries enter at hilus, travel through medullary cords, and form capillary beds in cortex
o Capillaries extend into paracortex and they form unique postcapillary venules
 High endothelial venules (HEVs)
o Lined by tall, simple cuboidal endothelium
o Join veins in medullary cords (trabeculae)
o Drain into veins that exit at hilus
o Express receptors for naïve lymphocytes or memory cells
 HEVs bind lymphocytes, inducing them to leave circulation and
migrate into the lymph node
o Most lymphocytes enter node via the HEV of the deep cortex (~90%)
 Only small numbers enter via lymphatics
Lymphocyte traffic pattern
o Lymphocytes percolate through sinuses
o Those entering through HEVs first migrate to superficial cortex
o During this process, they engage in immunosurveillance
Immune response
o B cell response
 Lymphoblasts migrate from germinal center to medullary cords and differentiate
 Differentiated plasma cells secrete antibodies into medullary sinuses
o T cell response
 T cells proliferate and differentiate in paracortex
 Cytotoxic T cells leave via efferent lymphatics
o





-
Spleen
 Functions
o Protection against blood borne infectious agents and other antigens
 Production of lymphocytes
 Filters pathogens and antigens
o Storage of RBCs and platelets (can be very useful in emergency injury and blood loss)
o Destruction of senescent and abnormal RBCs
o Recycles iron
Hematopoiesis
 Especially important in fetus
 Can back up bone marrow
 General architecture
o Capsule (thick CT)
 Amount of smooth muscle varies with species (large amounts in dog and horse)
o Splenic blood flow
 Splenic artery enters at hilus and branches into trabecular arteries (run within
trabeculae CT), which then branch into individual central arteries (really arteriole)
 Central artery sends branches to the lymphatic nodules and marginal sinuses
 The marginal sinus is where blood borne antigens will first contact lymphoid cells
 Some capillaries are surrounded by macrophages and reticular cells
o Sheathed capillaries (ellipsoids), periarterial macrophage sheaths (PAMS)
 Important for clearing blood-borne infections
 Large and abundant in cat and pig spleens
o Parenchyma of spleen is known as “splenic pulp”
 White pulp
o Lymphoid cells that appear “white” upon gross examination
 Histologically stains blue
o Organized to maximize interaction b/w antigens and immune cells
o Nodular and diffuse lymphoid tissue organized around arteries
o Periarteriolar lymphatic sheath (PALS)
 Sleeve of lymphocytes surrounding central artery
 T cells predominate near vessel
 Red pulp
o Bulk of spleen volume
o Organized to promote RBC and macrophage interactions
o Pulp cords, sinuses, and venules
 Network of reticular fibers that slows blood flow
 Variety of cells present
 Reticular cells, RBCs, macrophages, lymphocytes, plasma
cells, other leukocytes
 Capillaries extending into red pulp lack lymphoid sheath
 Marginal zone
o Zone between white and red pulp
o Contains marginal sinus
o Supplied by capillaries from white and red pulp
o Because the spleen lacks HEVs and afferent lymphatics, lymphocytes
migrate into the spleen from capillaries in the marginal zone
Sinusoidal spleen (dogs, primates, rodents)
 Central artery branches into venous sinusoids lined by discontinuous endothelium
 Red pulp contains sinuses lined by longitudinal endothelial cells with bands of basal lamina
 Two mechanisms for removing abnormal RBCs
o Endothelial cells effectively trap defective RBCs and act as a mechanical filter
 Can pinch off RBC membrane defects (RBC pitting)
 Can prevent their re-entry into circulation
o When remove spleen from dogs, often see increased number of defective RBCs
o Macrophages within reticular meshwork red pulp
 Act as a biological filter, recognizing and removing old or damaged RBCs
 Can possibly change from open to closed circulation (depends on situation)
o Terminal capillaries connect directly to sinuses (closed)
o Terminal capillaries empty into and must cross a network of splenic reticular meshwork
before entering sinues (open)
o
-
-
Non-sinusoidal spleen (cat, horse, ox, pigs)
 Pulp venules (nonanastomosing, short, thin-walled) are open-ended vessels that originate in the
reticular meshwork near trabeculae and drained into trabecular veins
 Wider splenic cords that lack splenic sinuses
 Strictly open circulation
 One mechanism for removing abnormal RBCs (macrophages)
o Without endothelial cell barrier, this spleen is less effective at removing abnormal RBCs