Host Defense Mechanisms and Lymphoid Tissues

Vocabulary-style flashcards covering key terms related to host defenses, lymphoid tissues, and immune system organization from the lecture notes.

Natural Barriers

Non-specific physical and chemical barriers (e.g., skin, mucous membranes) that protect against infection.

Features of a Normal Thymus

only primary lymphoid organ before birth; only makes T cells; has a cortex, medulla, and Hassell’s Corpuscles; no B cell nodules; no reticular fibers

Inborn Host Defenses

Natural body components that block infectious agents; first line of defense, not organism-specific; physical, chemical and genetic barriers

Acquired Immunity

Specific immunity developed uniquely for each organism, may be active (infection or vaccines) or artificial (immunoglobulins/antibodies); has memory

Nonspecific Defense

first line of defense against pathogens; skin, mucous membranes, respiratory tract, mouth, GI tract, UG tract; mechanical barriers, chemical barriers, enzymes, and flushing actions

Natural Barriers- Skin

most difficult barrier to penetrate; epidermis contains keratin which repels water; dermis contains woven connective tissue that prevents spread of infection; tough, dry, salty, oily, thick; sweat glands secrete salt and fatty acids

Natural Barriers- Mucous Membranes

Secretions with antimicrobial properties (IgG, secretory IgA) that prevent pathogen attachment and spread.

Mucociliary Escalator

Ciliated epithelium and mucus that move organisms out of airways, aided by secretory IgA.

Natural Barriers- Respiratory Tract

turbulent air flow in the nose throws dust, pollen, and microbes onto mucous lining; microbes then follow mucociliary elevator; alveolar macrophages and tissue histeocytes will phagocytize if needed

Natural Barriers- Gastrointestinal Tract

acidic stomach pH can kill microbes; antibacterial pancreatic enzymes, bile, and intestinal secretions; peristalsis removes organisms; normal bowel flora can inhibit pathogens

Natural Barriers- Genitourinary Tract

longer male urethra; kidneys produce mucoproteins which bind to bacteria and cause excretion; vaginal acidic pH (due to lactobacillus) inhibits growth of bacteria and candida; continuous discharge from vagina expels microbes

Natural Barriers- Chemicals

various bodily secretions kill organisms; lysozymes, peroxidase, lactoferrin, defensins; acidic skin, stomach, vagina pH; mucous secretions from epithelium; sebaceous secretions coat skin and hair to impeded pathogen entry

Alveolar Macrophages

Phagocytes in the alveoli that ingest and digest inhaled pathogens.

Lysozyme

Enzyme that degrades peptidoglycans; found in tears, saliva, blood, and phagocytes.

Peroxidase

Enzyme that breaks down hydrogen peroxide to produce reactive oxygen; found in saliva, body tissues, and phagocytes

Lactoferrin

Protein that sequesters iron from microorganisms, limiting their growth; found in saliva, blood, tissue fluids, and some phagocytes

Defensins

Antimicrobial peptides inserted into microbial membranes; found on mucous membranes and in phagocytes.

Lymphatic System

Drains extracellular fluid (lymph) and returns it to blood; travels from tissues to to heart; moves lymph through skeletal muscle contractions; have valves; surveillance, recognition, and protection against foreign material

Interstitial Fluid

extracellular fluid; returned to circulation by lymphatics; contains oxygen, glucose, amino acids, and other nutrients

Lymph Capillaries

Smallest lymphatic vessels that absorb interstitial fluid to form lymph.

Lymph Nodes

Aggregations of lymphoid tissue; encapsulated immune filters where lymph is tested for antigens; stroma contains reticular fibers and reticular cells (antigen trapping)

Lymphatic Fluid

plasmalike liquid carried by lymphatic circulation; formed when blood components enter extracellular space; made of water, dissolved salts, and proteins; transports WBCs, fast, cell debris, and infectious agents

Lymphatics & Cancer

lymph channels are throughout the body, so they can carry cancer cells around the body (metastasis); intervening lymph nodes can trap cancer cells; cancer cells remain in lymph nodes and can spread

Lymphoid System

protective and immunologic; source of immunocompetent cells that react and neutralize foreign substances; lymphocytes, plasma cells, and macrophages directly neutralize antigens

Lymphoid System Development

originates from mesenchyme derived yolk sac islands in embryonic development; vasculature causes multipotent cells to migrate around the body (liver and spleen); cells proliferate and differentiate along leukocytic lines; bone marrow becomes primary source of lymphoid stem cells

Lymphoid Tissue

parenchyma of lymphoid tissue and loose connective tissue throughout the body; lymphoid cells are predominant cells; usually diffuse or nodular densities

Nodular Lymphatic Tissue

B cell zones; denser and more organized lymphoid tissue

Diffuse Lymphatic Tissue

T CELLS; loose aggregates of lymphoid tissue

Nodule

B CELLS; consists of dense lymphoid tissue with a light central region (germinal center) and a darker and peripheral surrounding cap

Primary Lymphoid Organs

Organs where lymphocytes develop; thymus and bone marrow (and Bursa Equivalent in some contexts).

Bursa of Fabricus

Primary lymphoid organ; derived from cloaca of birds; site of B cell development in mammals is the Bursa Equivalent

Bursa Equivalent

primary lymphoid organ before birth, secondary after birth; includes GALT, tonsils, peyers patches, and appendix; equivalent to Bursa of Fabricius

Thymus

Primary lymphoid organ where T lymphocytes mature; derived from embryonic foregut; only diffuse

Thymic Involution

physiological atrophy; occurs at puberty; mediated by adrenocortical and sex hormones

Thymus Structure

surrounded by capsule with trabeculae separating thymus into lobules; lobule is separated into cortex and central medulla; cortex is aggregated T lymphocytes; medulla has fewer lymphocytes and more reticuloendothelial cells

Secondary (Peripheral) Lymphoid Organs

Sites where immune responses are initiated: lymph nodes, spleen, and GALT (tonsils, appendix, Peyer’s Patches)

GALT (Gut-Associated Lymphoid Tissue)

includes tonsils, Peyer’s patches, and appendix; becomes secondary lymphoid after birth

Primary vs Secondary Tissue

primary: stroma with reticular fibers, parenchyma is diffuse with no nodules

secondary: stroma rich in reticular fibers and reticuloendothelial cells, parenchyma is diffuse AND nodular, diffuse is T cells, nodules is B cells

T Cell Lymphocytes

involved in cell mediated immunity; diffuse tissue; eliminate antigens by releasing lymphokines; activate neutrophils, monocytes, and macrophages

B Cell Lymphocytes

form plasma cells and secrete antibodies; nodules; involved in humoral immunity

Germinal Center

Site within lymphoid nodules where B cells proliferate and mutate during an immune response; contain lymphocytes and lymphoblasts

Hassall’s Corpuscles

Thymic medullary structures whose function is not fully known but may relate to T-cell maturation/death; more of them with older ages

Hilus

concave aspect of lymph node made of connective tissue; nodal blood supply and efferent lymphatics located here

Afferent Lymphatics

penetrate lymph node capsule and become confluent with system of sinuses

Lymph Node Cortex

Outer region of a lymph node rich in nodules (B-cell zones) and distinct germinal centers; surrounded by diffuse lymphoid tissue (T cell zones)

Paracortical Region

Middle zone of a lymph node rich in T cells (diffuse lymphoid tissue) between cortex and medulla.

Lymph Node Medulla

Inner portion of a lymph node containing medullary cords and sinuses with macrophages; large number of macrophages

Lymphatic Sinuses

lymph that enters lymph nodes via afferent lymphatics pass through sinuses

Dendritic Cells

phagocytic reticular cells; trap and maintain antigens on plasma membrane; removes 99% of impurities

White Pulp

splenic pulp; made of diffuse and modular lymphoid tissue; organized around arteries of parenchyma; for immunity

Red Pulp

splenic pulp; reflects predominance of RBCs in the blood; erthyrophagocytosis; parenchyma is diffuse lymphoid tissue organized in cords

Periarterioloar Lymphoid Sheath

diffuse lymphoid tissue that forms a cuff around arterial branches; nodular lymphoid tissue extends from sheath; populated by T cells

Marginal Zones

between red and white pulp; receive majority of blood entering spleen; antigen trapping and distribution of incoming cells; large number of macrophages and phagocytes

Spleen

largest lymphoid organ; acts as a blood filter; traps old RBCs and blood borne antigens

Peyer’s Patches

GALT; ileum of small intestines; contain solitary nodules surrounded by diffuse lymphoid tissue

Tonsils

GALT; secondary lymphoid organ; lined with stratified squamous epithelium; deep crypts; has B cell nodules AND T cell zones; reticular fibers house lymphoid tissue

Appendix

GALT; blind evagination of cecum at proximal end of colon; lamina propria is lymphoid tissue

Erythroid Cell Destruction

globin disassembled into amino acids; iron transported to bone marrow by transferring; heme converted to bilirubin and excreted by liver in bile

LPS

Component of Gram-negative bacteria; potent activator of macrophages and inflammatory cytokine production; stimulus for TNF alpha and interleukins

TNF-α (Tumor Necrosis Factor-Alpha)

Inflammatory cytokine produced by macrophages and other cells; contributes to fever, vascular permeability, and septic shock.

Interleukins

stimulated by LPS binding to LPS binding protein; modulate endothelial cell-leukocyte adhesion, leukocyte recruitment, and other immune functions

IL-1, 6, 8, and 12

Macrophages

Phagocytic cells that engulf pathogens, present antigens, and secrete cytokines.

Reticular Fibers

Fibers forming a supportive meshwork in SECONDARY lymphoid organs, supporting reticuloendothelial cells.

Reticuloendothelial Cells

Phagocytic cells (including macrophages) that trap and process antigens in lymphoid tissues.

Lactobacillus

maintains acidic vaginal pH; prevents overgrowth of other bacteria and Candida

Lymphatic Vessel & Vein Commonalities

have valves; dependent on skeletal muscle contraction; look the same

Parts of a Normal Lymph Node

afferent arteriole; efferent arteriole; T cell zone;B cell nodules; cortex and medulla

no marginal zones or corpuscles

Organs Producing Lymphoid Cells

before birth: yolk sac mesenchyme, liver, spleen, bone marrow

after birth: bone marrow