Lymphatic & Immune System — revision sheet

A one-way network of lymphatic vessels returns about 3 L of filtered fluid and leaked plasma protein to the blood every day, absorbs dietary fat, and carries lymph past some 450–600 lymph nodes. Lymphoid organs and circulating leukocytes form the immune system: an innate first line that reacts within minutes, and an adaptive line of T and B lymphocytes that recognises specific antigens and remembers them.

Lymph returned to blood: ≈ 2–4 L/day (classic figure ≈ 3 L/day; resting flow ≈ 120 mL/h)Number of lymph nodes: ≈ 450–600 (estimates vary widely; most lie in the abdomen, pelvis and neck)Thoracic duct: ≈ 38–45 cm long, ≈ 2–5 mm wide; drains ≈ ¾ of the bodyTotal leukocyte count (adult): 4.0–11.0 × 10⁹/LDifferential (approx.): Neutrophils 40–75%, lymphocytes 20–45%, monocytes 2–10%, eosinophils 1–6%, basophils < 1%Blood lymphocyte subsets: T cells ≈ 70–80%, B cells ≈ 10–15%, NK cells ≈ 5–15%; CD4:CD8 ≈ 2:1CD4 T-cell count: ≈ 500–1500 cells/µL (AIDS-defining < 200/µL)Spleen: ≈ 150 g (80–300 g), ≈ 11–12 cm long; ribs 9–11; holds ≈ ⅓ of the platelet poolThymus: Largest relative to body size in infancy; ≈ 30–40 g at puberty, then fatty involutionSerum immunoglobulins: IgG ≈ 75–80%, IgA ≈ 10–15%, IgM ≈ 5–10%, IgD < 1%, IgE traceIgG half-life: ≈ 21–23 days (longest of all isotypes)Blood cells produced: ≈ 5 × 10¹¹ per day by red bone marrow

Lymphatic vessels & ducts

Lymph Fluid: The clear, straw-coloured fluid inside lymphatic vessels: interstitial fluid that has entered lymphatic capillaries, carrying leaked plasma protein, lymphocytes, antigen and — from the gut — fat as milky chyle.
Lymphatic capillaries Lymphatic vessel: Blind-ended, highly permeable vessels that begin the lymphatic system in the interstitium; their overlapping endothelial flaps act as one-way valves letting fluid, protein, cells and microbes in.
Lymphatic vessels Lymphatic vessel: The network of valved channels — capillaries, collecting vessels, trunks and ducts — that carries lymph one way from the tissues, through lymph nodes, to the great veins of the neck.
Lymphatic trunks Lymphatic vessel: Nine large lymph vessels formed by the efferent vessels of the most proximal node groups; each drains a major body region into the thoracic duct, the right lymphatic duct or directly into the venous angles.
Cisterna chyli Lymphatic vessel: A thin-walled, dilated lymph sac on the posterior abdominal wall at about L1–L2 that receives the lumbar and intestinal trunks and gives rise to the thoracic duct.
Thoracic duct Duct: The largest lymphatic vessel (≈ 38–45 cm): it rises from the cisterna chyli through the aortic hiatus and posterior mediastinum to empty into the left venous angle, draining about three-quarters of the body.
Right lymphatic duct Duct: A short (≈ 1–2 cm) vessel at the right root of the neck, formed by the right jugular, subclavian and bronchomediastinal trunks, that drains the right upper quadrant of the body into the right venous angle.

Lymph nodes & regional groups

Lymph node Lymphoid organ: Small, bean-shaped, encapsulated secondary lymphoid organs placed along lymphatic vessels that filter lymph and provide the meeting place where antigen, dendritic cells and lymphocytes interact to start adaptive immune responses.
Lymphoid follicle and germinal centre Tissue: Spherical B-cell aggregates found in all secondary lymphoid tissues; after antigen stimulation they develop a pale germinal centre where B cells proliferate, mutate their antibody genes and are selected for high affinity.
Cervical lymph nodes Lymph node group: The lymph nodes of the head and neck: a superficial "collar" at the junction of head and neck and the deep cervical chain along the internal jugular vein, through which all lymph from the head and neck ultimately passes.
Supraclavicular lymph nodes Lymph node group: The lowest deep cervical nodes in the supraclavicular fossa, near the termination of the thoracic and right lymphatic ducts; a palpable supraclavicular node is always abnormal and often malignant.
Axillary lymph nodes Lymph node group: About 20–40 nodes in the axillary fat, arranged in five groups (three surgical levels), that drain the upper limb, most of the breast and the thoracoabdominal wall above the umbilicus.
Thoracic (mediastinal and hilar) lymph nodes Lymph node group: Parietal (parasternal, intercostal, diaphragmatic) and visceral (pulmonary, hilar, tracheobronchial, paratracheal, mediastinal) nodes that drain the thoracic wall and viscera into the bronchomediastinal trunks.
Mesenteric (pre-aortic) lymph nodes Lymph node group: The largest node group in the body: hundreds of nodes in the mesentery and around the coeliac, superior and inferior mesenteric arteries that filter lymph and chyle from the gut before it reaches the intestinal trunk.
Para-aortic (lumbar) lymph nodes Lymph node group: Retroperitoneal nodes around the abdominal aorta and inferior vena cava that drain the paired abdominal organs (kidneys, adrenals, gonads), the posterior abdominal wall and, via the iliac nodes, the pelvis and lower limbs.
Iliac (pelvic) lymph nodes Lymph node group: Nodes along the external, internal and common iliac vessels that drain the lower limb (via the inguinal nodes), pelvic viscera, perineum and gluteal region towards the para-aortic nodes.
Inguinal lymph nodes Lymph node group: Superficial (≈ 10) and deep (1–3) nodes in the femoral triangle that drain the lower limb, the abdominal wall and back below the umbilicus, the gluteal region, the perineum, external genitalia and lower anal canal.
Popliteal lymph nodes Lymph node group: A small group of about 5–7 nodes in the fat of the popliteal fossa that drain the lateral foot, posterolateral calf (small saphenous territory) and deep tissues of the leg and knee.

Primary lymphoid organs

Bone marrow Lymphoid organ: The soft tissue filling the medullary cavities and spaces of spongy bone: red marrow makes all blood cells and B lymphocytes (a primary lymphoid organ), while yellow marrow is mostly fat that can revert to red marrow under demand.
Thymus Lymphoid organ: A bilobed primary lymphoid organ in the anterior superior mediastinum where bone-marrow-derived precursors mature into T lymphocytes and are selected to recognise self-MHC but tolerate self-antigens; it is largest in childhood and involutes after puberty.
Haematopoiesis Physiological process: The formation of all blood cells from self-renewing haematopoietic stem cells, which branch into myeloid and lymphoid lineages under the control of growth factors and cytokines.

Secondary lymphoid organs

Spleen Lymphoid organ: The largest secondary lymphoid organ: a fist-sized, intraperitoneal organ in the left hypochondrium deep to ribs 9–11 that filters blood, removes old red cells, recycles iron and mounts immune responses to blood-borne antigens, especially encapsulated bacteria.
Tonsils (Waldeyer ring) Lymphoid organ: A ring of partially encapsulated lymphoid tissue guarding the entrances to the digestive and respiratory tracts: the pharyngeal tonsil (adenoids), paired tubal tonsils, paired palatine tonsils and the lingual tonsil.
Palatine tonsil Lymphoid organ: Paired almond-shaped masses of lymphoid tissue in the tonsillar fossae of the oropharynx, covered by stratified squamous epithelium that dips into 10–20 deep crypts; the site of tonsillitis and quinsy.
Pharyngeal tonsil (adenoids) Lymphoid organ: A single midline mass of lymphoid tissue in the roof and posterior wall of the nasopharynx; prominent in early childhood, when enlargement can obstruct the nose and the auditory tubes.
Mucosa-associated lymphoid tissue (MALT) Tissue: Unencapsulated lymphoid tissue in the mucosae of the gut, airways and urogenital tract — from scattered lymphocytes to organised follicles such as the tonsils, Peyer patches and appendix — specialised for producing secretory IgA and maintaining tolerance to harmless antigens.
Peyer patches Lymphoid organ: Oval clusters of lymphoid follicles in the mucosa and submucosa of the small intestine, most numerous in the distal ileum on the antimesenteric border, covered by an epithelium containing M cells that sample gut antigens.

Leukocytes — myeloid (innate)

Leukocytes (white blood cells) Cell: The nucleated cells of the immune system that travel in the blood: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes), normally 4.0–11.0 × 10⁹/L in adults.
Neutrophil Cell: The most abundant leukocyte (≈ 40–75%): a short-lived, highly motile phagocyte with a 3–5-lobed nucleus that is first to arrive at sites of acute bacterial infection and kills microbes with oxidants, granule enzymes and extracellular traps.
Eosinophil Cell: A granulocyte with a bilobed nucleus and large orange-red granules (≈ 1–6% of leukocytes) specialised for killing helminth parasites and a major effector of allergic and asthmatic inflammation.
Basophil Cell: The rarest leukocyte (< 1%): a granulocyte whose large dark-purple granules, rich in histamine and heparin, obscure its nucleus; it binds IgE and releases mediators of allergic inflammation.
Mast cell Cell: Long-lived, tissue-resident sentinel cells packed with histamine- and tryptase-containing granules, located near blood vessels and nerves beneath the skin and mucosae; IgE-bound mast cells trigger immediate allergic reactions and anaphylaxis.
Monocyte Cell: The largest blood leukocyte (≈ 2–10%), with a kidney-shaped nucleus and grey-blue cytoplasm, which circulates for 1–3 days before entering tissues to become macrophages or dendritic cells.
Macrophage Cell: Large, long-lived tissue phagocytes that engulf microbes, dead cells and debris, present antigen to T cells, orchestrate inflammation with cytokines and direct tissue repair; they take tissue-specific forms such as Kupffer cells and alveolar macrophages.
Dendritic cell Cell: Professional antigen-presenting cells with long branching processes that capture antigen in the tissues, carry it through afferent lymph to lymph nodes and are uniquely able to activate naive T cells — the bridge between innate and adaptive immunity.

Leukocytes — lymphoid (adaptive)

Lymphocyte Cell: The second most common blood leukocyte (≈ 20–45%): small round cells with a dense nucleus and scant cytoplasm comprising T cells, B cells and NK cells — the cells of adaptive immunity and of innate lymphoid defence.
Natural killer (NK) cell Cell: Innate cytotoxic lymphocytes (CD56⁺, CD16⁺, CD3⁻) that kill virus-infected and tumour cells within hours, without prior sensitisation, by detecting stress ligands and missing MHC class I.
B lymphocyte Cell: Lymphocytes that mature in the bone marrow, recognise intact antigen through a membrane-bound antibody (the B-cell receptor) and, when activated, become antibody-secreting plasma cells and memory B cells; they also present antigen to helper T cells.
Plasma cell Cell: Terminally differentiated B cells specialised as antibody factories, recognisable by an eccentric "clock-face" nucleus, intensely basophilic cytoplasm full of rough ER and a pale perinuclear Golgi zone.
T lymphocyte Cell: Lymphocytes that mature in the thymus and recognise peptide antigens only when displayed on MHC molecules; they orchestrate (CD4 helper), execute (CD8 cytotoxic) and restrain (regulatory) cell-mediated adaptive immunity.
Helper T cell (CD4⁺) Cell: CD4⁺ T cells that recognise peptide on MHC class II and direct the immune response through cytokines and CD40 ligand, differentiating into subsets (Th1, Th2, Th17, Tfh, Treg) tailored to different classes of pathogen.
Cytotoxic T cell (CD8⁺) Cell: CD8⁺ T cells that recognise foreign peptides presented on MHC class I — which every nucleated cell displays — and kill virus-infected, tumour and foreign cells by inducing apoptosis with perforin and granzymes.
Regulatory T cell (Treg) Cell: CD4⁺CD25⁺FOXP3⁺ T cells (≈ 5–10% of CD4 cells) that suppress other immune cells, maintaining tolerance to self, food and commensal antigens and limiting immune-mediated damage.
Memory lymphocytes Cell: Long-lived antigen-experienced B and T cells, left behind after an immune response, that are more numerous, more easily activated and faster-acting than naive cells — the cellular basis of immunological memory and vaccination.

Immune molecules

Antigen Molecule: Any molecule that can be specifically bound by an antibody, B-cell receptor or T-cell receptor; the precise part that is recognised is the epitope, and an antigen able to provoke a response by itself is an immunogen.
Antibody (immunoglobulin) Molecule: Y-shaped glycoproteins made by B cells and plasma cells, built from two heavy and two light chains, whose Fab arms bind antigen and whose Fc stem recruits effector functions; there are five classes — IgG, IgA, IgM, IgE and IgD.
Complement system Molecule: A cascade of more than 30 plasma and membrane proteins, mostly made by the liver, that are activated on microbial surfaces by three pathways (classical, lectin, alternative) converging on C3, and that opsonise, inflame, lyse microbes via the membrane attack complex and clear immune complexes.
MHC molecules (HLA) Molecule: Highly polymorphic cell-surface glycoproteins (HLA in humans) that bind peptide fragments and display them to T cells: class I on all nucleated cells presents to CD8 T cells; class II on antigen-presenting cells presents to CD4 T cells.
Cytokines Molecule: Small secreted proteins — interleukins, interferons, tumour necrosis factors, chemokines, colony-stimulating factors and TGF-β — that carry messages between immune cells and other tissues, controlling inflammation, cell growth, differentiation and migration.
Pattern-recognition receptors Molecule: Germline-encoded receptors of innate immune cells — Toll-like, NOD-like, RIG-I-like and C-type lectin receptors — that detect conserved microbial molecules (PAMPs) and danger signals from damaged cells (DAMPs), triggering inflammation and interferons.

Immune processes & responses

Innate immunity Physiological process: The inborn, rapid (minutes–hours) first line of defence — barriers, phagocytes, NK cells, complement, cytokines and inflammation — which recognises conserved microbial patterns with germline-encoded receptors and responds the same way at every encounter.
Adaptive immunity Physiological process: The specific, slower-starting arm of immunity mediated by T and B lymphocytes, whose clonally distributed receptors recognise virtually any antigen, which expands selected clones and which remembers each encounter.
Inflammation Physiological process: The protective vascular and cellular response of vascularised tissue to infection or injury — redness, heat, swelling, pain and loss of function — that delivers leukocytes and plasma proteins to the site, eliminates the cause and initiates repair.
Antigen processing and presentation Cell process: The breakdown of protein antigens into peptides and their display on MHC molecules — cytosolic proteins on class I to CD8 T cells, endocytosed proteins on class II to CD4 T cells — together with the co-stimulatory signals needed to activate T cells.
Clonal selection and expansion Cell process: The central principle of adaptive immunity: each lymphocyte is born with a single, randomly generated receptor; antigen selects the rare matching cells, which proliferate into a clone of identical effector and memory cells, while self-reactive clones are eliminated.
Primary and secondary immune responses Physiological process: The first encounter with an antigen produces a slow, modest, IgM-dominated primary response; later encounters produce a rapid, larger, longer-lasting, high-affinity IgG-dominated secondary response because of memory lymphocytes.
Vaccination (immunisation) Physiological process: Deliberate exposure to a harmless form of a pathogen's antigens to induce protective antibody and memory without disease — artificial active immunity — which also protects the unvaccinated through herd immunity.
Immunological tolerance Physiological process: Specific unresponsiveness to an antigen — above all to the body's own antigens — established centrally in the thymus and bone marrow and maintained peripherally by anergy, regulatory T cells, deletion and immune privilege; its failure causes autoimmune disease.
Hypersensitivity reactions Physiological process: Excessive or misdirected immune responses that damage the host, classified by Gell and Coombs into type I (IgE-mediated, immediate), type II (antibody against cell-bound antigen), type III (immune complexes) and type IV (T-cell-mediated, delayed).

Mnemonics

Spleen — "1, 3, 5, 7, 9, 11": Spleen measures ≈ 1 × 3 × 5 inches, weighs ≈ 7 oz (≈ 150–200 g) and lies deep to ribs 9–11 on the left.
Leukocyte differential — "Never Let Monkeys Eat Bananas": In order of abundance: Neutrophils (≈ 60%) > Lymphocytes (≈ 30%) > Monocytes (≈ 6%) > Eosinophils (≈ 3%) > Basophils (< 1%) — roughly "60, 30, 6, 3, 1".
Hypersensitivity types — "ACID": Type I Anaphylactic/allergic (IgE, minutes); Type II Cytotoxic (IgG/IgM against cell-bound antigen); Type III Immune-complex; Type IV Delayed (T-cell, 48–72 h).
Immunoglobulin classes — "GAMED": G crosses the placenta (Gestation) and dominates secondary responses; A guards mucosae (Airways, gut) as a secretory dimer; M is the Mammoth pentamer, Made first; E Elicits allergy and fights helminths via mast cells; D is on naive B cells with IgM (Don't know much else).
Cytokines — "Hot T-bone stEAK": IL-1 = hot (fever); IL-2 = stimulates T cells; IL-3 = stimulates bone marrow; IL-4 = IgE; IL-5 = IgA (and eosinophils); IL-6 = aKute-phase proteins (CRP).
MHC "rule of 8": MHC I × CD8 = 8; MHC II × CD4 = 8. Class I (all nucleated cells) presents to CD8 cytotoxic T cells; class II (antigen-presenting cells) presents to CD4 helper T cells.
Encapsulated organisms in asplenia — "SHiN": Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis — the classic killers after splenectomy (also Salmonella, Klebsiella, group B streptococcus).
DiGeorge syndrome — "CATCH-22": Cardiac defects, Abnormal facies, Thymic hypoplasia (T-cell deficiency), Cleft palate, Hypocalcaemia (hypoparathyroidism) — deletion on chromosome 22 (22q11.2).

From the Corpus anatomy atlas — for education only.