Lymphatic System

Overview and Main Divisions

The lymphatic system is a network of vessels, nodes, and organs that helps maintain fluid balance and supports immune function. It absorbs excess interstitial fluid and returns it to the bloodstream, while lymphoid organs and tissues play key roles in immunity.

• Lymphatic system: Absorbs fluid from tissues and returns it to the blood.

• Lymphoid organs and tissues: Provide sites for immune cell development and function.

General Organization

• Lymphatic capillaries: Small, blind-ended vessels with flaplike minivalves that allow fluid entry.

• Collecting lymph vessels: Larger vessels that transport lymph toward lymph nodes.

• Lymph nodes: Filter lymph and activate immune responses.

• Lymphatic trunks and ducts: Larger vessels that return lymph to the venous system.

Major Drainage Pathways

• Right lymphatic duct: Drains lymph from the right upper limb, right side of head and thorax.

• Thoracic duct: Drains lymph from the rest of the body.

• Both ducts empty into the junction of the subclavian and jugular veins.

Cells of the Lymphatic System

• Lymphocytes: Main immune cells; include B lymphocytes (produce antibodies) and T lymphocytes (cell-mediated immunity).

• Macrophages: Phagocytic cells that engulf pathogens and debris.

• Dendritic cells: Antigen-presenting cells that activate lymphocytes.

• Reticular cells: Produce the stroma (supporting network) of lymphoid organs.

Lymphoid Organs and Tissues

• Primary lymphoid organs:

  • Thymus: Site of T cell maturation.

  • Red bone marrow: Site of hematopoiesis and B cell maturation.

• Secondary lymphoid organs:

  • Lymph nodes: Filter lymph, activate immune responses.

    • Structure: Cortex, medulla, afferent/efferent vessels, hilum.

    • Groups: Cervical, axillary, inguinal, mesenteric.

  • Spleen: Filters blood; red pulp (RBC recycling), white pulp (immune function).

  • MALT (Mucosa-Associated Lymphoid Tissue):

    • Tonsils: Pharyngeal (adenoid), palatine, lingual; contain tonsillar crypts and germinal centers.

    • Peyer's patches: Lymphoid follicles in the small intestine.

    • Appendix: Contains lymphoid tissue.

Immune System

Overview

The immune system protects the body from pathogens and abnormal cells. It is divided into innate (nonspecific) and adaptive (specific) defenses.

Innate Defenses

• Surface barriers: Physical (skin, mucosa) and chemical (secretions) barriers to entry.

• Internal defenses:

  • Phagocytes: Engulf and digest pathogens (phagocytosis); opsonins enhance phagocytosis.

  • Natural Killer (NK) cells: Destroy infected or abnormal cells.

  • Inflammation: Local response to injury/infection.

    • Signs: Redness, heat, swelling, pain.

    • Process: Chemical release, vasodilation, increased permeability, phagocyte mobilization (leukocytosis, margination, diapedesis, chemotaxis).

  • Antimicrobial proteins: Interferons (antiviral), complement system (enhances immune response).

  • Fever: Systemic response; pyrogens reset hypothalamic thermostat.

Adaptive Defenses

• Antigen: Substance that provokes an immune response; must be immunogenic and reactive; contains antigenic determinants.

• Humoral immunity (B cells):

  • B cell activation: Leads to antibody production.

  • Primary vs secondary response: Faster and stronger upon re-exposure.

  • Active immunity: Body produces antibodies (natural: infection; artificial: vaccination).

  • Passive immunity: Antibodies received from another source (natural: mother to fetus; artificial: injection).

  • Antibody structure: Y-shaped proteins; several types (IgG, IgM, IgA, IgE, IgD).

  • Antibody actions: Neutralization, agglutination, precipitation, complement activation.

• Cellular immunity (T cells):

  • T cell selection: Positive (recognize self-MHC) and negative (do not react strongly to self-antigen) selection.

  • T cell types: CD4 (helper, TH), CD8 (cytotoxic, TC).

  • Activation: Antigen binding, costimulation, proliferation, differentiation.

  • TH action: Activate B cells, TC cells, and macrophages.

  • TC action: Destroy infected cells via perforins and granzymes.

• Antigen Presenting Cells (APCs): Dendritic cells, macrophages, and B cells present antigens to T cells.

Endocrine System

Overview and Function

The endocrine system consists of glands that secrete hormones into the bloodstream to regulate body functions. It works with the nervous system but acts more slowly and has longer-lasting effects.

Hormones

• Hormones vs nervous system: Hormones travel via blood, act on distant targets; nervous system uses electrical signals, acts rapidly.

• Chemical structure:

  • Amino acid-based hormones: Bind to cell surface receptors; use second messengers (e.g., cAMP).

  • Steroid hormones: Lipid-soluble; bind to intracellular receptors and activate genes.

• Hormone function: Change activity within target cells.

• Stimuli for hormone release: Humoral (blood levels), neural (nerve signals), hormonal (other hormones).

• Target cells: Cells with specific receptors for a hormone.

• Hormone interactions: Permissiveness (one hormone enables another), synergism (amplify effects), antagonism (oppose effects).

Endocrine Glands and Hormones

• Hypothalamus: Produces releasing/inhibiting hormones (e.g., CRH).

• Pituitary gland:

  • Anterior pituitary: Releases GH, TSH, ACTH, FSH, LH, PRL (tropic hormones stimulate other glands).

  • Posterior pituitary: Releases oxytocin and ADH.

• Pineal gland: Secretes melatonin (regulates sleep).

• Thyroid gland:

  • Produces TH (T3, T4), calcitonin (from parafollicular cells).

  • Follicular cells surround colloid-filled follicles.

  • Clinical: Myxedema (hypothyroidism), goiter, congenital hypothyroidism, Graves’ disease (hyperthyroidism).

• Parathyroid glands: Secrete parathyroid hormone (PTH) to regulate calcium.

• Adrenal glands:

  • Cortex:

    • Zona glomerulosa: Mineralocorticoids (aldosterone; regulates Na+/K+ balance).

    • Zona fasciculata: Glucocorticoids (cortisol; stress response).

    • Zona reticularis: Gonadocorticoids (androgens).

  • Medulla: Catecholamines (epinephrine, norepinephrine).

• Pancreas:

  • Alpha cells: Glucagon (raises blood glucose).

  • Beta cells: Insulin (lowers blood glucose).

  • Homeostatic regulation of blood glucose; diabetes results from insulin deficiency or resistance.

• Gonads and placenta: Ovaries (estrogens, progesterone), testes (testosterone), placenta (various hormones during pregnancy).

• Other hormone-secreting organs: Heart (ANP), kidneys (erythropoietin), GI tract, etc.

Table: Comparison of Major Endocrine Glands and Their Hormones

Example: Hormonal Regulation of Blood Glucose

• When blood glucose rises, insulin is released to promote glucose uptake by cells and storage as glycogen.

• When blood glucose falls, glucagon is released to stimulate glycogen breakdown and glucose release into blood.

• Imbalance can lead to diabetes mellitus.

Additional info:

• Second messengers such as cAMP are common in amino acid-based hormone signaling:

• Antibody structure: are the five main classes.

• Inflammation process: