Blood, Immune Response, and Lymphatic System

Blood: Function and Components of Blood

  • Blood is classified as an atypical connective tissue that circulates throughout blood vessels in the body.

  • Composition of blood:

    • Approximately 55% plasma.

    • Approximately 45% formed elements, which include:

      • erythrocytes (red blood cells),

      • leukocytes (white blood cells),

      • platelets.

  • Functions of blood include:

    • Transporting oxygen from the lungs to body cells and bringing carbon dioxide (waste) from cells to the lungs.

    • Transporting nutrients and hormones.

    • Regulating body temperature.

    • Protecting the body from pathogens.

    • Clotting to prevent blood loss at sites of injury.

Blood Cell Production in Red Bone Marrow

  • Blood cells and platelets are produced in red bone marrow, which is a vascular connective tissue that fills the spaces between trabecular in spongy bone.

  • Within the marrow, stem cells known as hemocytoblasts differentiate to form:

    • Red blood cells,

    • White blood cells,

    • Megakaryocytes (precursors to platelets).

  • As red blood cells develop:

    • They produce hemoglobin.

    • They eject their organelles (including nuclei) and enter the bloodstream as biconcave discs specialized for oxygen transport.

Platelet Production in Red Bone Marrow

  • Within red bone marrow, the hormone thrombopoietin causes hemocytoblasts to differentiate into megakaryocytes.

  • Megakaryocytes break off into fragments called platelets, which enter the bloodstream. These are irregular disc-shaped cells with many vesicles but no nucleus.

  • The granules of platelets contain chemicals released at injury sites to promote blood clotting.

Blood Plasma

  • Plasma is the largest component of blood and is a yellowish liquid that is 90% water.

  • Plasma carries suspended blood cells and other substances.

  • Nutrients passing from the digestive system are transported through blood plasma, including:

    • Glucose,

    • Amino acids,

    • Vitamins,

    • Minerals,

    • Fatty acids.

  • Waste products from body tissues are filtered from blood plasma by the kidneys, including:

    • Urea,

    • Uric acid,

    • Creatinine.

  • Plasma transports:

    • Electrolytes,

    • Components of the immune system,

    • Enzymes,

    • Hormones.

  • Plasma maintains homeostasis by releasing heat.

Plasma Protein Production

  • The liver produces plasma proteins and secretes them into the blood.

  • This blood leaves the liver through hepatic veins and returns to the heart to be pumped throughout the body.

  • Plasma proteins are responsible for transporting hormones, fatty acids, and other substances in the bloodstream.

Production of Red Blood Cells (Erythrocytes)

  • Red blood cells (erythrocytes) comprise 40-45% of blood volume and function primarily to transport oxygen from the lungs to body cells.

  • They are produced in the red bone marrow found in bones of the skeletal system (vertebrae, sternum, ribs, pectoral/pelvic girdles).

  • Stem cells, known as hemocytoblasts, give rise to red blood cells during the differentiation process.

    • Hemocytoblasts eject their nucleus to increase oxygen-carrying capability.

  • Mature red blood cells enter the bloodstream via enlarged capillaries called sinusoids.

Erythropoietin and RBC Production

  • Erythropoietin is a hormone produced by cells in the kidneys and liver, which stimulates red blood cell production.

Function of Red Blood Cells

  • Red blood cells deliver oxygen to cells and remove carbon dioxide through internal respiration.

  • In the lungs, oxygen is absorbed into erythrocytes. These cells contain hemoglobin, which binds to oxygen.

  • Oxygenated blood flows from the lungs to the heart, which pumps it throughout the body.

  • At capillaries in body tissues:

    • Red blood cells release oxygen, which diffuses into tissues.

    • Carbon dioxide waste diffuses into the bloodstream and is transported back to the lungs, where it is expelled.

White Blood Cell Histology

Function of White Blood Cells (Leukocytes)

  • White blood cells defend the body against disease.

  • They are categorized as:

    • Granulocytes.

    • Agranulocytes.

  • Together, leukocytes account for about 1% of circulating blood and their numbers rise during infection or inflammation.

Granulocytes

  1. Neutrophils:

    • Consume bacteria through phagocytosis.

    • Make up approximately 70% of all white blood cells.

  2. Eosinophils:

    • Destroy parasites and combat histamine effects.

  3. Basophils:

    • Involved in controlling allergic reactions.

Agranulocytes

  1. Monocytes:

    • Develop into macrophages and remove debris after infection.

  2. Lymphocytes:

    • Second most common leukocytes. They mainly move through lymphatic tissue and briefly through bloodstream.

    • Types of lymphocytes:

      • B cells: Produce antibodies.

      • T cells: Target viruses, fungi, cancer cells, and transplanted cells.

      • Natural killer cells: Attack and destroy foreign microbes.

Neutrophils

  • Granular leukocytes with small, evenly distributed neutral-staining granules containing hydrolytic enzymes or antimicrobial proteins, appearing pale lilac when absorbing acidic dyes.

  • Referred to as polymorphonuclear leukocytes due to their nuclei having 2 to 5 lobes of varying shapes.

  • Predominantly found at sites of inflammation, where they kill and ingest bacteria.

Lymphocytes

  • Agranular leukocytes lacking visible cytoplasmic granules.

  • Each consists of a large, spherical nucleus (dark purple stain) surrounded by a thin layer of pale blue cytoplasm.

  • Include B cells, T cells, and natural killer cells, all crucial for the immune response.

Eosinophils

  • Granular leukocytes with large, uniform-sized cytoplasmic granules containing digestive enzymes, staining red-orange with acidic dyes.

  • Their nuclei consist of two lobes interconnected by nuclear material.

  • Located in loose connective tissue of skin and mucosal surfaces; they attack parasites.

Basophils

  • Granular leukocytes with large coarse granules that contain histamine and heparin, staining purplish-black with basic dyes.

  • Granules can obscure their lobed nuclei (two lobes typically forming an S or U shape).

  • Present at sites of inflammatory allergic reactions, releasing granule contents to dilate blood vessels and attract other white blood cells.

Monocytes

  • Agranular leukocytes lacking visible cytoplasmic granules.

  • Characterized by kidney-shaped nuclei (staining dark purple) surrounded by pale blue cytoplasm containing lysosomes.

  • When they leave the bloodstream and enter tissues, they differentiate into macrophages (large phagocytic cells that defend against viruses, bacteria, parasites, chronic infections).

Function of Platelets

  • Cell fragments called platelets (thrombocytes) make up about 2% of blood.

  • Functions include:

    • Stopping blood loss from damaged vessels by adhering to vessel walls at the injury site.

    • Changing shape to connect with one another and promote blood clotting.

    • Releasing chemicals that activate the coagulation system to form fibrin threads, resulting in platelet plug formation (stopping blood loss).

Locations of Platelet Production

  • Platelets originate in red bone marrow, similar to blood cells.

  • Megakaryocytes in the red bone marrow shed cell fragments that enter circulation as platelets.

Lymphatic System: Overview

  • The lymphatic system includes the thymus, spleen, and a network of lymph vessels, ducts, and nodes, functioning to protect the body by removing pathogens.

  • Lymphatic vessels and capillaries surround tissues in the body.

  • Water and substances exchanged between tissues and bloodstream allow unwanted substances to enter the lymphatic network, which travels towards nodes.

  • Lymph nodes are clustered in:

    • Axillary region,

    • Groin,

    • Neck,

    • Intestinal region.

  • Lymph nodes contain lymphocytes, which support immunity by eliminating pathogens that cause infections.

  • Clean lymphatic fluid returns to the bloodstream through the thoracic cavity.

Lymph Circulation

  • Lymph circulates throughout the body, beginning as interstitial fluid between cells, filtering into lymphatic capillaries and flowing into larger vessels (trunks) before returning to venous blood.

  • Lymph empties into the bloodstream from:

    • Thoracic duct,

    • Right lymphatic duct, at the junction between the subclavian and internal jugular veins.

Thoracic Duct

  • Also known as the left lymphatic duct, it starts at the cisterna chyli and collects lymph from the left upper body and the entire body beneath the ribs.

Right Lymphatic Duct

  • Collects lymph from the right upper body.

Lymph Trunks

  • Major lymphatic vessels emptying into the thoracic and right lymphatic ducts include:

    • Jugular trunks and nodes,

    • Subclavian trunks,

    • Bronchomediastinal trunks,

    • Lumbar trunks,

    • Intestinal trunk and nodes.

Lymphocyte Production

  • B cells mature in red bone marrow (located in spongy bone of vertebrae, sternum, ribs, pectoral/pelvic girdles), then enter the bloodstream before colonizing lymphatic tissues (spleen, lymph nodes) or residing in the bloodstream.

    • T cells develop in the thymus. After maturing, they enter the bloodstream, colonize lymphatic tissues such as the spleen and lymph nodes, with some remaining in the bloodstream until needed.

Spleen

  • The spleen contains macrophages that destroy abnormal blood cells and lymphocytes that carry out immune responses.

  • It also stores iron from blood cells.

Spleen Histology and Structure

  • Surrounded by a dense connective tissue capsule, with connective tissue strands called trabeculae extending into the spleen.

  • Composed of two types of lymphatic tissue:

    • White pulp (contains lymphocytes and macrophages).

    • Red pulp (contains splenic cords made of reticular fibers, red blood cells, macrophages, lymphocytes, plasma cells, and granulocytes).

    • Responsible for immune surveillance of the blood and removal of damaged blood cells and platelets.

Spleen: White Pulp

  • Comprises lymphocytes and macrophages that cluster around small branches of splenic arteries (central arteries).

  • Lymphocytes perform immune functions while macrophages destroy blood-borne pathogens.

Spleen: Red Pulp

  • Consists of splenic cords located between blood-filled venous sinuses.

  • Macrophages in the red pulp clean the blood by removing defective blood cells and debris.

  • The spleen also produces blood cells and stores platelets in red pulp.

Lymph Node Function

  • Lymph nodes filter lymph, containing lymphocytes that destroy pathogens.

Lymph Node Distribution

  • Distributed in areas where limbs meet the torso, notably near the intestines, defending against pathogens entering from extremities or ingested food.

Lymph Node Histology

## Structure

  • Lymph nodes filter lymph before it reenters the bloodstream.

  • Lymph enters through afferent vessels and passes through a network of chambers (trabecular, subcapsular, cortical, and medullary sinuses), where macrophages and lymphocytes neutralize or destroy pathogens.

  • Filtered lymph leaves through efferent vessels.

Cortex and Medulla

  • Surrounded by dense connective tissue capsule; trabeculae extend into the node.

  • Contains a reticular fiber network supporting an outer cortex and inner medulla:

    • The outer cortex has densely packed clusters of B cells (lymphatic nodules/follicles) that activate when antigens are detected, forming germinal centers.

    • Germinal centers are sites for activated B cells, follicular dendritic cells, and macrophages, leading to antibody-producing plasma cells and memory B cells.

    • Inner cortex contains T cells and dendritic cells that divide when antigens are present, forming new T cells that migrate to destroy antigens in the body.

Lymph Node Medulla

  • Located interior to the cortex, lymphatic tissue from cortex extends into the medulla, forming medullary cords that host antibody-producing plasma cells, macrophages, and B cells.

  • Medullary cords are separated by large lymphatic capillaries (medullary sinuses) encircled by a network of reticular fibers with embedded macrophages; lymph passing through these sinuses allows the ingestion of antigens by the macrophages.

Physical Defenses

  • Skin and mucous membranes provide a physical barrier to pathogens.

  • Additional mechanisms include:

    • Tears,

    • Saliva,

    • Urine,

    • Gastric juices - aid in the destruction and removal of microbes.

Innate Immunity

  • Provides a rapid, general defensive response against invading pathogens.

  • Fever elevates body temperature to accelerate the immune response following infection.

  • Inflammation at injury sites is caused by dilated blood vessels, allowing white blood cells to exit the bloodstream and enter tissues, some of which are phagocytes consuming invading microbes.

  • The innate immune response features physical defenses, antimicrobial substances, fever, inflammation, and phagocytes that consume pathogens.

Phagocytosis

  • Involves the consumption of bacteria or other pathogens by certain white blood cells (phagocytes), protecting the body from infection.

  • The process starts when phagocytes are attracted to pathogen chemicals.

  • Phagocytes attach, engulf the microbe in a vacuole, and digest it, releasing indigestible remains from the cell.

Adaptive Immunity

  • Adaptive immunity enables the body to fight unknown invaders via antibody or T cell-mediated responses.

  • Upon recognition of a specific pathogen, a B cell activates, dividing to form plasma cells and memory cells.

  • Plasma cells release antibodies neutralizing pathogens; memory cells persist in the body for future responses.

  • T cells, during cell-mediated immunity, seek antigens on infected cells, bind, and destroy them, with some T cells becoming memory cells.

  • The adaptive immune response is a targeted action where B and T lymphocytes neutralize invading microbes within the lymphatic system and bloodstream.

B Cells

  • B cells develop and mature in red bone marrow before becoming active in the spleen, lymph nodes, and bloodstream.

  • They produce antibodies which recognize antigens on foreign microbes, tagging these invaders for future destruction.

T Cells

  • T cells develop in red bone marrow and mature in the thymus before becoming active in the spleen, lymph nodes, and bloodstream.

  • Upon activation by antigens (substances on foreign microbes), they seek out and destroy infected cells.