Anatomy M11

Functions of the Lymphatic System

The lymphatic system produces lymphocytes, transports lymph throughout the body, maintains blood volume by returning excess interstitial fluid to the bloodstream, and helps remove toxins and waste products from tissues. :contentReference[oaicite:0]{index=0}

Types of Lymphocytes

B lymphocytes mature in the bone marrow and produce antibodies (humoral immunity). T lymphocytes mature in the thymus and provide cell-mediated immunity. Natural Killer (NK) cells destroy abnormal or infected cells without prior sensitization.

Order of Lymph Flow

Excess tissue fluid enters lymphatic capillaries → small lymphatic vessels → major lymphatic vessels → lymphatic trunks → lymphatic ducts → venous circulation.

Thoracic Duct vs. Right Lymphatic Duct

The right lymphatic duct drains the right side of the body superior to the diaphragm. The thoracic duct drains the entire body inferior to the diaphragm plus the left side of the body superior to the diaphragm.

Lymph Nodes

Lymph nodes filter and purify lymph before it returns to the venous circulation. They remove pathogens, debris, and abnormal cells while providing a site where immune cells encounter foreign antigens.

Lymph Node Structure

Afferent lymphatic vessels carry lymph into the lymph node. Efferent lymphatic vessels carry filtered lymph away from the node through the hilum, the indented region where blood vessels, nerves, and efferent lymphatic vessels enter or exit the node.

Afferent vs. Efferent Lymphatic Vessels

Afferent vessels bring unfiltered lymph into the lymph node from surrounding tissues, while efferent vessels carry filtered lymph away from the node toward larger lymphatic vessels and eventually the bloodstream.

Hilum

The hilum is the indented region of a lymph node (and many other organs) where blood vessels, nerves, and efferent lymphatic vessels enter or leave the organ.

B Cells vs. T Cells vs. NK Cells

B cells differentiate into plasma cells that produce antibodies and provide humoral immunity. T cells coordinate immune responses or directly destroy infected cells through cell-mediated immunity. NK cells rapidly kill virus-infected and cancerous cells without requiring prior exposure.

Humoral vs. Cell-Mediated Immunity

Humoral immunity is mediated by B cells and antibodies circulating in body fluids, whereas cell-mediated immunity is carried out by T cells that recognize and destroy infected or abnormal cells.Bone Marrow

Hematopoiesis

Hematopoiesis is the process of blood cell formation. Hematopoietic stem cells (HSCs) are multipotent cells capable of self-renewal and differentiation into all formed elements of the blood, including erythrocytes, leukocytes, and platelets.

Bone Marrow Stroma vs. Hematopoietic Tissue

The hematopoietic tissue contains stem cells that generate blood cells, while the stroma consists of supportive connective tissue, blood vessels, and stromal cells that provide the environment necessary for stem cell growth and development.

B-Cell Maturation

B lymphocytes complete their maturation in the bone marrow. As they mature, they migrate toward the center of the marrow before entering the bloodstream and secondary lymphoid organs where they can respond to antigens.

Thymus

The thymus is a primary lymphoid organ located in the anterior mediastinum, superior to the heart. It is where immature T lymphocytes mature and learn to distinguish self from non-self before entering circulation.

Thymus Cortex vs. Medulla

The cortex is densely packed with immature developing T cells undergoing selection, while the medulla contains more mature T cells that have successfully completed development and are preparing to leave the thymus.

Positive & Negative Selection of T Cells

During maturation, T cells undergo positive selection to ensure they can recognize self-MHC molecules and negative selection to eliminate cells that react strongly against the body's own tissues. Together, these processes help prevent autoimmunity.

Thymic Involution

The thymus is largest and most active during childhood. After puberty, it gradually shrinks and is replaced by adipose tissue, although it continues to function at a reduced level throughout adulthood.

Spleen

The spleen is the largest secondary lymphoid organ. Unlike lymph nodes, it filters blood rather than lymph, removes old or damaged red blood cells, stores platelets, recycles iron, and monitors the blood for pathogens.

White Pulp vs. Red Pulp of the Spleen

White pulp contains lymphocytes surrounding blood vessels and is responsible for immune surveillance and activation against blood-borne pathogens. Red pulp contains macrophages and venous sinuses that remove aging red blood cells, recycle iron, and store platelets.

Functions of the Spleen

The spleen filters blood, removes worn-out erythrocytes, stores platelets, recycles iron from hemoglobin, serves as a site for immune activation, and helps protect the body from blood-borne infections.

Primary Lymphoid Organs Review

Primary lymphoid organs are where lymphocytes develop and mature. Bone marrow produces all blood cells and matures B cells, while the thymus matures T cells before they migrate to secondary lymphoid organs.Innate vs. Adaptive Immunity

Components of Innate Immunity

Innate immunity includes physical barriers (skin and mucous membranes), chemical barriers (lysozyme, stomach acid, antimicrobial secretions), phagocytic cells, natural killer (NK) cells, complement proteins, interferons, inflammation, and fever.

Complement System

The complement system is a cascade of plasma proteins that enhances immune defense by promoting inflammation, coating pathogens for phagocytosis (opsonization), attracting immune cells (chemotaxis), and forming the membrane attack complex (MAC), which lyses pathogens.

Interferons

Interferons are antiviral signaling proteins released by virus-infected cells. They warn neighboring cells, stimulate antiviral defenses, slow viral replication, and enhance the activity of natural killer cells and macrophages.

Inflammation vs. Fever

Inflammation is a localized protective response that increases blood flow, vascular permeability, and immune cell recruitment to injured tissues. Fever is a systemic response that raises body temperature, slowing microbial growth while enhancing immune cell activity.

Natural Killer (NK) Cells

NK cells are innate lymphocytes that recognize and destroy virus-infected and cancerous cells without prior sensitization by releasing perforins and granzymes that trigger apoptosis.

Humoral vs. Cell-Mediated Immunity

Humoral immunity is carried out by B cells and antibodies that target pathogens in body fluids. Cell-mediated immunity is carried out by T cells that coordinate immune responses and directly destroy infected or abnormal cells.

Properties of Adaptive Immunity

The adaptive immune system is characterized by specificity (recognizes unique antigens), diversity (responds to millions of different antigens), memory (responds more rapidly after repeat exposure), and self-tolerance (normally avoids attacking the body's own tissues).

Active vs. Passive Immunity

Active immunity develops when the body produces its own antibodies and memory cells following infection or vaccination, resulting in long-lasting protection. Passive immunity is acquired through antibodies produced by another individual, providing immediate but temporary protection.

Natural vs. Artificial Immunity

Natural active immunity develops after recovering from infection, while artificial active immunity develops after vaccination. Natural passive immunity occurs when maternal antibodies are transferred through the placenta or breast milk, whereas artificial passive immunity results from antibody injections or immune globulin therapy.

Putting the Immune System Together

Pathogens first encounter innate defenses such as skin, mucus, complement, inflammation, interferons, fever, and NK cells. If pathogens persist, adaptive immunity is activated through B cells, T cells, antibodies, and memory cells, providing specific long-term protection against future infections.

Anatomy M11 High-Yield Review

Know the order of lymph flow; thoracic duct vs. right lymphatic duct; afferent vs. efferent lymphatic vessels; bone marrow vs. thymus; red pulp vs. white pulp; innate vs. adaptive immunity; complement; interferons; fever; humoral vs. cell-mediated immunity; and active vs. passive immunity.