Lymphatic System

Structure of lymphatic vessels

thin-walled vessels that carry lymph. They are similar to veins but have thinner walls. They contain one-way valves that prevent lymph from flowing backward. They begin as tiny lymphatic capillaries in tissues.

Capillaries merge into collecting vessels ➡ lymphatic trunks ➡lymphatic ducts.

function of lymphatic vessels

collect excess interstitial fluid from tissues & return it to the blood. Transport lymph, immune cells and other substances to lymph nodes. Help absorb & transport dietary fats from the small intestine

what is lymph

interstitial fluid that has entered a lymphatic vessel. It is clear fluid that is eventually returned to the bloodstream. Its main job is to transport fluid, immune cells & other substances through the lymphatic system.

what does lymph contain ?

water, electrolytes, proteins, WBCs (especially lymphocytes), fats/lipid and cellular waste & foreign particles

How does lymph move through the body?

lymph does not have a heart/pump.

It moves because of:

• skeletal muscle contractions ➡ squeeze lymphatic vessels

• one-way valves ➡prevent backward flow

• respiratory pump ➡pressure changes during breathing help move lymph

• smooth muscle contraction in lymphatic vessel walls

what is the pathway of lymph through the body?

lymphatic capillaries ➡ collecting lymphatic vessels ➡lymphatic trunks ➡lymphatic ducts ➡subclavian veins ➡bloodstream

What are the 2 main lymphatic ducts

right lymphatic duct ➡drains the right upper body

thoracic duct ➡drains the rest of the body

What is lymphoid tissue?

Tissue that contains many lymphocytes & other immune cells. Its main function is to help protect that body from pathogens. Found throughout the lymphatic system & in areas exposed to the outside environment

What is diffuse lymphoid tissue?

lymphocytes scattered throughout connective tissue.

does not form a distinct cluster or structure.

provides quick immune protection in tissues. F

found in various organs, diffuse lymphoid tissue supports the immune response by allowing lymphocytes to interact with pathogens.

What is follicular lymphoid tissue?

Lymphocytes grouped together into small clusters called follicles

often contains many B cells.

Provide a more organized area for immune responses.

What is a lymph node?

A small, bean-shaped lymphoid organ. Filters lymph before it returns to the bloodstream. Contains many lymphocytes & macrophages that help detect and destroy pathogens

what is the basic structure of a lymph node?

outer capsule ➡ surrounds & protects the node

cortex ➡ outer region

Paracortex ➡ middle region

medulla ➡ inner region

lymph enters through afferent vessels & leaves through efferent vessels

What is found in the cortex of a lymph node?

contains lymphatic follicles. mainly associated with B cells.

B cells can become activated when they encounter antigens.

what is found in the paracortex?

mainly contains T cells. Important area for T-cell activation. Located between the cortex and medllua

What is found in the medulla?

Contains medullary cords & medullary sinuses. Contains B cells, plasma cells, and macrophages. Lymph flows through the medullary sinuses before leaving the node.

What are afferent lymphatic vessels ?

Vessels that bring lymph INTO a lymph node. Enter along the outer surface of the node.

What are efferent lymphatic vessels?

Vessels that carry lymph OUT of a lymph node. Leave through the hilum.

What are the functions of lymph nodes?

Filter lymph, trap pathogens & foreign particles, Provide a place for B & T cells to encounter antigens, activate immune responses and macrophages & other immune cells help destroy pathogens.

What are the primary lymphoid organs?

Bone marrow and thymus

These are where lymphocytes develop and mature.

What is the function of bone marrow?

produces blood cells, including lymphocytes.

B cells mature in the bone marrow.

T cells are produced here but mature in the thymus.

What is the function of the thymus?

Site where T cells mature

Helps T cells develop immunocompetence & self-tolerance

most active during childhood and become smaller with age

what are secondary lymphoid organs?

places where mature lymphocytes encounter antigens & become activated.

• lymph nodes

• spleen

• MALT

What does the spleen do?

filter blood rather than lymph

• white pulp ➡ immune defense; contains lymphocytes

• red pulp➡removes old/damaged red blood cells and stores blood

What is MALT?

Mucosa-associated lymphoid tissue.

Located in mucous membranes

Protects area where pathogens can enter the body.

• tonsils

• Peyer’s patches

• appendix

What are physical/surface barriers?

They are the body’s first line of defense that prevent pathogens from entering.

• skin ➡ physical barrier

• mucous membrane ➡ traps pathogens

• mucus ➡ traps microorganisms

• cilia ➡ move mucus & trapped particles out

• Flushing actions ➡ tears, saliva, urine, and coughing help remove pathogens

What are chemical barriers?

Substances that kill, destroy or inhibit pathogens

• lysozyme ➡ breaks down bacterial cell walls

• Defensins ➡ damage pathogen cell membranes

• Stomach acid ➡ destroys many microorganisms

• sebum ➡ creates an enironment that inhibits microbial growth

• Antimicrobial substances in mucus, tears and saliva

What is phagocytosis?

A process where a phagocyte engulfs & destroys a pathogen or foreign particle. Mainly performed by neutrophils & macrophages.

What is chemotaxis?

The phagocyte is attracted toward the pathogen by chemical signals. Step 1 in phagocytosis

What happens during attachment?

The phagocyte binds to the pathogen. This allows the phagocyte to prepare to engulf it.. Step 2 in phagocytosis.

What happens during engulfment?

The phagocyte’s cell membrane surrounds & takes in the pathogen. The pathogen becomes enclosed inside the cell. stage 3 in phagocytosis.

What is phagosome?

A vesicle inside the phagocyte that contains the engulfed pathogen. It forms when the pathogen is taken into the cell. Stage 4 of phagocytosis.

What happens when a lysosome fuses with a phagosome?

The lysosome releases digestive enzymes into the phagosome. Together they form a phagolysosome. Step 5 in phagocytosis

What happens during digestion?

Enzymes break down & destroy the pathogen. The remaining waste can be removed from the cell. step 6 in phagocytosis.

What are natural killer (NK) cells?

NK cells are white blood cells of the innate immune system.

They recognize & destroy virus-infected cells & abnormal cells, including some cancer cells.

They do not need prior exposure to a specific pathogen. They release chemicals that cause the target cell to die.

How are NK cells different from phagocytes?

Phagocytes ➡engulf & digest pathogens

NK cells ➡ kill infected or abnormal body cells without engulfing them.

What is inflammation?

A protective immune response to injury, or harmful substances. It helps contain the damage, destroy pathogens & begin tissue repair.

It causes increased blood flow & brings immune cells to the affected area.

What are the 5 cardinal signs of inflammation?

• Redness ➡increased blood flow

• Heat ➡ increased blood flow/metabolism

• Swelling ➡ fluid moves into the tissue

• Pain ➡ chemicals stimulate nerve endings

• Loss of function ➡ swelling & pain can interfere with normal function

What does histamine do during inflammation?

Causes vasodilation ➡ more blood blow

Increases capillary permeability ➡ fluid & immune cells can leave the blood & enter tissues.

Released mainly by mast cells

What do prostaglandins do during inflammation?

Help cause pain & inflammation. Contribute to vasodilation and can contribute to fever.

What do leukotrienes do during inflammation?

Help attract white blood cells to the site of inflammation. Increase vascular permeability. Can cause bronchoconstriction.

What are cytokines?

Chemical messengers released by immune cells. Help immune cells communicate with each other.

Can promote inflammation & recruit immune cells to an infected or injured area.

What is complement?

A group of proteins in the blood that help destroy pathogens.

• can be activated through 3 pathways.

  • Classical

  • Lectin

  • Alternative

• Major effects include:

  • Opsonization ➡ marks pathogens for phagocytosis

  • Inflammation ➡ attracts/activates immune cells

  • Membrane attack complex (MAC) ➡ creates holes in pathogen membranes

What are antimicrobial substances?

Substances that destroy or inhibit pathogens. They are part of the body’s innate defenses.

What does lysozyme do?

Breaks down bacterial cell walls, helping destroy bacteria. Found in tears, saliva, mucus and other secretions

What does defensins do?

Small antimicrobial proteins. Damage the cell membrane of pathogens, causing them to die.

What does complement do?

Complement proteins help defend against pathogens by:

• Opsonization ➡ marks pathogens for phagocytosis

• Inflammation ➡ recruits/activates immune cells

• MAC➡ creates holes in pathogen membranes

What do interferons do?

Proteins released by virus-infected cells. Help protect nearby cells from viral infection. Help activate immune defenses.

How does stomach acid protect the body?

The stomach’s low pH destroys or inhibits many microorganisms that are swallowed. Acts as chemical barrier against pathogens.

How does a fever happen?

An infection or inflammation causes immune cells to release pyrogens.

• pyrogens act on the hypothalamus, which raises the body’s temperature set point.

• The body then produces & conserves heat until it reaches the new set point

Why can a fever help the body?

A moderately elevated temperature can slow the growth of some pathogens.

• it can increase certain immune system activities.

• It helps the body fight infection.

What is an antigen?

A substance that the immune system recognizes as foreign & can trigger an immune response.

• examples include molecules found on bacteria, viruses, toxins & foreign cells

What is hapten?

A small molecule that is not immunogenic by itself. It can trigger an immune response when it binds to a larger carrier protein.

What is an antigenic determinant (epitope) ?

The specific part of an antigen that an antibody or lymphocyte receptor recognizes and binds to.

• one antigen can have multiple epitopes.

Where do B cells originate?

B cells originate from hematopoietic stem cells in the red bone marrow. They are a type of lymphocyte

Where do B cells mature?

Mature in the red bone marrow. During maturation, they develop immunocompetence & self-tolerance.

What is the function of B cells?

Responsible for humoral (antibody-mediated) immunity

When activated B cells undergo clonal selection/expansion.

They can become:

• Plasma cells ➡ produce antibodies

• Memory B cells ➡provide faster response if the same antigen appears again

Where do T cells originate?

originate from hematopoietic stem cells in the red bone marrow. They are a type of lymphocyte.

Where do T cells mature?

Travel to the thymus to mature.

During maturation, they develop:

• immunocompetence ➡able to recognize antigens

• self-tolerance ➡ do not normally attack the body’s own cells

What is the function of T cells?

Mainly responsible for cell-mediated immunity.

They include

• Helper T cells (CD4) ➡ coordinate/activate other immune cells

• Cytotoxic T cells (CD8) ➡ destroy infected or abnormal body cells

• Regulatory T cells ➡ help control immune responses

• Memory T cells ➡ provide a faster response to a familiar antigen.

What is immunocompetence?

The ability of a lymphocyte to recognize & respond to a specific antigen.

• B & T cells develop immunocompetence during maturation.

What is self-tolerance?

The ability of the immune system to avoid attacking the body’s own cells & tissues.

• During lymphocyte development, cells that strongly recognize self are eliminated or controlled.

What are antigen-presenting cells (APCs) ?

Immune cells that process antigens & display pieces of them on their cell surface.

• They present antigen fragments using MHC (major histocompatibility complex) molecules

• This allows T cells to recognize the antigen and become activated.

How do macrophages acts as APCs?

Engulf pathogens through phagocytosis.

• digest/process the pathogen

• Display antigen fragments on MHC II (major histocompatibility complex)

• Present them to helper T cells (CD4)

How do dendritic cells act as APCs?

• capture & process antigens.

• present antigen fragments on MHC II (major histocompatibility complex)

• Activate Naive T cells

• They are especially important for starting adaptive immune responses.

How do B cells act as APCs?

• B cells can bind a specific antigen

• They take it inside & process it.

• present the antigen fragment on MHC II

• Present it to helper T cells.

• This helps activate the B cell & promote antibody production

What is MCH I?

• Found on almost all nucleated body cells.

• Presents intracellular antigens (things produced inside the cell, such as viral proteins)

• Present antigens to CD8 cytotoxic T cells

What is MHC II?

• found mainly on antigen-presenting cells.

• Present extracellular antigens that were engulfed & processed.

• Presents antigens to CD4 helper T cells.

What is humoral immunity?

• The branch of adaptive immunity involving B cells & antibodies.

• B cells become plasma cells, which produce antibodies.

• Mainly protects against pathogens & toxins in body fluids.

What is B-cell activation?

• A B cell recognizes its specific antigen

• Usually receives additional signals from a Helper T cell

• The B cell becomes activated & begins clonal expansion

What is clonal expansion?

• The selected B cell rapidly divides, producing many genetically identical cells.

• These cells become plasma cells & memory B cells

What is clonal selection?

• An antigen binds to a B cell with a matching receptor.

• That specific B cell is selected for activation

What do plasma cells do?

• produce & secrete large amounts of antibodies.

• They are the antibody-producing cells of humoral immunity

What do memory B cells do?

• Remain in the body after the initial immune response.

• Remember the specific antigen.

• Allow a faster & stronger response if the same antigen returns.

What is the primary immune response?

• the immune response to an antigen the first time it enters the body.

• Takes longer to develop

• produces memory cells

What is the secondary immune response?

• Response when the same antigen is encountered again.

• Faster & stronger because of memory cells.

What is active immunity?

• Your own immune system produces the antibodies

• Creates memory cells

• usually provides longer-lasting protection

What is passive immunity?

• You receive antibodies made by another source.

• Provides immediate protection

• Does not create immune memory

What is naturally acquired active immunity?

You become immune after naturally encountering an antigen, such as becoming infected & developing an immune response

What is artificially acquired active immunity?

• immunity produced after vaccination

• The vaccine exposes the immune system to antigen so it can develop memory.

What is naturally acquired passive immunity ?

• antibodies are naturally transferred from mother to baby

• Examples: antibodies across the placenta & through breast milk

What is artificially acquired passive immunity?

• pre-made antibodies are given to a person

• Provides immediate but temporary protection

What is the basic structure of an antibody?

• antibodies are Y-shaped proteins.

• Made of: 2 heavy chains, 2 light chains

• have 2 antigen-binding sites.

What are heavy chains?

• The two longer protein chains that form the main structure of an antibody

• Help determine the class of antibody

What are light chains?

• The two shorter protein chains.

• Help form the antigen-binding sites.

What are antigen-binding sites?

• located at the tips of the antibody’s Y-shaped arms

• Bind specifically to an epitope on an antigen

What is the variable region?

• The part of the antibody that differs between antibodies.

• Determines which antigen/epitope the antibody can bind

What is the constant region?

• The part of the body that is similar within an antibody class

• Determines many of the antibody’s functions.

What does IgG do?

• Most abundant antibody in blood

• provides long-term protection

• can cross the placenta

• important in secondary immune responses

• IgG is responsible for opsonization, neutralization of toxins, and activating complement.

Memory: IgG= General long-term protection

What does IgA do?

• Found mainly in mucous membranes & secretions

• Found in saliva, tears, mucus & breast milk

• Protects body surfaces from pathogens

• Plays a crucial role in mucosal immunity

• Helps prevent pathogen adherence and invasion.

Memory: IgA= Areas exposed to outside.

What does IgM do?

• Usually the first antibody produced during a primary immune response

• Very effective at activating complement

Memory: IgM = first

What does IgE do?

• Involved in allergic reactions

• Important in defense against parasites

• Activates mast cells & basophils

Memory: IgE = allergies

What does IgD do?

• Found mainly on the surface of B cells

• Functions as a B-cell receptor

• Helps with B-cell activation

Memory: IgD= B-cell receptor

What is antigen processing?

• A cell takes in or produces an antigen, breaks it into smaller pieces, and displays those pieces on MHC molecules

• Allows T cells to recognize the antigen

What is the MHC I pathway?

Antigen inside cell ➡ broken into peptide fragments ➡ loaded onto MHC I ➡ Displayed on cell surface ➡ recognized by CD8 cytotoxic T cell

What is the MHC II pathway?

Antigen outside cell ➡ APC engulfs antigen ➡antigen is digested ➡ Loaded onto MHC II ➡ displayed on cell surface ➡ recognized by CD4 Helper T cell

What is cell-mediated immunity?

• adaptive immunity involving T cells

• T cells directly or indirectly help eliminate infected, abnormal, or foreign cells

What happens during T-cell activation?

A T cell must recognize:

• a specific antigen

• Presented on an MHC molecule

Additional signals activate the T cell, causing it to undergo clonal expansion.

What is T-cell clonal selection ?

• An antigen-MHC complex binds to the T cell with the matching receptor.

• That specific T cell is selected for activation

What is T-cell clonal expansion?

• The activated T cell rapidly divides

• Produces many cells with the same antigen specificity

• These become effector T cells & memory T cells

What do Helper T cells do?

• usually CD4+

• Coordinate immune response

• Activate B cells, cytotoxic T cells, and other immune cells