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Chapter 21: The Lymphatic and Immune Systems

1 Lymphatic system structure and function

1 Components

1 Lymph

The fluid within lymphatic vessels. Blood plasma is filtered by capillary beds and produces interstitial fluid. Interstitial fluid flows back into the lymphatic vessels and is then called lymph.

2 Lymphatic vessels

Vessels transporting lymph.

3 Structures containing lymphatic tissue

There are many organs and tissues that contain lymphatic tissue, which is a term that describes a specialized connective tissue with many lymphocytes present. Remember, lymphocytes are the B and T cells.

4 Red bone marrow

Red bone marrow is considered part of the lymphatic system because all of the immune cells, including lymphocytes, develop here.

2 Functions of the lymphatic system

1 Draining excess interstitial fluid

Excess fluid from around tissue cells flows into the lymphatic system and then back into the blood.

2 Transporting dietary lipids

Lymphatic vessels carry the absorbed lipids and lipid-soluble vitamins from the small intestine.

3 Carrying out immune responses

The lymphatic system is involved in all of the immune responses that are generated in the body. Although many cells are transported in the blood and may reside in tissues, the cells themselves or the chemical messages that they produce eventually pass into the lymphatic system. Specialized tissues within the lymphatic tissue provide environments for interaction of cells and chemical signals.

1 Cell-mediated immune responses

T cells kill foreign or altered self cells (viral infected, cancer) by releasing chemicals or causing the target cells to lyse (break open).

2 Antibody-mediated immune responses

B cells synthesize and sometimes release large proteins called antibodies which can recognize foreign or altered cells, or molecules. They eliminate these cells or molecules through a variety of mechanisms.

3 Lymphatic vessels and lymph circulation

1 Lymphatic capillaries

Tiny vessels located between cells. These are closed at one end (unlike capillaries). The walls of the lymphatic capillaries overlap, which function as valves in keeping the interstitial fluid in the lymphatic capillaries. They join together to form larger lymphatic vessels in a similar way to capillaries and venules.

2 Lymphatic vessels

Similar to veins except have thinner walls and more valves. In superficial tissues, they follow the same route as veins and in deep tissues they follow the routes of arteries.

3 Lymph nodes

At intervals along the lymphatic vessels, lymph flows through lymph nodes, which are essentially masses of B and T cells.

4 Lymph trunks and ducts

Lymphatic vessels merge after lymph nodes into larger vessels called lymphatic trunks. From the main lymphatic trunks, the lymph passes into two main ducts, the left and right lymphatic ducts. The left duct or thoracic duct is the major duct for returning lymph to the blood and dumps it into the venous blood at the junction of the left internal jugular and left subclavian veins. The situation is similar on the left side but less tissue is drained by the left duct.

5 Formation and flow of lymph

About three liters per day of blood plasma leaks into the interstitial spaces of the body tissues. This becomes lymph when it flows into the lymphatic capillaries. The same two pumps move lymph as venous blood.

1 Skeletal muscle pump

Movement of skeletal muscle against the lymphatic vessels and prevention of backflow by valves.

2 Respiratory pump

Pressure changes resulting from inhalation push on the abdominal region, moving lymph along. The valves prevent backflow when muscles relax during exhalation.

4 Lymphatic organs and tissues

1 Primary lymphatic organs

The primary lymphatic organs are where stem cells divide and become competent immune cells i.e. capable of mounting an immune response.

1 Red bone marrow

B cells arise and become competent in the red bone marrow.

2 Thymus

T cells become competent in the thymus.

2 Secondary lymphatic organs and tissues

1 Lymph nodes

Lymph nodes are the sites where B and T cells encounter foreign substances (antigens). In response to this recognition, they develop into effective immune cells that specifically target that antigen.

1 Afferent lymphatic duct

This is the lymphatic duct that brings lymph from the tissues.

2 Efferent lymphatic duct

This is the duct that leaves the lymph node and carries the lymph toward the lymphatic trunks. Activated lymphocytes will travel in the efferent lymphatic duct, eventually reach the circulatory system and travel throughout the body looking for the specific antigen.

2 Spleen

The largest lymphatic tissue in the body. Acts in a similar fashion to lymph nodes: B and T cells recognize antigen and become active here. Also, macrophages destroy blood-borne pathogens and some damaged and dead RBCs by phagocytosis.

3 Lymphatic nodules

For your information only.

2 Development of lymphatic tissues

For your information only.

3 Nonspecific resistance: innate defenses

Nonspecific resistance means the same thing as innate defenses. There are several mechanisms of nonspecific resistance, but they all have two things in common: 1. they are present at birth and don’t change for the life of the individual; and 2. they are available for immediate protection against foreign cells and substances.

Nonspecific resistance does not recognize any specificity of the pathogen. That is, the response to one bacteria will be the same as to another. Descriptions of the mechanisms of nonspecific resistance follow.

1 First line of defense: skin and mucous membranes

The skin and mucous membranes provide a dumb, mechanical barrier against infection. If the surface of the skin is broken through a cut or other trauma, pathogens can enter underlying tissues.

1 Fluids

1 Mucus

Traps microorganisms.

2 Tears

Washes pathogens out of the eyes into the nasal cavity. Contains lysozyme, which breaks down the cell walls of bacteria.

3 Saliva

Washes microbes off teeth.

4 Urine

Flow of urine washes microbes out of the urethra.

5 Sebum

Forms a protective film over the surface of the skin. Contains compounds that inhibit the growth of some bacteria, unfortunately stimulating the growth of others.

6 Gastric juice

pH 2…nuff said.

7 Others

Include vaginal secretions, vomiting, defecation, perspiration. All result in the flow of potential pathogens out of the body.

2 Second line of defenses: internal defenses

1 Antimicrobial proteins

Blood and interstitial fluids contain three types of antimicrobial proteins that inhibit microbial growth.

1 Interferons

Produced by lymphocytes, macrophages, and fibroblasts infected with viruses. Generally anti-viral.

2 Complement system

Found in the blood plasma and on plasma membranes, these proteins become activated in response to an infection and produce holes in bacterial membranes, encourage phagocytosis and contribute to general inflammation.

3 Transferrins

Reduce the amount of iron that is required for bacterial growth.

2 Natural killer cells and phagocytes

1 Natural killer cells

Natural killer cells are a type of lymphocyte that scans tissues looking for altered cells, either because of viral infection or mutations leading to cancer. Binding to these altered cells results in killing of the target cell by producing holes in the plasma membranes.

2 Phagocytes

Phagocytes identify and eat foreign or altered cells or substances.

1 Macrophages and neutrophils

2 Phases of phagocytosis

1 Chemotaxis

2 Adherence

3 Ingestion

4 Digestion

5 Killing

3 Inflammation

Inflammation is a non-specific response to tissue injury, regardless of whether pathogens are present. Many alterations in homeostasis result in initiation of inflammation. In general, inflammation creates an environment that inhibits microbial growth and eliminates any microbes that are present (ideally).

1 Four characteristics

1 Redness

2 Pain

3 Heat

4 Swelling

2 Three stages of inflammation

1 Vasodilation and increased permeability of blood vessels

Dilation of blood vessels provides more blood to the area, bringing anti-microbial chemicals to the area.

Increased permeability allows chemicals and cells to get into interstitial spaces, where they can inhibit the growth of pathogens.

Heat, redness and swelling are all due to vasodilation and increased permeability.

2 Emigration of phagocytes

Within an hour of the start of inflammation, phagocytes enter the tissue, neutrophils first. They die of rapidly and are replaced by macrophages.

Damaged tissue and dead and living phagocytes form pus.

3 Fever

Abnormally high body temperature occurring during infection and inflammation. Fever-causing chemicals are released by macrophages and inhibits the growth of microorganisms.

4 Specific resistance: Immunity

The immune system can also respond specifically to microorganisms by recognizing specific molecules from the microbes. The molecules recognized by the specific immune system are called antigens. There are two fundamental properties of the specific immune system: 1. specificity for the antigen, and 2. memory of the previously encountered antigen.

1 Maturation of T cells and B cells

2 Types of immune responses

Specific immunity involves two major branches, which actually overlap in a significant way.

1 Cell-mediated immune responses

T cells attack foreign cells. Cell-mediated responses tend to be directed against intracellular pathogens, cancer cells and transplants (Note: transplants have no evolutionary significance in that our immune system has only recognized foreign human tissues very recently).

2 Antibody-mediated immune responses

B cells produce antibodies, which bind to and inactivate antigens and the cells from which they're derived. Antibody-mediated immunity tends to work best against extracellular pathogens and antigens that are free in body fluids.

3 Antigens and antigen receptors

1 Chemical nature of antigens

2 Diversity of antigen receptors

4 Major histocompatibility complex antigens

5 Pathways of antigen processing

1 Processing of exogenous antigens

2 Processing of endogenous antigens

6 Cytokines

5 Cell-mediated immunity

1 Activation, proliferation, and differentiation of T cells

2 Types of T cells

1 Helper T cells

2 Cytotoxic T cells

3 Memory T cells

3 Elimination of invaders

4 Immunological surveillance

6 Antibody-mediated immunity

1 Activation, proliferation, and differentiation of B cells

2 Antibodies

1 Antibody structure

2 Antibody actions

3 Role of the complement system in immunity

3 Immunological memory

7 Self-recognition and self-tolerance

8 Stress and immunity

9 Aging and the immune system

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