B AND T LYMPHOCYTES MEDIATERS OF ADAPTIVE IMMUNE

RESPONSE

Dr.Mohammed Sharique Ahmed QuadriAssistant professor Physiology

Al Maarefa College

Objectives

• Describe the different types of lymphocytes

• Discuss the formation and preprocessing of the different types of lymphocytes

• Define clonal selection theory

• Explain the concept of humoral(antibody mediated ) immunity .

• Describe mechanism of action of antibodies

• Emphasis the importance of memory cell ( primary and secondary response)

Immunity • Body’s ability to resist or eliminate potentially

harmful foreign materials or abnormal cells• Immune system activities

– Defends against invading pathogens– Removes “worn-out” cells and tissue damaged by

trauma • Identify and destroy abnormal or mutant cells• Immune surveillance

– Identifies and destroys abnormal or mutant cells that have originated in the body

– Mounts inappropriate immune responses that lead either to allergies or to autoimmune diseases

Leukocytes • Effectors of the immune system• 5 Types

– Neutrophils• Highly mobile phagocytes that engulf and destroy unwanted materials

– Eosinophils• Secrete chemicals that fight parasites • Involved in allergic reactions

– Basophils• Release histamine and heparin• Involved in allergic reactions

– Monocytes • Transformed into macrophages (tissue-bound phagocytic specialists)

– Lymphocytes • Β lymphocytes (β cells)

– Transformed into plasma cells that secrete antibodies• T lymphocytes (T cells)

– Responsible for cell-mediated immunity

Lymphocytes • Mainly produced from lymphoid

colonies in lymphoid tissues• Lymphoid tissues

– Tissues that produce, store, or process lymphocytes

– Include• Bone marrow• Lymph nodes• Spleen• Thymus• Tonsils• Adenoids• Appendix• Peyer’s patches (GAIT)

IMMUNITY• Mechanism by which the body is able to

resist or overcome disease causing micro-organisms.

Broadly there are 2 types - • I. Innate/Non-specific/Natural Immunity • II. Acquired /Specific/Adaptive

Immunity

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I. Innate/Non-specific/Natural Immunity. Provides first line of defense against infections.

• Intact skin & mucous membranes.• Secretions such as tears, saliva, HCl acid.• Phagocytes & Natural Killer Cells. • Complement system,(a group of 20 proteins

activated by immune complexes)

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Immune Responses• Innate immune system

– Nonspecific– Responses work immediately when body is

exposed to threatening agent– Nonselectively defend against foreign invaders– First line of defense– Rapid but limited responses– Neutrophils, macrophages, several plasma

proteins are important in innate defense

Immune Responses• Adaptive or acquired immune system

– Specifically targets foreign material to which body has already been exposed

– Body has taken time to prepare to attack– Ultimate weapon against most pathogens– Responses are mediated by β and T lymphocytes– Formation of memory cells allows system to react

more swiftly against specific invaders in the future

Adaptive Immunity• 2 classes of adaptive immunity

– Antibody-mediated or humoral immunity• Involves production of antibodies by B lymphocyte

derivatives known as plasma cells

– Cell-mediated immunity• Involves production of activated T lymphocytes

– Directly attack unwanted cells

Adaptive ImmunityOrigins of β and T Cells

After early childhood most new lymphocytes are derived from peripheral lymphocyte colonies rather than from bone marrow

Lymphocytes

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Adaptive Immunity• Antigen

– Large, foreign, unique molecule

– Induces an immune response against itself

– In general, the more complex a molecule is, the greater its antigenicity

β Lymphocytes: Antibody-Mediated Immunity

• Each lymphocyte has surface receptors for binding with one particular type of possible antigens- named as BCRs- B-cell receptors

• These are eyes of adaptive immune system

T-Dependent and T-independent Antigen

• Binding with Antigens stimulate B cells to convert into plasma cells that produce antibodies

• T-independent: after binding with polysaccharide antigen B cells are activated without assistance from T- helper cells

• T-dependent: polypeptide antigen cannot stimulate B cells without the help of T- helper cells The majority of antigens to which B cell respond

are T-dependent

β Lymphocytes: Antibody-Mediated Immunity

• On binding with processed and presented antigen– Most B cells differentiate into active plasma cells– Other B cells become dormant memory cells

• A mature plasma cell then produces antibodies rapidly (2000 antibodies/sec)

• All antibodies eventually enter blood where they are known as gamma globulins or immunoglobulin's

B -Lymphocytes

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Plasma Cells• Antibody (Immunoglobulin) subclasses

– IgM• Serves as the β cell surface receptor for antigen attachment• Secreted in early stages of plasma cell response

– IgG• Most abundant immunoglobulin in blood• Produced in large amounts when body is exposed to same antigen

– IgE• Helps protect against parasitic worms• Antibody mediator for common allergic responses

– IgA• Found in secretions of digestive, respiratory, and genitourinary

systems; also in milk and tears– IgD

• Present on surface of many β cells• Function is uncertain

Antibodies• Y-shaped molecules• Composed of 4 interlinked

polypeptide chains– Two long, heavy chains– Two short, light chains

• Properties of tail portion determine functional properties of the antibody( what it will do after binding with antigen)

• Identical antigen-binding fragments (Fab) at tip of each arm (unique for each different antibody)

• Tail (constant region) regions within each subclass are identical

Mechanism of Action of Antibodies

• Can physically hinder antigens– By neutralization, they prevent harmful

chemicals from interacting with susceptible cells– Can bind to foreign cells by agglutination

• Enhance activity of other defense systems by– Activating complement system– Enhancing phagocytosis– Stimulating natural killer (K) cells

Mechanism of Action of Antibodies

Mechanism of Action of Antibodies

Immune Complex Disease• Occurs when overzealous antigen-antibody response

causes damage to normal and invading foreign cell

• Antigen-antibody complex normally form in response to foreign invasion and are removed by phagocytic cells

• If large numbers of these complexes are continuously produced, phagocytes can’t clear away all immune complexes formed– Antigen-antibody complexes not removed continue to

activate complement system– Excessive activated complement damage normal cells

• Can occur in bacterial, viral, or parasitic infections

Clonal Selection Theory(how matching B cell responds to its antigen)

• During fetal life B cells are genetically preprogrammed to produce antibody against particular antigen , before ever being exposed to it.

• All offspring of particular b lymphocyte form family of identical cells, on exposure to specific antigen . i.e. ( clonal expansion ).

Clonal Selection Theory(how matching B cell responds to its antigen)

– Memory cells• Small percentage of β lymphocytes become

memory cells• Remain dormant• Upon re exposure to same antigen, they are

more ready for immediate action than the original lymphocytes of the clone

• Secondary response is quicker, more potent, and longer-lasting

– Can be induced by disease or vaccination

Formation of memory cell can occurs through the person either actually having disease or being vaccinated

Primary and Secondary Response

Primary and Secondary Immune Responses

Active and Passive Immunity• Active immunity

– “self-generated”– Results from exposure to an antigen

• Passive immunity– “borrowed immunity”– Results from transfer of preformed antibodies– Can provide immediate protection – Example of passive immunity is transfer of IgG

antibodies from mother to fetus– Tetanus toxins, anti snake venom, rabies virus

References

• Human physiology by Lauralee Sherwood, seventh edition

• Text book physiology by Guyton &Hall,11th edition• Text book of physiology by Linda .s contanzo,third edition