BLOOD AND BODY DEFENCE Dr. Amel Eassawi Dr. Abdelrahman Mustafa 1

HMIM 224 L 8: B LYMPHOCTE 2

OBJECTIVES The student should be able to:  Define clonal selection theory  Explain the concept of humoral (antibody mediated) immunity.  Describe the mechanism of action of antibodies.  Emphasize the importance of memory cell.  Differentiate between primary and secondary response.

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

5 B Lymphocytes: Antibody-Mediated Immunity

B LYMPHOCYTES 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 antigens to which B cell respond are T-dependent.

B LYMPHOCYTES B 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.

8 B -Lymphocytes

IMMUNOGLOBULIN Antibody (Immunoglobulin) subclasses –IgM Serves as the B 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 B cells Function is uncertain

ANTIBODIES 10 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

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ANTIBODIES 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

13 Mechanism of Action of Antibodies

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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).

17 Clonal Selection Theory

PRIMARY AND SECONDARY RESPONSE –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

19 Primary and Secondary Immune Responses

REFERENCES  Human Physiology, Lauralee Sherwood, seventh edition.  Text book Physiology by Guyton &Hall,11 th edition.  Text book of Physiology by Linda S. Contanzo, third edition.  Physiology by Berne and Levy, sixth edition. 20