9.3.3 Role of B Cells in Third Line of Defence (humoral/antibody Mediated Immune Response)

B cells are differentiated in bone marrow. B cells express specific receptors on their cell membrane, the B cell receptors (BCRs). BCRs allow the B cell to bind a specific antigen, against which it will initiate an antibody response. Like T cells, there are millions of B cells types, found in the body; each is specific for one particular antigen.

Definition

B Cells: B cells are differentiated in bone marrow. B cells express specific receptors on their cell membrane, the B cell receptors (BCRs).

Humoral/Antibody Mediated Immune Response: The stimulation of B cells to divide into plasma clone cells and memory B cells and the secretion of antibodies by plasma clone cells is called humoral mediated immune response.

Main Idea

• B cells are differentiated in bone marrow.
• B cells express specific receptors on their cell membrane, the B cell receptors (BCRs).
• BCRs allow the B cell to bind a specific antigen, against which it will initiate an antibody response.
• Like T cells, there are millions of B cells types, found in the body; each is specific for one particular antigen.

Activation of B Cells

• B cell activation begins when the B cell binds to an antigen via its BCR.
• Actually, B cells are stimulated to bind with specific antigen by interleukin-2 proteins, which are secreted by helper T cells.
• After binding with specific type of antigen the B cells divide to produce two type of cells: plasma clone cells and memory B cells.
• The plasma clone cells are specialized to secrete bulk quantity of antibodies.
• After B cells become plasma cells they live only for a few days but secrete a great deal of antibody during the time.
• A plasma cell can produce more than 10 million molecules of antibody per hour.
• If the same antigen enters the body later, the memory B cells divide to make more plasma cells and memory cells that can protect against future attacks by the same antigen.
• The stimulation of B cells to divide into plasma clone cells and memory B cells and the secretion of antibodies by plasma clone cells is called humoral mediated immune response.

Structure of an Antibody

• Antibodies (also called immunoglobulins or Ig's) are Y-shaped proteins that circulate through the blood stream and bind to specific antigens, thereby attacking microbes.
• The antibodies are transported through the blood and the lymph to the pathogen invasion site.
• A typical antibody is a Y-shaped molecule, which consists of four polypeptide chains: two identical long chains called heavy chains, and two identical short chains called light chains.
• Each chain has a constant segment, a functional segment, and a variable segment.
• In the constant segment (C) of the heavy chains, the amino acid sequence is constant within a particular immunoglobulin class.
• On the other hand, the variable segments (V) consist of different amino acid sequences in every antibody.
• Therefore, they act as antigen-binding site.
• Each antibody has two antigen-binding sites.

Mode of Action of an Antibody

Antibodies work in different ways:

• Antibodies can combine with virus and toxins to neutralize them. This prevents binding of pathogens with host cells.
• The antibody can bind to an antigen, forming an antigen-antibody complex thus promote phagocytosis by macrophages. This mode of action is called opsonization.
• They can activate complement system to form membrane attack complex. As a result, target cell either bursts or undergoes phagocytosis.
• Antibodies can attach with virus infected cells or tumour cells to start antibody dependent cell mediated cytotoxicity. NK cells attach with target cell and induce apoptosis to kill that cell.

Primary and Secondary Immune Response

• The action of the immune system can be classified into two parts: the primary and the secondary immune response.
• The primary immune response displays the first contact of the immune system with an infectious agent whereas all following contacts with the same pathogen are named secondary immune response.
• Secondary immune response is faster and stronger than primary response.
• Primary response forms memory cells, while secondary response uses existing memory cells.
• The primary response sets the stage for the secondary response, allowing the immune system to "remember" and mount a more effective defence against future infections.

Summary Table of B Cells

Summary of Humoral Immune Response

Summary of Antibody Structure

Summary of Antibody Action

Summary of Primary and Secondary Immune Response

Intermediate Level

Role of B Cells in Third Line of Defence

Humoral Immune Response

Antibody Structure

Antibody Action

Primary and Secondary Immune Response

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. B cells are differentiated in bone marrow.
• 2. B cells express B cell receptors (BCRs).
• 3. BCRs allow B cell to bind specific antigen.
• 4. Millions of B cell types exist.
• 5. Each is specific for one particular antigen.
• 6. B cell activation begins when B cell binds to antigen via BCR.
• 7. Interleukin-2 from helper T cells stimulates B cell.
• 8. B cell divides to produce plasma clone cells and memory B cells.
• 9. Plasma cells secrete bulk quantity of antibodies.
• 10. Plasma cells live only for a few days.
• 11. A plasma cell can produce more than 10 million molecules of antibody per hour.
• 12. Memory B cells protect against future attacks.
• 13. Humoral immune response is B cell mediated.
• 14. Antibodies are Y-shaped proteins.
• 15. Antibodies are also called immunoglobulins.
• 16. Antibodies have two heavy chains and two light chains.
• 17. Antibodies have two antigen-binding sites.
• 18. Antibodies work by neutralization, opsonization, complement activation, and ADCC.
• 19. Primary immune response is the first contact.
• 20. Secondary immune response is faster and stronger.

Remember

• - B cells = differentiated in bone marrow
• - BCRs = B cell receptors
• - Function = bind specific antigen; initiate antibody response
• - Types = millions; each specific for one antigen
• - Humoral immune response = B cell mediated
• - Plasma cells = secrete antibodies
• - Memory B cells = provide long-lasting immunity
• - Antibodies = Y-shaped proteins
• - Antibodies = two heavy chains; two light chains
• - Antibodies = two antigen-binding sites
• - Antibody action = neutralization; opsonization; complement activation; ADCC
• - Primary response = first contact; slower; forms memory cells
• - Secondary response = following contacts; faster; stronger

Common Mistakes

• 1. Confusing B cells and T cells — B cells = humoral; T cells = cell-mediated.
• 2. Mixing up the types of B cells — Plasma cells and memory B cells.
• 3. Forgetting the role of interleukin-2 — Stimulates B cell.
• 4. Mixing up the antibody structure — Two heavy chains; two light chains.
• 5. Forgetting the number of antigen-binding sites — Two per antibody.
• 6. Mixing up the modes of antibody action — Neutralization; opsonization; complement activation; ADCC.
• 7. Believing plasma cells live long — Plasma cells live only for a few days.
• 8. Mixing up primary and secondary responses — Primary = first; secondary = following.
• 9. Thinking B cells are not important — They are essential for humoral immunity.
• 10. Mixing up the functions — Plasma cells secrete antibodies; memory B cells provide long-lasting immunity.

Quick Revision

B cells are differentiated in bone marrow. B cells express specific receptors on their cell membrane, the B cell receptors (BCRs). BCRs allow the B cell to bind a specific antigen, against which it will initiate an antibody response. Like T cells, there are millions of B cells types, found in the body; each is specific for one particular antigen. B cell activation begins when the B cell binds to an antigen via its BCR. Actually, B cells are stimulated to bind with specific antigen by interleukin-2 proteins, which are secreted by helper T cells. After binding with specific type of antigen the B cells divide to produce two type of cells: plasma clone cells and memory B cells. The plasma clone cells are specialized to secrete bulk quantity of antibodies. After B cells become plasma cells they live only for a few days but secrete a great deal of antibody during the time. A plasma cell can produce more than 10 million molecules of antibody per hour. If the same antigen enters the body later, the memory B cells divide to make more plasma cells and memory cells that can protect against future attacks by the same antigen. The stimulation of B cells to divide into plasma clone cells and memory B cells and the secretion of antibodies by plasma clone cells is called humoral mediated immune response. Antibodies (also called immunoglobulins or Ig's) are Y-shaped proteins that circulate through the blood stream and bind to specific antigens, thereby attacking microbes. The antibodies are transported through the blood and the lymph to the pathogen invasion site. A typical antibody is a Y-shaped molecule, which consists of four polypeptide chains: two identical long chains called heavy chains, and two identical short chains called light chains. Each chain has a constant segment, a functional segment, and a variable segment. In the constant segment, (C) of the heavy chains, the amino acid sequence is constant within a particular immunoglobulin class. On the other hand, the variable segments (V) consist of different amino acid sequences in every antibody. Therefore, they act as antigen-binding site. Each antibody has two antigen-binding sites. Antibodies work in different ways: (1) Antibodies can combine with virus and toxins to neutralize them. This prevents binding of pathogens with host cells. (2) The antibody can bind to an antigen, forming an antigen-antibody complex thus promote phagocytosis by macrophages. This mode of action is called opsonization. (3) They can activate complement system to form membrane attack complex. As a result, target cell either bursts or undergoes phagocytosis. (4) Antibodies can attach with virus infected cells or tumour cells to start antibody dependent cell mediated cytotoxicity. NK cells attach with target cell and induce apoptosis to kill that cell. The action of the immune system can be classified into two parts: the primary and the secondary immune response. The primary immune response displays the first contact of the immune system with an infectious agent whereas all following contacts with the same pathogen are named secondary immune response. Secondary immune response is faster and stronger than primary response. Primary response forms memory cells, while secondary response uses existing memory cells. The primary response sets the stage for the secondary response, allowing the immune system to "remember" and mount a more effective defence against future infections.

60-Second Revision

• B cells = differentiated in bone marrow
• BCRs = B cell receptors
• Function = bind specific antigen; initiate antibody response
• Types = millions; each specific for one antigen
• Humoral immune response = B cell mediated
• Plasma cells = secrete antibodies
• Memory B cells = provide long-lasting immunity
• Antibodies = Y-shaped proteins
• Antibodies = two heavy chains; two light chains
• Antibodies = two antigen-binding sites
• Antibody action = neutralization; opsonization; complement activation; ADCC
• Primary response = first contact; slower; forms memory cells
• Secondary response = following contacts; faster; stronger
• Warning: B cells are humoral; T cells are cell-mediated

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