Humoral Immunity

Pinakiranjan Chakrabarti

Vijaygarh Jyotish Ray College

B Cells and Humoral immunity

• The humoral response is carried out by antibodies which are produced by Plasma cells.

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• Plasma cells are derived from activated B-cells that are produced in the bone marrow

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The innate immune system activates Acquired immunity

• Cells of the innate immune system activate the specific immune response.
• A group of cells called Antigen presenting cells (APC) activate the acquired immune system.
• Macrophages, Dendritic cells and B-cells are examples of types of APCs.
• APCs turn on the acquired immune system by activating T-Helper cells (T_H-cells).
• T_H-cells in turn activate either the cell mediated or the humoral immune system.

Humoral immunity

APC

APC

APC

T_H

T_H

B

APC

The Microbial antigen is ingested by an APC and partially digested. Fragments from microbe bind with the MHC II to form a MHC II /Ag complex on the surface of the APC

APC

APC

T_H

A Helper T cell, specific for the presented antigen, binds to the MHC II/Ag complex

APC

APC

T_H

APC

T_H

B

The helper T cell then activates an appropriate B cell by releasing IL-2 to it.

• The interaction between the T_H-cell and the B-cell causes the B- cell to differentiate into Plasma cells and memory cells.

T_H

APC

Memory cells

• Memory cells do not react right away but are held in reserve for later infections. The secondary response that is carried out by memory cells is different in 3 ways.
• Memory cells produce antibodies that bind with greater affinity to their antigens than the antibodies produced in the initial response.

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• The response time is much vaster than the primary response

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• A greater number of antibodies are produced.

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Function of Antibodies

Function of Antibodies

• Antibodies function in 6 ways to protect the body
• Aggltination: Enhances phagocytosis and reduces number of infectious units to be dealt with
• Opsonization: Coating antigen with antibody enhances phagocytosis
• Neutralization: blocks adhesion of bacteria and viruses to mucosa. Also blocks active site of toxin

Function of Antibodies Cont

• Activation of complement

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• Increases inflammation through the byproducts of the complement system (C5a and C3a)

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• Antibody dependant cell mediated cytotoxicity: Antibodies attached to target cell cause destruction by non specific immune system cells.

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Structure of Antibodies

Structure of an Antibody

• Structure of an Antibody
• Antibody composed of two heavy chains and two light chains. These chains bind together to make a Y shaped molecule. See figure 17.5.

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Structure of Antibodies

• The two sections located at the ends of Y’s arms are called variable (V) regions.
• The variable region is structurally identical for all antibodies synthesized by a particular plasma cell.
• The Antibodies from each plasma cell however are different or unique from all other antibodies produced by other plasma cell.

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Structure of Antibodies

• The stem of the antibody molecule as well as the lower portion of the arms called constant (c) regions.
• There are 5 major types of C regions which correspond to the 5 different classes of antibodies.
• All plasma cells in the body are producing one of these classes of antibodies.
• A particular plasma cell may switch the particular class of Antibody that it is producing in order to fight an infection in a different way.

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• The structure of Antibodies may be described by the way they are cut and digested by proteases.

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• The stem portion is referred to as the FC region

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• The Y portion with the top third of the stem is referred to as the Fab region.

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• The FC region often acts as the receptor for phagocytes during opsonization or Antibody dependant cell mediated cytotoxicity.

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• The FC region contains the antigen binding region

IgM

• IgM expressed as membrane bound anitbodies on B-cells
• Pentamer
• 5 units held together by disulfide bonds
• J (Joining) chain functions in the polymerization of monomers
• First immunoglobulin class produced in a primary response to an antigen
• Has 10 anitgen binding sites
• More effective at stimulating complement
• Large-size - does not diffuse well
• The F_C receptors on phagocytes bind IgM (opsinization)

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IgD

• Found on surface of mature B-cells.

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• Biological function unknown (thought to function in activation of B-cells)

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IgG

• Most abundant isotype in serum (80%)
• Cross placenta and play important role in protecting fetus
• Provides passive immunity to unborn fetus.
• Placental cells bind the Fc portion of IgG and transfer Ab across the placental membrane.
• Activate complement system
• Opsonin—phagocytosis

IgE

• Mediate the immediate hypersensitivity reactions (hayfever, asthma, hives, anaphylactic shock)
• Mast cells and basophils bind fc portion of IgE
• Cross-linkage of receptor bound IgE molecules by antigen, induces degranulaltion of the Mast and basophil cells
• Parasitic response
• Eosinophils express receptors for IgE

IgA

• Most abundant Ab in the body
• Found Predominantly in external secretions i.e. Breast Milk, Saliva, tears, mucus.
• Serum form is a monomer
• Secretory form is a dimer or tetramer linked together via a “secretory component” and a J chain.
• J (Joining) chain functions in the polymerization of monomers.
• Plasma cells that release IgA Abs are concentrated along the Mucus Membrane surface.
• Provides passive immunity to infants through mothers breast milk