Antibodies

Antibodies

- Are immunoglobulin (Ig) or gamma globulin – proteins that binds to antigens. - Primarily migrate in the beta and gamma regions during protein electrophoresis.

- Structure of Antibody (composed of two heavy and two light polypeptide chains)
o Light (polypeptide) chain – 220 amino acid residues in length.
o Heavy (polypeptide) chains - 440-5508 amino acid residues in length.

Functions of the antibodies

o Neutralize toxins and viruses.
o Opsonize microbes so that they will be readily recognized and more easily phagocytosed.
o Activate complement system.
o Prevent microbe attachment to mucosal surface.

Antibody Structure

- A typical immunoglobulin is shaped like a “Y” letter consisting of polypeptide chains linked by disulfide bonds.

o Variable region
o Constant region
o Hinge region
o J-chain (joining chain)

Variable region

– contains the hypervariable region that represents the antigen binding site of the antibody in both L and H chains.

 The antigen-antibody binding site is therefore composed of the variable regions of both heavy and light chains. Interchain disulfide bonds hold together two heavy chains. Intrachain disulfide bonds are found within each of the polypeptide chains.

Constant region

 Made up of a carboxyl-terminal, and it has a homologous primary structure. These regions are located at the Y-shaped immunoglobulin monomer's tail on both the light and the heavy chains.

 The constant region in heavy chains is abbreviated as CH, while in light chains abbreviated as CL.

 Determine the immunoglobulin class and allow the binding of complement proteins.

Hinge region

 A flexible amino acid stretch in the central part of the heavy chains of the IgG and IgA immunoglobulin classes, which links these 2 chains by disulfide bonds.

 Connecting CH1 and CH2

 Hinge region is particularly susceptible to protease cleavage such as papain and pepsin.

J-chain (joining chain)

is a protein component that links monomers of antibodies IgM and IgA to form polymeric antibodies capable of secretion.

Immunoglobulin Classes

1. IgG
2. IgM
3. IgA
4. IgD
5. IgE

IgG

a. Predominantly serum antibody, approximately 73% is in the blood.
b. Subclasses include IgG1 (most common), IgG2, IgG3, IgG4
c. Only immunoglobulin that crosses placenta
d. Produced in secondary antibody response.
e. IgG1, IgG2, and IgG3 activate the classical complement pathway.

IgM

a. Five monomers linked together by a J-chain and interchain disulfide bonds.
b. 10% of the total serum immunoglobulins
c. First antibody produced against an immunogen.
d. Produced in the high levels in the primary response.
e. It is the last activator of the classical pathway of complement – only one molecule of IgM is required.

IgA

a. Serum and secretory forms: serum IgA is a single immunoglobulin molecule, whereas secretory IgA is a dimer held together by a J-chain.
b. 2 subclasses: IgA1 and IgA2
c. Accounts for 15-20% of total serum antibody
d. Functions of serum IgA are antigen clearance and immune regulation.
e. The function of IgA in mucous membranes is to block attachment of viruses, bacteria and toxins to host’s cells.

IgD

a. Primarily a cell membrane surface component of B-lymphocytes
b. Short half-life (2-3 days), 1% serum antibody
c. A monomer that has no known antibody function but as antigen receptor.

IgE

a. Responsible for allergic reaction (Type I hypersensivity)
b. The Fc portion binds to receptors on mast cells and basophils. Once attached to mast cells, IgE binding an allergen triggers degranulation of the cell and release of allergic mediators such as histamine and leukotrienes.
c. Elevated IgE concentrations are often found during parasitic infections.

Monoclonal Antibodies

- These are identical antibodies produced from a single dose of plasma cells.

- Found in individuals with multiple myeloma (a type of bone marrow cancer often affects several areas of the body, such as the spine, skull, pelvis and ribs)

- Produced in industry by infusing an antigen-sensitized, splenic B-lymphocyte with nonsecreting myeloma cell, thus creating immortal cell line that secretes an antibody of a single idiotype.

Quantitation of Antibodies

- The purpose is to provide information about the functional immune status of an individual.

- IgG, IgM, and IgA are quantified using radial immunodiffusion, nephelometry or turbidimetry.

Immune system

is composed of molecular and cellular components that are derived from the central (primary) and peripheral (secondary) lymphoid organs, that defends the body against infection while protecting the body’s own cells.

Central lymphoid organs

Peripheral lymphoid organ

Central lymphoid organs

Primary sites for cells differentiation and maturation and plays important role in adaptive immunity (T- and B-lymphocytes)

 Bone marrow – sites for cell origination

 Thymus – maturation site

Peripheral lymphoid organ

o Reactivity sites of lymphoid cells
o Organs of reactivity sites:

 Lymph nodes – migration of B-cells to the cortex and T-cells to the paracortex  Spleen - filters blood, contains both T- and B- cells
 Mucosa-associated lymphoid tissue (MALT) – includes tonsils, adenoids, Peyer’s patches in the ileum and the appendix – they interact with the environment and can begin the immune response early.

(waits for entry of antigen)

Monocytes and macrophages

________ in the peripheral blood; _________ in the tissue

Functions:
 Phagocytosis of invaders
 Present immunogens to T-helper cells – the 1st step in an immune response  Release cytokines that affect other cells’ activities

Granulocytes

• Neutrophils

• Eosinophils

• Basophils

• Lymphocytes

Neutrophils

 60 to 70% of WBCs in circulation
 Function: Phagocytosis and contributes to inflammatory response

Eosinophils

 1-3% of circulating WBCs
 Mediate IgE allergic response

Basophils

 0-1% of circulating WBCs
 Has receptors for IgE and granules responsible for allergic reactions

Lymphocytes

 20-40% of circulating WBCs
 B cells (B-lymphocytes) o 20% circulating lymphocytes.

o Express surface molecules such as CD 19 and CD20 (cluster differentiation) o After birth, B cells mature in the bone marrow

o B cell differentiate into either a plasma cell (produce antibody) or a memory B cell.

 T cells (T-lymphocytes)
o 80% circulating lymphocytes
o Express surface molecules such as CD2 and CD3
o Functions: Lyse host’ cells infected with virus and tumor cells and produce lymphokines; T cells stimulates or suppresses other cells
o T cells begins in the marrow and matures in the thymus
o Natural killer (NK) cells are slightly larger than T or B cell and have cytoplasmic granules

 Other cells that assist in the immune response
 Dendritic cells: present antigens to T cells
 Langerhans cells: dendritic cells found in the dermis and squamous epithelia  Mast cells: granulocyte resembling basophil that contains many chemicals that affect the immune response.

Cell-Mediated Immunity

1. Provide resistance and aid in recovery from infections due to intracellular organism.
2. Defense against fungi, parasites, and bacteria.
3. Involved in transplant and graft rejections.
4. Main defense against tumor cells.

Complement System

- part of your body's immune system, cleans up damaged cells, helps your body heal after an injury or an infection and destroys microscopic organisms.
- It is the front line of defense for the immune system.
- Consists of serum proteins, C1-C9.

3 Main effects to activate complement system

o Cell lysis
o Generation of inflammatory mediators
o Opsonization leading to enhanced phagocytosis

4 basic steps

o Initiation
o Formation of C3 convertase
o Formation of C5 convertase
o Formation of membrane attack complex (MAC)

3 pathways of Complement System

o Classical pathway – activated by antigen-antibody complexes.

o Alternative or properdin pathway – activated by bacterial products such as endotoxin or complexes of immunoglobulins.

o Lectin pathway or MBL (mannose-binding lectin) pathway – activated by specific patterns of sugars found on the bacterial cell wall.

Classical pathway

– activated by antigen-antibody complexes.

Alternative or properdin pathway

– activated by bacterial products such as endotoxin or complexes of immunoglobulins.

Lectin pathway

– activated by specific patterns of sugars found on the bacterial cell wall.

Hypersensitivity

– overactive immune response to non-toxic substances on re-exposure that can result in tissue damage.

Allergens

– molecules that trigger any hypersensitivity reaction.

Type 1

= Allergic Anaphylaxis and Atopy

Type II

= Antibody

Type III

= Immune Complex

Type IV

= Delayed