Immunoglobulins – Structure, Classes, Functions of Immunoglobulins

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Immunoglobulins (IG)

The immunoglobulins are γ-globulins, called antibodies.

  • All antibodies are immunoglobulin but all immunoglobulins may not be antibodies.
  • They constitute about 20 percent of all the plasma proteins.
  • Immunoglobulins are produced by plasma cells and to some extent by lymphocytes.
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Structure of Immunoglobulins

Immunoglobulins are glycoproteins made up of light (L) and heavy (H) polypeptide chains. The term “light” and “heavy” refer to molecular weight. Light chains have a molecular weight of 25,000 whereas heavy chains have a molecular weight of 50,000 to 70,000. All immunoglobulins have the very same standard structure.

The fundamental immunoglobulin is a ‘Y’ shaped particle and consists of 4 polypeptide chains, two H and two L chains. The four chains are connected by disulfide bonds. L chain may be either of 2 types, Kappa (κ) or Lambda (λ) however not both. The heavy chains may be of 5 types and are designated by Greek letter:

  1. Alpha (α).
  2. Gamma (γ).
  3. Delta (δ).
  4. Mu (μ).
  5. Epsilon (ε).

Immunoglobulins are named based on their heavy chain type as IgA, IgG, IgD, IgM, and IgE. The L and H chains are further divided into variable and constant regions. L chain consists of one variable (VL) and one constant (CL) domain or region. Most H-chains consist of one variable (VH) and 3 constant (CH-1, CH-2, and CH-3) domains.

Each immunoglobulin molecule has a hinge region between CH-I and CH-2, which allows a better fit with the antigen surface. The variable areas of both the light and heavy chains form the antigen-binding sites. The enzyme (papain) food digestion splits the immunoglobulin particle into two pieces called Fab (Fragment for antigen binding) and Fc (crystallizable fragment or fragment for complement binding).

Immunoglobulin Classes
IgG (Heavy chain γ)

IgG is a smaller sized monomeric molecule with two antigen-binding sites. IgG is the major class of immunoglobulin which represents 70 percent of the total serum immunoglobulins. IgG is the only antibody that crosses the placenta and is the maternal antibody that protects the fetus. It is produced mainly in the secondary response and provides defense against bacteria and viruses.

IgA (Heavy chain α)

IgA is the second most abundant class constituting about 20 percent of serum immunoglobulins. It is monomer or dimer. Dimeric IgA particles formed by two monomer units, joined together at their carboxy terminals by a protein described J-chains (J for joining). IgA is found in external secretions such as colostrum, saliva, tears, and respiratory, digestive, and genital system secretions.

It prevents the attachment of microorganisms like bacteria and viruses to mucous surface areas and assists to protect mucous surfaces from antigenic attack and prevent access of foreign substances to the circulation.

IgM (Heavy chain μ)

It is a pentamer including 5 similar immunoglobulin molecules, collaborated by disulfide bridges. This pentamer is closed in a ring structure by J-chain.

IgM is the very first antibody to be produced in reaction to an antigen and is necessary for defense against bacteria and viruses. IgM present on the surface of B lymphocytes is a monomer, where it operates as an antigen-binding receptor.

IgD (Heavy chain δ)

It is a monomer and looks like IgG structurally. IgD has no known antibody function but might work as an antigen receptor. Like IgM, it is present on the surface of numerous B lymphocytes. The circulating concentration of IgD in the blood is extremely low.

IgE (Heavy Chain ε)

IgE is a monomeric particle comparable to IgG. It is sometimes called reagin. Although IgE exists in trace quantities, (roughly 0.004%) in normal persons with allergic activity have considerably increased amounts. IgE is accountable for the anaphylactic (instant) type of hypersensitivity and allergic reaction. Its activity is moderated by histamine. IgE also takes part in defense against particular parasites, e.g., helminths (worms).

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Functions of Immunoglobulins (Antibody)

The main function of antibodies is to protect against contagious representatives.

  • In addition to these functions, antibodies can function as an enzyme to catalyze the synthesis of ozone (O3) that has microbicidal activity.
Disorders of Immunoglobulins

Abnormally big quantities of specific immunoglobulins may be found in the plasma in several diseases of humans. As well as deficiency of γ-globulins is also found in rare hereditary diseases.

Multiple Myeloma

Multiple myeloma, a plasma cell cancer results due to an abnormally high concentration of serum immunoglobulins, normally IgG or IgA.

Amyloidosis

Amyloidosis is the accumulation of immunoglobulin light chain fragments in the tissues of numerous myeloma patients.

Bence Jones Proteins

In several myelomas, more light chains are produced than heavy chains and enter into the bloodstream, because they are of relatively low molecular weight, they pass through the glomerular membrane and appear in the urine. These protein chains of low molecular weight are referred to as Bence Jones Proteins.

Bence Jones proteins have the remarkable characteristic of precipitating on heating urine from 45 ° to 60 ° C and redissolve when the heating is continued above 80 ° C. Multiple myeloma with Bence Jones protein in the urine is called “light chain disease.” The chain may be of either of the κ-(kappa) or the λ-( lambda) type.

Multiple Choice Questions (MCQs) – Immunoglobulins Tutorial

  1. What are immunoglobulins?
    • A. Proteins
    • B. Antibodies
    • C. Enzymes
    • D. Hormones
  2. What percentage of all plasma proteins do immunoglobulins constitute?
    • A. 5%
    • B. 10%
    • C. 15%
    • D. 20%
  3. What are the light and heavy chains in immunoglobulins based on?
    • A. Molecular charge
    • B. Molecular weight
    • C. Binding affinity
    • D. Functional role
  4. Which enzyme digests immunoglobulins into Fab and Fc fragments?
    • A. Amylase
    • B. Papain
    • C. Pepsin
    • D. Lipase
  5. Which immunoglobulin is the major class and represents 70% of total serum immunoglobulins?
    • A. IgM
    • B. IgA
    • C. IgG
    • D. IgE
  6. Which immunoglobulin is found in external secretions and helps prevent the attachment of microorganisms to mucous surfaces?
    • A. IgD
    • B. IgE
    • C. IgA
    • D. IgG
  7. What is the structural form of IgM, and how many immunoglobulin molecules does it contain?
    • A. Dimer, 2 molecules
    • B. Monomer, 1 molecule
    • C. Pentamer, 5 molecules
    • D. Trimer, 3 molecules
  8. Which immunoglobulin is the first antibody to be produced in response to an antigen?
    • A. IgA
    • B. IgD
    • C. IgE
    • D. IgM
  9. What is the primary function of antibodies (immunoglobulins)?
    • A. Energy storage
    • B. Cellular respiration
    • C. Protection against infectious agents
    • D. Hormone synthesis
  10. In which disease is an abnormally high concentration of serum immunoglobulins, usually IgG or IgA, observed?
    • A. Amyloidosis
    • B. Bence Jones disease
    • C. Multiple Myeloma
    • D. Light chain disease
  11. What is the term for the accumulation of immunoglobulin light chain fragments in the tissues of multiple myeloma patients?
    • A. Amyloidosis
    • B. Bence Jones Proteins
    • C. Hypergammaglobulinemia
    • D. Agammaglobulinemia
  12. What characteristic do Bence Jones Proteins exhibit when heated?
    • A. Precipitation
    • B. Polymerization
    • C. Denaturation
    • D. Crystallization
  13. In multiple myeloma with Bence Jones protein in the urine, what is it referred to as?
    • A. Amyloidosis
    • B. Light chain disease
    • C. Hyperproteinemia
    • D. Gamma-globulinemia
  14. Which immunoglobulin is responsible for anaphylactic hypersensitivity and allergic reactions?
    • A. IgA
    • B. IgD
    • C. IgE
    • D. IgM
  15. What is the enzyme that moderates the activity of IgE in allergic reactions?
    • A. Insulin
    • B. Histamine
    • C. Lipase
    • D. Amylase
  16. How many antigen-binding sites does IgG have?
    • A. 1
    • B. 2
    • C. 3
    • D. 4
  17. What is the role of the hinge region in immunoglobulins?
    • A. Catalysis
    • B. Antigen binding
    • C. Allows flexibility
    • D. Enzymatic degradation
  18. Which of the following is NOT a heavy chain type in immunoglobulins?
    • A. Alpha (α)
    • B. Beta (β)
    • C. Gamma (γ)
    • D. Delta (δ)
  19. Which immunoglobulin can cross the placenta and provide protection to the fetus?
    • A. IgA
    • B. IgD
    • C. IgG
    • D. IgM
  20. What is the primary function of IgA in external secretions?
    • A. Energy production
    • B. Prevention of microbial attachment
    • C. Hormone synthesis
    • D. Cellular respiration
  21. Which region allows a better fit with the antigen surface in immunoglobulins?
    • A. Fab
    • B. Fc
    • C. Variable region
    • D. Constant region
  22. What are the two types of L chains in immunoglobulins?
    • A. Alpha and Beta
    • B. Delta and Epsilon
    • C. Kappa and Lambda
    • D. Gamma and Mu
  23. What is the term for the coordinated effect of two or more genes in producing a phenotypic trait?
    • A. Gene interaction
    • B. Epistasis
    • C. Codominance
    • D. Polygenic inheritance
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Answers:

  1. B, 2. D, 3. B, 4. B, 5. C, 6. C, 7. C, 8. D, 9. C, 10. C, 11. A, 12. A, 13. B, 14. C, 15. B, 16. B, 17. C, 18. B, 19. C, 20. B, 21. C, 22. C, 23. A

 

Frequently Asked Questions (FAQs) – Immunoglobulins Tutorial

1. What are immunoglobulins (IG)?

  • Immunoglobulins, also known as γ-globulins, are proteins commonly referred to as antibodies. They make up about 20 percent of all plasma proteins and are produced by plasma cells and, to some extent, by lymphocytes.

2. How are immunoglobulins structured?

  • Immunoglobulins are glycoproteins composed of light (L) and heavy (H) polypeptide chains. The terms “light” and “heavy” refer to their molecular weights, with light chains around 25,000 and heavy chains ranging from 50,000 to 70,000. They have a standard ‘Y’ shape and consist of four polypeptide chains connected by disulfide bonds.

3. What are the types of heavy chains in immunoglobulins?

  • The heavy chains in immunoglobulins come in five types, designated by Greek letters: Alpha (α), Gamma (γ), Delta (δ), Mu (μ), and Epsilon (ε).

4. How are immunoglobulins named?

  • Immunoglobulins are named based on their heavy chain type, resulting in designations such as IgA, IgG, IgD, IgM, and IgE.

5. What are the variable and constant regions in immunoglobulin chains?

  • Both light (L) and heavy (H) chains are divided into variable (V) and constant (C) regions. L chains consist of one variable (VL) and one constant (CL) domain, while most H chains consist of one variable (VH) and three constant (CH-1, CH-2, and CH-3) domains.

6. How are immunoglobulins digested by enzymes?

  • The enzyme papain can digest immunoglobulin molecules into two pieces known as Fab (Fragment for antigen binding) and Fc (crystallizable fragment or fragment for complement binding).

7. What is the hinge region’s function in immunoglobulins?

  • The hinge region, located between CH-I and CH-2, allows a better fit with the antigen surface and provides flexibility to the immunoglobulin molecule.

8. Which immunoglobulin is the major class and crosses the placenta to protect the fetus?

  • IgG is the major class of immunoglobulin, representing 70 percent of total serum immunoglobulins. It is the only antibody that crosses the placenta and protects the fetus.

9. What is the primary function of IgA in external secretions?

  • IgA, the second most abundant class of immunoglobulin, prevents the attachment of microorganisms to mucous surfaces and protects against antigenic attack in external secretions.

10. How does IgM function in response to an antigen? – IgM, present on the surface of B lymphocytes, is a pentamer that is the first antibody to be produced in response to an antigen, playing a crucial role in defense against bacteria and viruses.

11. What is Multiple Myeloma? – Multiple Myeloma is a plasma cell cancer resulting from an abnormally high concentration of serum immunoglobulins, usually IgG or IgA.

12. What is Amyloidosis? – Amyloidosis is the accumulation of immunoglobulin light chain fragments in the tissues of multiple myeloma patients.

13. What are Bence Jones Proteins, and how are they characterized? – Bence Jones Proteins are light chains produced in excess in certain myelomas, entering the bloodstream and appearing in urine. They precipitate on heating urine and redissolve when heating continues above 80 °C.

14. What is the function of IgE, and when is it notably increased? – IgE is responsible for anaphylactic hypersensitivity and allergic reactions. It is notably increased in individuals with allergic activity.

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15. Can antibodies function as enzymes, and what is one example of such activity? – Yes, antibodies can function as enzymes. One example is their ability to catalyze the synthesis of ozone (O3), which has microbicidal activity.

 

Summary: Immunoglobulins Tutorial

The Immunoglobulins tutorial provides an in-depth exploration of the structure, classes, and functions of these essential proteins, commonly known as antibodies. Here’s a concise summary:1. Immunoglobulins (IG):
  • Immunoglobulins, or γ-globulins, are antibodies constituting approximately 20 percent of all plasma proteins. They are produced by plasma cells and lymphocytes.

2. Structure of Immunoglobulins:

  • Immunoglobulins are glycoproteins composed of light (L) and heavy (H) polypeptide chains. They share a fundamental ‘Y’ shape with four interconnected chains, and the L chain can be either Kappa (κ) or Lambda (λ). Heavy chains, designated by Greek letters, include Alpha (α), Gamma (γ), Delta (δ), Mu (μ), and Epsilon (ε).

3. Immunoglobulin Classes:

  • IgG (Gamma): Monomeric, with two antigen-binding sites, representing 70 percent of serum immunoglobulins. Crosses the placenta to protect the fetus.
  • IgA (Alpha): Second most abundant, monomer or dimer, found in external secretions, preventing microbial attachment.
  • IgM (Mu): Pentamer crucial for the initial immune response, especially against bacteria and viruses.
  • IgD (Delta): Monomer resembling IgG, present on B lymphocytes with an unknown antibody function.
  • IgE (Epsilon): Monomeric, involved in hypersensitivity reactions and defense against parasites.

4. Functions of Immunoglobulins (Antibody):

  • Primary function is defense against infectious agents.
  • Additionally, antibodies can act as enzymes, catalyzing the synthesis of microbicidal ozone (O3).

5. Disorders of Immunoglobulins:

  • Abnormal quantities of specific immunoglobulins may be present in various human diseases.
  • Deficiency of γ-globulins is observed in rare hereditary diseases.

6. Multiple Myeloma:

  • Plasma cell cancer resulting from abnormally high concentrations of serum immunoglobulins, typically IgG or IgA.

7. Amyloidosis:

  • Accumulation of immunoglobulin light chain fragments in tissues, associated with multiple myeloma patients.

8. Bence Jones Proteins:

  • Excess production of light chains in some myelomas, entering the bloodstream and appearing in urine.
  • Precipitates on heating urine and redissolves above 80 °C, characteristic of “light chain disease.”

This comprehensive tutorial offers valuable insights into the diverse roles and characteristics of immunoglobulins, from their structural aspects to their pivotal functions and associated disorders.

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