Immunoglobulins (Antibodies): Structure, Characteristics and Functions

Structure of Antibodies

Structure and Functions of Immunoglobulins (Antibodies)
The Organization of Heavy and Light Chains in an Immunoglobulin (Ig)

In the previous post, we have discussed the introductory features of antibodies. There we have also discussed the reason for calling Antibodies (Ab) as Immunoglobulins (Ig).  In this lesson, we will see the detailed molecular structure and organization of immunoglobulins (antibodies).

Immunoglobulin ShapeWhat are Immunoglobulins (Ig)?

As we discussed earlier, the Antibodies or Immunoglobulins are globular proteins present in the serum and tissue fluids. They are produced by the plasma cells (B-cells) and are used in the immune system of the body to neutralize pathogenic microbes or other toxic foreign components. Antibodies play a very crucial role in the immune system of an organism. Antibodies bind to definite molecules of microbes called antigens with high affinity and specificity. This enables our immune system to detect foreign organisms such as invading pathogens, of its products and initiate the mechanism to eliminate these foreign particles. The production of antibodies by the plasma cells is also stimulated by the antigens.  

How Immunoglobulins (Ig) are classified?

The immunoglobulins constitute about 20 – 25% of the total serum proteins. Based on the Physiochemical and Antigenic differences, the immunoglobulins are classified into FIVE categories. These immunoglobulins variants are called as Isotypes. The five isotypes or classes of the immunoglobulins are given below.

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Difference between Antibody and Immunoglobulin

Antibodies vs Immunoglobulins

Antibodies vs Immunoglobulins
(Why are Antibodies called as Immunoglobulins?)

All Antibodies are Immunoglobulins but all Immunoglobulins are NOT Antibodies.

Antibodies are the antigen binding proteins found on the B-cell membrane and secreted by the plasma cells of the immune system. Antibodies are commonly called as ‘IMMUNOGLOBULINS’. In the present post we will see, what is the exact difference between an Immunoglobulin and an Antibody, and also why antibodies are called immunoglobulins?

What are Antibodies?

Ø  Antibodies are globular proteins (globulins) present in the serum and tissue fluids.

Ø  They form one of the major components of the blood plasma proteins.

Ø  In the blood, three types of globulin proteins are present and they are named as alpha (α), beta (β) and gamma (γ) globulins.

Ø  All antibodies are gamma (γ) globulins.

Ø  Antibodies confer protection against microbial pathogens and, they act as the first line defense against infections.

Ø  Antibodies are highly specific and they specifically bind to foreign particles called antigens.

Ø  The antibodies can protect us from the invading microbes in four different ways:

1.  They can prevent the attachment of microbes to the mucosal surface of the host.

2.  They reduce the virulence of the pathogen by neutralizing the toxins and viruses.

3.  They facilitate phagocytosis by opsonization of microbes.

4.  They can activate the complement system.

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Difference between Active Immunity and Passive Immunity – Comparison Table

Compare active and passive immunity

Active Immunity vs Passive Immunity
(Similarities and Differences between Active and Passive Immunity)

The acquired immunity is the immunity acquired by an organism during its life. The acquired immunity against a particular microbe may be induced by the host’s response to the microbe or by the transfer of antibodies or lymphocytes specific for the microbes. Based on the above criteria, the acquired immunity is categorized into two types – (1) Active Immunity and (2) Passive Immunity.

Active Immunity: The active immunity is the direct response of your body against the pathogens. It is induced by the exposure to a foreign antigen such as the antigen of microbes. It is an adaptive response of the individual after contact with specific pathogen or antigen.

what is active immunity

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Difference between Cell-mediated and Humoral Immunity (Comparison Table)

Compare Cell-mediated & humoral

Cell-mediated vs Humoral Immunity
(Similarities and Differences between Cell-Mediated and Humoral Immunity)

The immunity induced in an organism by the exposure of a foreign antigen is called Active Immunity. The active immunity is mediated through two distinct mechanisms, and they are named as (1) Cell-mediated immunity and (2) Humoral immunity. These two immune pathways show considerable differences in their components, their targets, and the method of killing of pathogens.

Cell-mediated immunity: The cell-mediated immunity is facilitated by the activated TH cells (T-Helper cells) and Cytotoxic T Lymphocytes (CTLs). Cytokines secreted by the TH cells activate the phagocytic cells. These activated phagocytic cells then phagocytosis and kill the microbes. The cell-mediated immunity is particularly important against the bacterial and protozoan pathogens.

Humoral Immunity (antibody-mediated immunity): The Humoral immunity is mediated through antibodies. Antibodies are produced by the B cells. These antibodies bound to specific microbial antigens. Binding of antibodies to antigens neutralize the microbes and target them for elimination by various effector mechanisms. The humoral immunity is the major defense mechanism against the extracellular microbes trying to invade the host systems.

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Difference between Type I and Type II Interferons (INF Alpha/Beta vs INF Gamma)

Difference between INF alpha beta and gamma

Type I Interferons vs Type II Interferons
(Similarities and Differences between Type I and Type II Interferons)

Interferons or INFs are glycoproteins with indirect antiviral properties. They are cytokines whose production in the cell is induced by viral infections. Interferons stimulate the synthesis of antiviral proteins in the cells which inhibit the multiplication of viral particles.  

Learn more: Interferons: Definition, Properties, Antiviral Properties and Clinical Uses

Interferons are classified into TWO categories based on their type of Receptors. They are Type I Interferons (INFα and INFβ) and Type II Interferons (INFγ). The present post describes the Similarities and Differences between Type I and Type II interferons.

Similarities between Type I and Type II Interferons

Ø  Both type I and type II interferons are short glyco-proteins.

Ø  Both possess indirect antiviral properties.

Ø  The production of both types of interferons is triggered by viral infections.

Ø  Both can elicit immunological responses in the organ.

Ø  Both types operate through receptors present on the plasma membrane.

Ø  Both can trigger the production of antiviral proteins.

Difference between Type I and Type II Interferons

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