Structure, Types and Functions of Antibodies

Definition of Immunity

Immunity is defined as the body’s defence mechanism against antigens. It is achieved by two ways, i.e. cellular immunity and humoral immunity.

Type of Immunity	Mediated By
Humoral (Antibody Mediated)	B-Cells, Plasma Cells
Cellular (Cell Mediated)	T-Cells, Dendritic Cells, NK Cells, Macrophages

Antibodies

Antibodies are the antigen binding glycoprotein, present on the B-Cell membrane and secreted by Plasma Cells.

Antibodies are produced in the body in response of any foreign molecules.

Antibodies are also called Immunoglobulines because the found in globulin fraction of serum (specially in γ-fraction of globulin protein) Because antibodies are involved in Immune functions, so the complete term becomes

                 Immune + Globulin = Immunoglobulin

Structure of Immunoglobulins

(a) Antibody molecules have a common structure of four peptide chains.
• Heavy Chain (Constant and Variable chains) and
• Light Chain (Constant and Variable chains)
(b) One immunoglobulin molecule consists of two identical light (L) chains (each chain having molecular weight approx. 25 kDa)
(c) The heavy chain also has two identical light (L) chains (each chain having molecular weight approx. 50 kDa).
(d) Each light chain is bound to a heavy chain by a disulfide bond, and by such noncovalent interactions as salt linkages, hydrogen bonds, and hydrophobic bonds, to form a heterodimer (H-L).
(e) The first 110 or so amino acids of the upper terminal region of a light or heavy chain varies greatly in different antibodies.
These segments of highly variable sequence are called Variable regions both in heavy and light chain: VL in light chains and VH in heavy chain. These regions are responsible for antigen binding, hence the are called antigen binding site of the antibody molecule and this fraction is termed as “Fab” (Fraction for antigen binding).
(f) There are two sub-isotypes of α heavy chains—α1 and α2 and thus two subclasses of IgG antibody are found IgA1 and IgA2.
(g) There are four sub-isotypes of У heavy chains: У1, У2, У3, and У4 (therefore four subclasses were found as IgG1, IgG2, IgG3, and IgG4.
(h) The hinge region of an antibody is a short, flexible segment of the heavy chains that connects the Fab region to the Fc region and allowing the antibody to bend.

Classes of Antibodies on the basis of Heavy chain Sequences

Constant region of Heavy chain has five type of Amino acid sequences. On the basis of amino acid sequence of constant region of Heavy chain, the antibodies are classified into 5 types.

The constant region of the light chain of an immunoglobulin (antibody) exists in two distinct types, known as kappa (κ) and lambda (λ).

Each antibody molecule contains only one type of light chain, either kappa (κ) or lambda (λ), but never both together. These κ and λ light chains differ in their amino acid sequence of the constant region. In humans about 60–65% of antibodies contain kappa (κ) light chains and about 35–40% contain lambda (λ) light chains.

Type of Antibody	Amino acid sequence of Heavy chain constant region (Isotype)	Sub-isotype	Antibody subclass	Constant region of light chain
IgA	α	α1	IgA1	kappa (κ) or lambda (λ)
		α2	IgA2
IgG	γ	γ1	IgG1	kappa (κ) or lambda (λ)
		γ2	IgG2
		γ3	IgG3
		γ4	IgG4
		γ5	IgG5
IgM	µ	—	—	kappa (κ) or lambda (λ)
IgE	ε	—	—	kappa (κ) or lambda (λ)
IgD	δ	—	—	kappa (κ) or lambda (λ)

Types of Antibodies

Antibodies, also known as immunoglobulins, are specialized proteins produced by the immune system to identify and neutralize foreign substances like bacteria, viruses, and toxins. There are several types of antibodies, each with distinct functions and characteristics.

The main classes of antibodies in humans are:

1. Immunoglobulin G (IgG) :

Most abundant; crosses placenta.

• Function: IgG is the most abundant antibody in the blood and extracellular fluid. It plays a crucial role in the long-term protection against infections by recognizing and neutralizing pathogens.

• Charateristics
  • Monomeric Structure IgG is a single Y-shaped unit.
  • Complement ActivationIt can activate the complement system, which helps in destroying pathogens.
  • Crosses Placenta: IgG is the only antibody class that can cross the placenta, providing passive immunity to the fetus.

2. Immunoglobulin A (IgA)

• Function: IgA is primarily found in mucosal areas, such as the respiratory and gastrointestinal tracts, as well as in saliva, tears, and breast milk. It plays a critical role in mucosal immunity by preventing pathogens from adhering to and penetrating epithelial surfaces.
• Charateristics
  • Dimeric or Monomeric In secretions (like saliva and breast milk), IgA often exists as a dimer (two Y-shaped units connected), while in the blood, it is primarily a monomer.
  • Secretory Component:The dimeric form includes a secretory component that protects it from degradation in mucosal environments.

3. Immunoglobulin M (IgM)

• Function: IgM is the first antibody produced in response to an infection and is effective in the early stages of immunity. It is highly efficient at forming complexes with antigens and activating the complement system.
• Charateristics
  • Pentamer Structure IgM is composed of five Y-shaped units linked together, forming a pentamer and can has 10-antigen binding sites and can bind 10 small antigens (haptens). However, due to steric restrictions, only five large antigen molecules can be bound by one IgM molecule.
  • Primary Response:It is the predominant antibody in the primary immune response and is usually the first to be produced by the body when it encounters a new pathogen.

Immunoglobulin E (IgE)

• Function: IgE is involved in allergic reactions and defence against parasitic infections. It binds to allergens and triggers histamine release from mast cells and basophils, leading to inflammation and allergic symptoms.
• Charateristics
  • Monomeric Structure: Structure IgE is a single Y-shaped unit.
  • Allergy and Parasitic Défense:High levels of IgE are associated with allergic diseases such as asthma and hay fever.

Immunoglobulin D (IgD)

• Function: IgD is present in small amounts in the blood and is primarily found on the surface of immature B cells. Its exact role is less well understood, but it is believed to be involved in the initiation and regulation of immune responses.
• Charateristics
  • Monomeric Structure: Structure IgD is a single Y-shaped unit.
  • B Cell Receptor: It acts as a receptor on B cells, helping to regulate their activation and response to antigens.

Type of Ab	Function / Special Feature
IgG	• Neutralizes toxins and viruses • Opsonization (helps phagocytes recognize pathogens) • Activates complement system (classical pathway) • Provides long-term immunity (memory response) • Only antibody that crosses the placenta (passive immunity to fetus) • Most abundant in blood
IgA	• Mucosal immunity (prevents pathogen attachment to mucosal surfaces) • Secretory antibody found in respiratory & gastrointestinal tracts, saliva, tears, and breast milk
IgM	• First antibody produced in primary immune response • Strongly activates the complement system • Exists as a pentamer (binds up to 10 antigens) • Largest antibody
IgE	• Defense against parasites (especially helminths) • Mediates allergic reactions (binds to mast cells and basophils → histamine release)
IgD	• Functions mainly as a B cell receptor (BCR) • Role in initiating B cell activation • Lowest concentration in blood