Explore Types of Antibodies in Blood, Their Structures & Functions

When a foreign substance such as a toxin or a pathogen enters the body, circulating B cells recognize certain cell-surface antigenic molecules and bind to them. This process causes the B cells to divide rapidly and form a group of identical mature B cells called a clone.

These mature B cells are called Plasma cells which then secrete millions of antibodies into the bloodstream, thus activating the humoral immune response. As these antibodies circulate through the body, they encounter and recognize specific antigens, aiding in their elimination.

Although there are different classes of antibodies, that is discussed further on in the article, the general structure of all antibodies is the same.

• They are fundamentally heavy globular proteins found in the plasma. The antibody molecule comprises four polypeptide chains – two heavy chains and two light chains.
• Each heavy chain comprises large polypeptides of around 50,000 Da. There are five types of heavy chains – Alpha, Delta, Epsilon, Gamma, and Mu. Each light chain is made up of polypeptides of around 20,000 Da.
• There are two types of light chains – Lambda and Kappa. The heavy and light chains are held together by disulfide bonds, giving the structure of the antibody molecule a Y shape.
• The portion of the heavy and light chains that contact the antigen is called the variable region. It consists of 100-110 amino acids that differ in each antibody molecule depending on the antigen encountered.

Due to this variability, this region is known as the variable region. The other portion of the heavy and light chains that define their subtypes is known as the constant region, and the amino acid content of this region remains constant for each subtype of the antibody molecule.

Properties:

• IgG is the most predominant antibody found in the body and constitutes 80% of the total antibody content in the serum.
• It is the only antibody that can cross the placental membrane and provide immunity to the fetus. There are four sub-classes of the IgG molecule: IgG1, IgG2, IgG3, and IgG4.
• Among these, IgG 3 and IgG 4 possess the ability to cross the placenta. IgG1 is the most common antibody subclass among the four.

Structure:

• The heavy chains of the IgG antibody are of the subclass Gamma, with two antigen binding sites. It is a monomer with a molecular weight of approximately 150,000 Da.

Functions:

• It provides immunity to the developing fetus.
• Activates the complement pathway of the immune response.
• Mediates a process known as opsonization which refers to antibodies coating a pathogenic cell to lure the phagocyte toward the antigenic surface of the pathogen.
• Facilitates the process of phagocytosis.
• Neutralization of toxins and pathogens.
• Since it is widely found circulating in the blood, it offers protection against pathogens in the blood and tissues.

Properties:

• IgA is found in the mucous membranes of the gastrointestinal and respiratory tracts. It contains mucus secretions, saliva, tears, and the colostrum.
• It constitutes 13% of the total antibody content found in the serum. There are two subclasses of the IgA antibody – IgA1 and IgA2.
• The IgA1 antibody, also called secretory immunoglobulin or sIgA, is most commonly found in secretions in high quantities.

Structure:

• The heavy chains of the IgA antibody are of the Alpha subclass, with four antigen binding sites.
• The molecular weight of IgA is around 385,000 Da. IgA exists in both monomeric and dimeric forms.

Functions:

• IgA is found in the secretions and provides the first line of defense against the uptake of microbes and antigens into the body.
• It limits inflammation.
• Participates in the immune response by activating the complement pathway.
• Provides immunity to the fetus and the newborn infant.

Properties:

• IgM is the largest antibody found in the body and is the first produced after an antigen enters the body.
• It is found in the blood and the lymph fluid. It constitutes 6% of the total antibody content of the serum.

Structure:

• The heavy chains of the IgM antibody are of the Mu subclass, and it has ten antigen-binding sites.
• The molecular weight of IgM is around 900,000 Da. IgM exists in the pentameric form and is the largest of all the antibodies.

Functions:

• IgM is the antibody found on the surface of the B-cell that helps in antigenic recognition.
• Activation of the complement pathway of the immune response.
• It is involved in opsonization and agglutination.
• Facilitates efficient immune system activation due to the more significant number of antigenic sites on its surface.
• The ABO system of blood grouping consists of IgM antibodies specific to the ABO antigens expressed on the surface of the RBCs.

Properties:

• IgE antibody, also known as the reaginic antibody, is involved in hypersensitivity reactions or allergic responses.
• It is found in the linings of the respiratory and intestinal tracts.
• It is the least abundant antibody, constituting about 0. 002% of the antibody content in serum.
• The antigenic site binds to mast cells or basophils that are known to be involved in hypersensitivity reactions.

Structure:

• The heavy chains of the IgE antibody are of the subclass Epsilon with two antigenic binding sites.
• It exists in the monomeric form and has a molecular weight of about 200,000 Da.

Function:

• Immune protection against pathogens is invading through the gastrointestinal or respiratory barriers.
• It is vital in Type 1 hypersensitivity or allergic responses.

Properties:

• IgD antibody makes up less than 1% of the total antibody content of serum.
• It is usually co-expressed on the surface of B cells with IgM.
• Its specific function is still unknown. However, it is thought to be involved in B cell activation.

Structure:

• The heavy chains of the IgD antibody comprise the subclass Delta, with two antigen binding sites.
• It is found in the body as a monomer and has a molecular weight of around 180,000 Da.
• It is present on the surface of the B cell and acts as a receptor.

Function:

• The known function of the IgD antibody is to act as a receptor on the surface of the B cell and participate in B cell activation and differentiation.