Antibody: Definition, Structure & Function

Introduction

Antibodies, also known as Immunoglobulin are glycoproteins produced by the immune system to protect our body from pathogenic infections.

• It is a large Y-shaped protein produced by the immune system in response to the presence of foreign substances in the body, such as pathogens (e.g., bacteria, viruses), toxins, or other antigens. Antibodies are a crucial component of the adaptive immune system and play a central role in defending the body against infections and other threats.
• The primary function of antibodies is to recognize and bind to specific antigens, which are molecules or structures on the surface of foreign invaders. Each antibody is highly specialized to recognize a particular antigen, and the body can produce a vast array of different antibodies to combat various pathogens.

Function of Antibodies

The first function is to recognize and bind to the foreign material(antigen) - antigenic determinants and thereby,

Neutralization: Antibody binds to antigens on the surface of pathogens that interact with host proteins, and blocks the receptors that the pathogen uses to gain entry into a cell. Antibody-pathogen complexes are then eliminated, often after phagocytosis.

Opsonization: Antibodies can act as signals to the immune system by coating antigen and interact with Fc receptors expressed by phagocytic cells. Thus, phagocytic cells (e.g., macrophages and neutrophils) recognize and engulf the marked pathogens.

Complement Activation: Antibodies can trigger the complement system, a group of proteins that enhance the immune response. Activation of the complement system can lead to the lysis (bursting) of pathogens and the recruitment of immune cells.

Antibody-dependent cell-mediated cytotoxicity(ADCC): In ADCC, antibodies bind to infected or abnormal cells, for destruction by natural killer (NK) cells and other immune cells.
(e.g., viral proteins on the surface of an infected cell), inducing apoptosis of the target cell.

Different Types of Antibodies and their Function
There are several different types of antibodies, each with its unique structure and function. Here, we will explore the main types of antibodies and their functions:

IgG (Immunoglobulin G):

• IgG is the most abundant antibody in the human body, accounting for about 75-80% of all antibodies.
• It is highly versatile and is involved in various immune responses, including neutralizing toxins, viruses, and bacteria.
• IgG can cross the placenta, providing passive immunity to newborns.
• It also plays a role in opsonization, where it marks pathogens for destruction by phagocytes.

IgM (Immunoglobulin M):

• IgM is the first antibody class produced during an initial infection, and it is primarily found in circulation.
• It forms pentameric structures that make it efficient at agglutinating (clumping) pathogens.
• IgM is essential for the early stages of immune responses and activates the complement system.

IgA (Immunoglobulin A):

• IgA is primarily found in mucous membranes, saliva, tears, and breast milk.
• It provides defense at the body's mucosal surfaces and prevents pathogens from entering the body.
• IgA is important for protecting against infections in the respiratory and gastrointestinal tracts.

IgD (Immunoglobulin D):

• IgD is found in small amounts in the blood and on the surface of B cells.
• Its precise function is not well-understood, but it is believed to play a role in activating B cells and the adaptive immune response.

IgE (Immunoglobulin E): 

• IgE is involved in allergic reactions and defense against parasites.
• It is found in low concentrations in the blood but can increase significantly during allergic responses.
• IgE binds to mast cells and basophils, triggering the release of histamines and other inflammatory mediators in response to allergens.

Structure of Antibody
Antibody or immunoglobulin(Ig) is a tetrameric glycoprotein molecule consisting of two identical heavy chains(H) of mol. Wt 50-73 kDa each and two identical light chains(L) of mol. Wt 25kDa each.

L chains is of two types -  kappa (𝛋) and lambda (𝝺)

H chains is of 5 types -

• Alpha (𝝰)
• Gamma (𝛄)
• Delta (𝝳)
• Epsilon (𝛆)
• Myu (𝝁 )

Each L chain is made up of 214 amino acids

And H chain ranges from 246 to 576 amino acids

The H chain and L chain interact at various domains to produce an overall Y-shaped structure 3-D structure in which the two arms of Y are connected to the base by hing region.

Production of Antibodies

Antigen Recognition: When the immune system encounters foreign invaders like bacteria, viruses, or other pathogens, specialised immune cells (B cells) recognize specific antigens on the surface of these invaders.

B Cell Activation: Upon recognizing the antigen, B cells become activated. These activated B cells undergo a process called clonal selection and begin to divide rapidly

Plasma Cell Formation: Some of the activated B cells differentiate into plasma cells. Plasma cells are antibody-producing factories

Antibody Production: Plasma cells synthesize and release antibodies, Y-shaped proteins. Each antibody is specific to a particular antigen and has binding sites that can attach to that antigen

CLICK HERE to download PDF