Saturday, 18 July 2020

'Basic Structure of an Antibody Molecule


 In our body there are two different modes of immune system, one is the cell mediated immune system, another one is known as a humoral immune system.

  Cell-mediated immunity involves T-lymphocytes. Cell mediated immunity fights against intracellular antigens. They kill infected cells tumor cells etc.

  But there are many pathogens which multiply in the extracellular spaces of the human body. These extracellular spaces are protected by B-lymphocytes. Which are responsible for humoral immunity.

Role of B lymphocytes in the basic structure of antibody

  B lymphocytes originated in bone marrow and they also complete their maturation. In the bone marrow after the maturation stage these cells are released into the blood. Mature B-cells keep recirculating between lymph, blood and secondary lymphoid tissues.

  When these mature B-cells recognise specific antigens they get activated. This recognition occurs via specific receptors present on B cells, which are known as B-cell receptors.

  Once activated these B cells proliferate and differentiate into  Memory b-cells and Plasma cells which are antibody secreting cells. Plasma cells produce and secrete antibodies specific to the antigen.

Antibodies

  Antibodies are glycoproteins. The basic structure of antibody resembles Y-shaped molecule having two antigen binding sites and a stock known as FC region.

   These antibodies circulate in the lymph and blood from there they reach the site of invasion by the pathogen. Antibodies bind to the pathogens or antigens and activate defense mechanisms that lead to the destruction of the pathogen.

  Antibodies are also known as immunoglobulins. this is because they belong to a group of glycoproteins known as globulins. The term immune reflects that these glycoproteins play major role in immunity.

Basic Antibody Structure

  All antibodies have a same core structure. It consists of four polypeptide chains
  - Two identical heavy chains designated as H chains and
  - Two identical light chains designated as L chains.

  Heavy chains of Antibody are the longer ones and light chains are the shorter ones. The term heavy and light refers to their molecular weights. Heavy chains have more molecular weights than light chains.

   Antibodies are polypeptides, so the N terminal of this polypeptide chain is present at the tip and C terminal is present at the base of each polypeptide chain.

Structure of an Antibody
   As per figure, image these chains are assembled into a Y-shaped structure.

• Each light chain is connected to a heavy chain via a disulfide bond.

• The heavy chains are connected to each other via two disulfide bonds.

• In the mid region besides these disulfide bonds a number of non covalent bonds are also present, which keep these chains together.

• The mid-region of antibody has considerable flexibility this region is known as the hinge region. Hinge region make possible the rotation and bending of antibody molecule. This hinge region of Antibody is reach in prolin amino acid.

• The stalk of this Y shaped antibody molecule is the stem region also known as FC region.

•  To each heavy chain short carbohydrate chains are attached. These carbohydrate chains serve many additional functions such as increasing the solubility of immunoglobulins.

Light Chains

   Light chains are the two shorter subunits of basic antibody molecule.
• Each light chain has a molecular weight of about 25 kD.
• Each chain contains about 220 amino acids.
• In humans there are two types of light chains
  -  Lambda(λ) chain and
  -  kappa(K) chains
Kappa chain is encoded on chromosome 2 and lambda chains are encoded on chromosome 22.
• Each antibody molecule produced by a B-cell will either have Kappa or lambda light chain but never both.
• In human 60% of light chains are Kappa and 40% are lambda.

Heavy Chains

heavy chains are the longer subunits of the antibody structure.
• Each heavy chain has a molecular weight of about 50 to 70 kD and each heavy chain contains about 440 amino acids.
• There are five types or classes of heavy chains in humans all encoded on chromosome 14.
• These five classes are designated by lowercase Greek letters gamma, alpha, mu, Delta and Epsilon. They are also returned as γ, α, μ, δ and ε respectively.

   Each light and heavy chain contained two distinct regions variable regions and constant regions.

Variable (V) region

• variable region refers to the first 110 amino acids of the N-terminal region
• In each heavy and light chain these regions are so called because the amino acid sequences in these regions have great variability.
• These regions are designated as VL in each light chain and VH in each heavy chain.
• It is the variable region of a light chain and a heavy chain, Which together form the antigen binding site
•  So there are two antigen binding sites in a core antibody molecule.
•  Variability of amino acid sequences is precisely organised.
•  These areas are called  hypervariable regions or complementarity determining regions (CDRs).
•  This is because these regions together form a structure which is complementary to the shape of specific antigen bound by the antibody.
• Scientists have found that there are three CDRs in the variable region of each chain. These are designated as CDR1, CDR2 and CDR3.
•  The intervening sequences between CDRs or hyper variable regions are known as framework residues. These intervening sequences have restricted variability. 
•  Antibodies can recognise diverse types of antigens because of these hypervariable regions

Constant (C) Region

   The region beyond the variable region of both heavy and light chain is known as constant region.
• They are so called because the amino acid sequence in these regions shows little variation among antibodies.
•  There is a single constant region in each light chain which is designated as CL.
•  Light chains are of two types Kappa chain and lambda chain. These chains differ from each other by minor differences in the constant region of light chain.
• Constant regions in heavy chains vary form 3 to 4.
• This depends on the antibody class and it is the constant region of heavy chain which forms the basis of this antibody classification.
•  In a particular class of antibody all antibodies have almost same constant region but constant region of one antibody class is different from the another class.
•  The constant regions of the heavy chains are designated as CH 1, CH 2, CH 3 and CH 4 starting from the N-terminal of the chain.
   Each variable and constant region in an antibody molecule also has at least one disulfide bond these are internal disulfide bonds.

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