An antibody is defined as a protein that is produced by B lymphocytes in response to the presence of an antigen. Antibody is gamma globulin in nature and it is also called immunoglobulin (Ig). Immunoglobulins form 20% of the total plasma proteins. Antibodies enter almost all the tissues of the body.

Types of Antibodies

Five types of antibodies are identified:

1. IgA (Ig alpha)

2. IgD (Ig delta)

3. IgE (Ig epsilon)

4. IgG (Ig gamma)

5. IgM (Ig mu).

Among these antibodies, IgG forms 75% of the antibodies in the body.

Structure of Antibodies

Antibodies are gamma globulins with a molecular weight of 1,50,000 to 9,00,000. The antibodies are formed by two pairs of chains, namely one pair of heavy or long chains and one pair of light or short chains. Each heavy chain consists of about 400 amino acids and each light chain consists of about 200 amino acids. Actually, each antibody has two halves, which are identical. The two halves are held together by disulfide bonds (S–S). Each half of the antibody consists of one heavy chain (H) and one light chain (L). The two chains in each half are also joined by disulfide bonds (S – S). The disulfide bonds allow the movement of amino acid chains. In each antibody, the light chain is parallel to one end of the heavy chain. The light chain and the part of heavy chain parallel to it form one arm. The remaining part of the heavy chain forms another arm. A hinge joins both the arms

Each chain of the antibody includes two regions:

1. Constant region

2. Variable region.

1. Constant Region

Amino acids present in this region are similar in number and placement (sequence) in all the antibodies of each type. So, this region is called constant region or Fc(Fragment crystallizable) region. Thus, the identification and the functions of different types of immunoglobulins depend upon the constant region. This region binds to the antibody receptor situated on the surface of the cell membrane. It also causes complement fixation. So, this region is also called the complement binding region.

2. Variable Region

Variable region is smaller compared to constant region. Amino acids occupying this region are different in

number and placement (sequence) in each antibody. So, it is called the variable region. This region enables the antibody to recognize the specific antigen and to bind itself with the antigen. So, this region of the chain is called antigen-binding region or Fab (Fragment antigen binding) region.

Functions of Different Antibodies

1. IgA plays a role in localized defense mechanism in external secretions like tear

2. IgD is involved in recognition of the antigen by B lymphocytes

3. IgE is involved in allergic reactions

4. IgG is responsible for complement fixation

5. IgM is also responsible for complement fixation.

Mechanism of Actions of Antibodies

Antibodies protect the body from invading organisms in two ways :

1. By direct actions

2. Through complement system.

1. Direct Actions of Antibodies

Antibodies directly inactivate the invading organism by any one of the following methods:

i. Agglutination: In this, the foreign bodies like RBCs or bacteria with antigens on their surfaces

are held together in a clump by the antibodies.

ii. Precipitation: In this, the soluble antigens like tetanus toxin are converted into insoluble forms

and then precipitated.

iii. Neutralization: During this, the antibodies cover the toxic sites of antigenic products.

iv. Lysis: It is done by the most potent antibodies. These antibodies rupture the cell membrane of

the organisms and then destroy them.

2. Actions of Antibodies through

Complement System

The indirect actions of antibodies are stronger than the direct actions and play more important role in defense mechanism of the body than the direct actions. Complement system is the one that enhances or

accelerates various activities during the fight against the invading organisms. It is a system of plasma

enzymes, which are identified by numbers from C1 to C9. Including the three subunits of C1 (C1q C1r C1s), there are 11 enzymes in total. Normally, these enzymes are in inactive form and are activated in three ways:

a. Classical pathway

b. Lectin pathway

c. Alternate pathway.

a. Classical pathway

In this the C1 binds with the antibodies and triggers a series of events in which other enzymes are activated in sequence. These enzymes or the byproducts formed during these events produce the following activities:

i. Opsonization: Activation of neutrophils and macrophages to engulf the bacteria, which are bound

with a protein in the plasma called opsonin.

ii. Lysis: Destruction of bacteria by rupturing the cell membrane.

iii. Chemotaxis: Attraction of leukocytes to the site of antigen-antibody reaction.

iv. Agglutination: Clumping of foreign bodies like RBCs or bacteria.

v. Neutralization: Covering the toxic sites of antigenic products.

vi. Activation of mast cells and basophils, which liberate histamine: Histamine dilates the blood

vessels and increases capillary permeability. So, plasma proteins from blood enter the tissues

and inactivate the antigenic products.

b. Lectin pathway

Lectin pathway occurs when mannose-binding lectin (MBL), which is a serum protein binds with mannose or fructose group on wall of bacteria, fungi or virus.

c. Alternate pathway

Complementary system is also activated by another way, which is called alternate pathway. It is due to a protein in circulation called factor I. It binds with polysaccharides present in the cell membrane of the invading organisms. This binding activates C3 and C5, which ultimately attack the antigenic products of invading organism.

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