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Antibodies (Immunoglobulins) Antigens (Immunogens)

1. Antibody is a protein molecule. Antigen is a protein or polysaccharide molecule.


2. It is synthesized to combat foreign material. It is usually a foreign material that stimulates antibody forma-
   tion.


3. Antibody occurs on the surface of a plasma cell
   and also in body fluids.

Antigen may occur on the surface of a microbe or as a free

molecule.

4. Antibody directly joins an antigen to destroy the
   latter.

Antigen binds to a macrophage to reach a helper T-cell and

initiate immune response.

Table : Differences between antibodies and antigens

• Most of the antibodies function as monomers. A few
(viz., IgA, IgM) can occur both as monomers and
polymers.

•    There are five classes of immunoglobulins – IgA (Ig

alpha), IgD (Ig delta), IgE (Ig epsilon), IgG (Ig

gamma) and IgM (Ig mu).

Characteristics IgM

pentamer

IgG

monomer

Secretory

IgA dimer

IgE

monomer

IgD

monomer

Number

of antigen

binding sites

10 2 4 2 2

Percentage of

total antibody

in serum

6% 80% 13% 0.002% 1%

Crosses

placenta

No Yes No No No

Fixes

complement

Yes Yes No No No

Function Main antibody of

primary responses, best

at fixing complement;

the monomer form of

IgM serves as the B-cell

receptor

Main blood antibody

of secondary

responses, neutralizes

toxins, opsonization

Secreted

into mucus,

tears, saliva,

colostrum

Antibody of

allergy and

antiparasitic

activity

B-cell

receptor

Location Blood, lymph,B- cell

surface (as monomer)

Blood, lymph, intes-

tine

Secretions

(tears, saliva,

mucus,

colostrum),

blood, lymph

Bound to

mast cells and

basophil cells,

blood

B cell sur-

face, blood,

lymph

Table : Immunoglobulin (Ig) classes