Biology Today — May 2017

N N

VH VH N

VL

CL

Variable

regions (V)

Constant

regions (C)

Heavy chain (H)

N

VL

Light chain (L)

CL

C

S–S

S–S

S–S

CH 3

CH 2 CH 2

CH 3

CC

CH 1 Hinge

region

CH 1

S – S

Interchain

disulfide

bonds

Antigen-binding site Antigen-binding site

Fig.: Structure of antibody molecule

Types of Immunoglobulins

IgM

•    Largest antibody.

•    Produced early in the primary response.

•    Most efficient in agglutination, complement fixation.

IgD

•    Functions as antigen receptor on surface of B cells.

•    Present on surface of B-cells and in blood and lymph.

IgE

•    Least abundant.

•    Bound to mast cells and basophils throughout the body.

•    Mediate immediate hypersensitivity reactions.

•    Involved in allergic reactions.

IgG

•    Most abundant, found in blood, lymph and

intestine.

•    Predominant antibody in the secondary response.

•    Only antibody to cross the placenta.

•    Opsonizes i.e., enhance phagocytosis, neutralises

toxins,fixes complement.

IgA

•    Second most abundant antibody.

•    Found in body secretions including saliva and tears.

•    Available in colostrum (first milk secreted by a

mother).

•    Localised protection in external secretions.

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