ANTIBODY

Prepared By: Ms. Hira Sakha 

Edited and Updated By: Mr. Bikram Malla

ANTIBODY

Antibodies are the immune system related proteins called immunoglobulins. They are secreted by B cells. They have antigen binding site which is identical to those of B cell membrane antigen receptor. In a schematic figure, the antibody molecule is Y-shaped in structure with two identical antigen binding regions at the tip of Y. All the antibodies share a common structure of four polypeptides i.e two heavy chain and two light chain. Each light chain is bound to its partner heavy chain by a disulphide bond. Light Chain (L) consist polypeptides about 22,000 Da and heavy chain with larger polypeptide of 50,000 Da or more.

There are five types of Ig heavy chain denoted by greek letters: α, δ , ε, µ and γ and two types of Ig light chain denoted by λ and κ. The remainder of each chain in the molecule- the constant region- exhibits the limited variation that defines two light chain subtype and five heavy chain subclasses.

Hinge Region: The γ, δ, and α heavy chain contain an extended peptide sequence between CH1 and CH2 domain that has no homology with other domain. It is rich in proline residues, rendering it particularly flexible, as a consequence, the two
antigen binding arms of antibodies can assume wide variety of angles that facilitates efficient antigen binding.

Carbohydrate chains: Immunoglobulin are glycosylated and some antibodies even have carbohydrate attached to their light chain.

Types of antibody

5 types –IgG, IgM, IgA, IgD, IgE.(classified according to type of heavy chain constant region,and are distributed differently in the body)

Pictures of different types of antibody (Source BioRender; Click to enlarge)

Properties	IgM pentamer	IgG monomer	Secretory IgA dimer	IgE monomer	IgD monomer
Heavy Chain	µ	γ	α	ε	δ
Sub class/es		IgG1, IgG2, IgG3 and IgG4	IgA1 and IgA2
Half-life in days	5	23 days except IgG3 (8 days)	6	2.5	3
Number of antigen binding sites	10	2	4	2	2
Molecular weight (Daltons)	900,000	150,000	385,000	200,000	180,000
Percentage of total antibody in serum	6%	80%	13%	0.002%	1%
Crosses placenta	No	Yes	No	No	No
Complement Fixation	Yes	Yes	No	No	No
Fc binds to		phagocytes		mast cells and basophils
Function	Main antibody of primary responses, best at fixing complement; the momomer form of IgM serves as the B cell receptor.	Main blood antibody of secondary responses, neutralizes toxins, opsonization	Secreted in mucus, tears, saliva, and colostrum	Antibody of allergy and anti-parasitic activity.	B-cell receptor