Antibody

An antibody (Ab), also known as an immunoglobulin (Ig),

is a large Y-shape glycoprotein produced by plasma cells (Type of B lymphocytes),

In the peripheral immune organs (spleen, Lymph nodes and bone marrow) to the blood circulation where is used by the immune system to:

 1-Inactivate and neutralize toxins

2-Iidentify and neutralize foreign objects such as viruses(consider as soluble antigens)

3-Iidentify and entangling bacteria prevent its spreading.

4- Opsonizing effect: enhance phagocytosis by changing bacterial – ve charge to be +ve charge make it more palatable for phagocytic cell.

As protein an Antibody consists 12 polypeptide units (domains) linked together by disulphid bonds (every unit has 110 amino acids) arranged in 4 chains of two types  

2 Heavy chains each one of 4 polypeptides units (domains)

2 Light chain each one of 2 polypeptides units (domains)

Under Electron microscope Y shape appear as follow:

Heavy chain consists of:

1-3 constant domains called CH1, CH2, CH3 with similar A.A

2-1 variable domain called VH with variable A.A

Light chain consists of:

1-1 constant domains called CL with similar A.A

2-1 variable domain called VL with variable A.A

The specificity of AB toward specific antigen refer to its specific part lies between HV and LV

Further analysis to AB is done by proteolytic enzyme called Papain that breakdown the Ab into 3 parts

2 FAB (Fragment Antigen Binding) portions (each one consists of one arm of light chain and half of heavy chain) form 2 arms of Y shape responsible binding specific Antigen

1 FC portion (consists of rest of heavy chain only) responsible about biological activity

And in between is the Hing region lies between CH1 and CH2 which in the midway between FAB and Fc portions,

composed of 20 A.A is like zipper offer high flexibility to FAB portion to bind antigen at any direction

Because of the difference in FAB portions according to the specific epitope (antigen) antibodies are classified into 5 types as follow:

GMADE

IgG

Monomeric, 180 K dalton, 2ndry produced AB, predominate and represent 85% of AB in circulation

The only passing AB to placenta as placenta has a receptor to its FC portion,

Additionally its FC portion has other receptor for complement that’s used in serological tests”

Precipitation, Complement Fixation, Agglutination and ELISA

IgM

Pentameric, 5 molecules connected together by disulphid bonds except one bond is J peptide chain, largest AB, 900 K Dalton, 1^st produced AB

Star shape

Important in diagnosis as

Presence of IgM represents an acute infection while Presence of IgG represent an chronic infection.

e.g in habitual abortion caused by Toxoplasmosis

Presence of IgM indicate acute infection and start treating while presence of IgG indicates that the women is vaccinated no treatment required 

IgA

Dimeric, 360 K Dalton, composed of two connected molecules via J peptide chain

Present locally in the sub mucosal layer of GIT and respiratory tract,

Secretory piece prevent its digestion from protolytic enzymes in GIT

It’s called as blocking AB as it blocks receptors against pathogen in GIT and other M.M

IgD

Unknown AB

From mature B lymphocytes in the bon marrow,

Its presence indicates B lymphocyte maturation.

IgE

Monomeric, 200 K Dalton, constant portion of its heavy chain consists of 4 domains not 3 as other AB,

Has specific affinity to bind to mast cells and basophils as mast cell has specific receptor to it.

So called cytophilic AB

Not found in the circulation as it bind directly to the mast cell

Responsible about hypersensitivity immune response

Has short life span 15-20 days then mast cell become free

In abnormal hypersensitive individual, plasma cell continue its production and once mast cells appear in the circulation,

It bind to it