What is Antibodies — Types, Functions, Importance

Definition :

Antibodies are immunoglobulins which are produced in the body in response to the antigen Are Foreign bodies.

Thus all antibodies are immunoglobulins but all immunoglobulins are not antibodies.

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Loca­tion and formation :

The anti­bod­ies May be bound to a cell mem­brane or they May remain free.

Anti­bod­ies are pro­duced by B lym­pho­cytes and plas­ma cells. 

In fact B lym­pho­cytes get trans­formed into plas­ma cell . 

The mature plas­ma cell pro­duced anti­bod­ies at an extreme­ly rapid rate — about 2000 mol­e­cules per second .

Anti­bod­ies direct the anti­body medi­at­ed immu­ni­ty ( = humoral immunity, )

Types of Antibodies : 

There are five types of antibodies viz:

1. Ig A (Ig alpha)
2. Ig D (Ig delta)
3. Ig E (Ig epsilon)
4. Ig G (Ig gamma)
5. Ig M (Ig mu)

Among tye anti­bod­ies , IgG forms 80 % of the anti­bod­ies in the body.

Anti­body Structure :

Ig G has been stud­ied exten­sive­ly and serves as a mod­el of basic struc­tur­al unit of all Igs.

An anti­body mol­e­cule con­sists of the fol­low­ing parts.

(I) Heavy and Light Chains 

An anti­body mol­e­cule is made up of 4 pep­tide chains , two small called light chain and two longer called heavy chain.

Hence an anti­body is rep­re­sent­ed as H₂L_2.

The heavy chain has larg­er num­ber of aminoacids while light chain has small­er num­ber of amino acids.

Heavy and light chains May chains May be either lamb­da or Kap­pa type.

(ii) Con­stant and Vari­able Regions

There are two dif­fer­ent regions the con­stant region and vari­able region in each chain of the antibody.

(iii) Disul­fide Bonds and Hinge Region

A disul­fide bond joins a light chain with a heavy chain.

Two disul­fide bonds also link the two heavy chains.

This part of the anti­body dis­plays con­sid­er­able flex­i­bil­i­ty ad is called the hinge region. Because the anti­body “arms” can move some­what as the hinge regions bends, an anti­body can assume a Y shaped molecule.

(iv) Frag­ment Anti­gen Bind­ing (Fab) and Frag­ment Crys­tal­liz­able (Fc).

Two iden­ti­cal frag­ments of Y shaped mol­e­cule poss­es the anti­gen bind­ing sites and are thus named frag­ment anti­gen bind­ing (Fab).

The anti­gen bind­ing sites bind to the spe­cif­ic anti­gens in a lock and key pat­tern, form­ing an anti­gen anti­body complex.

The third frag­ment which lacks the abil­i­ty to bind to anti­gen and can be crys­tal­lized, is , there­fore known as frag­ment crys­tal­liz­able (Fc).

The stem of. The Y shaped anti­body monomer is called the Fc region, so named because when anti­body struc­ture was first being iden­ti­fied , it was a frag­ment (F) that crys­tal­lized © in cold storage.

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Char­ac­ter­is­tics and Func­tions of Immunoglobulins

Anti­bod­ies show that fol­low­ing Char­ac­ter­is­tics and. Per­form dif­fer­ent functions.

(I) IgA

It is the sec­ond most abun­dant class, con­sti­tut­ing about 10 to 15 per­cent of anti­bod­ies of serum. 

It is main­ly found in Sweat ‚Tears ‚sali­va ‚mucus colostrum (first milk secret­ed by a moth­er) and gas­troin­testi­nal secretions.

Small­er quan­ti­ties are present in blood and lymph. 

Ig A Has an extra polypep­tide called change join­ing chain and extra pro­tein known as secre­to­ry com­po­nent. Lev­els decrease dur­ing stress, low­er­ing resis­tance to infec­tion . pro­vides localised pro­tec­tion in exter­nal secre­tions ( tears, intesti­nal secre­tions etc) against bac­te­ria and viruses . 

When IgA is excret­ed through fae­ces, it is called Coproantibody.

(ii) Ig D

It is main­ly found on the sur­face of B cells as anti­gen recep­tors, where it acti­vates B cells for anti­gen recog­ni­tion. It is about 0.2 % of all anti­bod­ies in the blood.

(iii) Ig E

It is less than 0.1 % of all anti­bod­ies in the blood ; Locat­ed on mast cell and basophils releas­ing his­t­a­mine from mast cells and basophils.

it is involved in aller­gic and hyper­sen­si­tiv­i­ty reac­tion; pro­vides pro­tec­tion against worms.This immunoglob­u­lins was dis­cov­ered in 1966 by Ishiza­ka. It exhibits unique prop­er­ties such as hard heat lia­bil­i­ty (in acti­vat­ed at 56°C in one hour). 

IgE medi­ates Type 1 hyper­sen­si­tiv­i­ty (ana­phy­lax­is). Praus­nitz and Kust­ner demon­strat­ed trans­mis­sion of IgE medi­ates type I hyper­sen­si­tiv­i­ty. It is called Praus­nitz Kust­ner (PK). Thus IgE acts as medi­a­tor in all aller­gic reaction.

(iv) Ig G

The most abun­dant class of IG in the body con­sti­tut­ing approx­i­mate­ly 80% of total Igs. 

It is found in the blood , lymph and intes­tine . it pro­tects against bac­te­ria and virus­es by enhanc­ing phago­cy­to­sis, neu­tral­iz­ing tox­ins and com­ple­ment activation. 

It is the only class of anti­body to cross the pla­cen­ta from moth­er to foe­tus there­by con­fer­ring con­sid­er­able immune pro­tec­tion. In newborns.

(v) Ig M

IGM is about 5 to 10% off all anti­bod­ies in the blood. It is also found in lymph . it is the largest Ig which secret­ed first by the plas­ma cells.

it is named because it is a macro glob­u­lin at least five times larg­er than IgG.

IgM is the old­est immunoglob­u­lins class. It is acti­vates rhe B cells . it is also the ear­li­est immunoglob­u­lins to be syn­the­sized by the foetus , 

IgM has J chain and its each dimer con­tains polype­tide called a secre­to­ry component . 

It can­not cross the pla­cen­tal bar­ri­er. IgM is 500 — 1000 times more effec­tive than IgG in Opson­i­sa­tion (to be described ahead) , in bac­te­r­i­al action and in agglu­ti­na­tion. But in neu­tral­iza­tion of Tox­ins and virus­es , it is less active than IgG. It helps in Com­ple­ment activation.

Mon­o­clon­al Anti­bod­ies ( MAbs)

Pro­duc­tion.

1. A mouse is inject­ed with the spe­cif­ic anti­gen that will induce anti­bod­ies against the anti­gen the antigen.
2. The spleen of the mouse is removed and a sus­pen­sion is made . the sus­pen­sion include B cells that pro­duces anti­bod­ies against the inject­ed antigen.
3. The spleen cells are then mixed with myelo­ma cells (can­cer cells ) that are capa­ble of con­tin­u­ous growth in cul­ture but have lost the abil­i­ty to pro­duce anti­bod­ies. Some of the anti­body pro­duc­ing spleen cells and myelo­ma cells fuse to form hybrid cells . These hybrid cells are now capa­ble of grow­ing con­tin­u­osly in cul­ture while pro­duc­ing antibodies.
4. The mix­ture of cells is placed in a selec­tive medi­um. That allows only hybrid cells to grow.
5. Hybrid cells pro­lif­er­ate into clones called hybrido­mas. The hybrido­mas are screened for pro­duc­tion of the desired antibody.
6. The select­ed hybrido­mas are then cul­tured to pro­duce large quan­ti­ties of mon­o­clon­al anti­bod­ies (MAbs) because they come from a sin­gle clone of iden­ti­cal cells.

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The method of production of Monoclonal antibodies (MAbs) was described by Jerne, Kohler and Milstein in 1975 for which they were awarded Nobel prize for Medicine in 1984.

Uses :

They rec­og­nize sev­er­al bac­te­r­i­al pathogens, diag­no­sis of preg­nan­cy, aller­gies and dis­ease such as hepati­tis, rabies and some sex­u­al­ly trans­mit­ted diseases. 

MAbs have also been used to detect can­cer at an ear­ly stage and to know the extent of metastasis.

MAbs are also being used since 1986 to min­i­mize rejec­tion of kid­ney transplant . 

For these pur­pose MAbs are pre­pared that react with the T cell that are respon­si­ble for rejec­tion of the trans­plant­ed tis­sue. The MAbs sup­press the T cell activ­i­ty. They May also be used to treat autoim­mune diseases.