structure,classification and function of antibodies

1. ANTIBODIES..!!!!
(IMMUNOGLOBULINS)
POOJA MISHRA
DEPARTMENT OF
MICROBIOLOGY

4. • ANTIBODIES ARE THE ANTIGEN BINDING PROTEINS PRESENT ON
THE B-CELL MEMBRANE AND SECRETED BY PLASMA CELLS.
• SECRETED ANTIBODIES CIRCULATE IN THE BLOOD, WHERE
THEY SERVE AS THE EFFECTORS OF HUMORAL IMMUNITY BY
SEARCHING OUT AND NEUTRALIZING ANTIGENS OR MARKING
THEM FOR ELIMINATION.
• ALL ANTIBODIES SHARE STRUCTURAL FEATURES, BIND TO
ANTIGEN, AND PARTICIPATE IN A LIMITED NUMBER OF
EFFECTOR FUNCTIONS.

5. • ANTIBODIES CAN BE MADE AGAINST
PROTEINS, CARBOHYDRATES, LIPIDS AND
NUCLEIC ACIDS.
• ANTIBODIES TO NUCLEIC ACIDS AND
LIPIDS CAN BE FOUND IN AUTOIMMUNE
DISEASES.
• ANTIBODIES TO SMALL ORGANIC
MOLECULES CAN CAUSE ALLERGIES TO
DRUGS

6. • ALTHOUGH DIFFERENT IMMUNOGLOBULINS CAN DIFFER
STRUCTURALLY, THEY ALL ARE BUILT FROM THE SAME BASIC
UNITS.
• A. HEAVY AND LIGHT CHAINS
• B. DISULFIDE BONDS.
• C. VARIABLE (V) AND CONSTANT (C) REGIONS.
• D. HINGE REGION.
• E. DOMAINS

9. • ALSO CALLED IMMUNOGLOBULINS CONSTITUTE THE GAMMA
GLOBULIN PORTION OF BLOOD PROTEINS.
• ANTIBODY MOLECULES HAVE A COMMON STRUCTURE OF FOUR
PEPTIDE CHAINS.
• THIS STRUCTURE CONSISTS OF TWO IDENTICAL LIGHT (L)
CHAINS, POLYPEPTIDES OF ABOUT 25,000 MOLECULAR WEIGHT,
AND TWO IDENTICAL HEAVY (H) CHAINS, LARGER
POLYPEPTIDES OF MOLECULAR WEIGHT 50,000 OR MORE.

10. • EACH LIGHT CHAIN IS BOUND TO A HEAVY CHAIN BY A
DISULFIDE BOND, AND BY SUCH NONCOVALENT INTERACTIONS
AS SALT LINKAGES, HYDROGEN BONDS, AND HYDROPHOBIC
BONDS, TO FORM A HETERODIMER (H-L).
• SIMILAR NONCOVALENT INTERACTIONS AND DISULFIDE
BRIDGES LINK THE TWO IDENTICAL HEAVY AND LIGHT (H-L)
CHAIN COMBINATIONS TO EACH OTHER TO FORM THE BASIC
FOUR-CHAIN (H-L)2 ANTIBODY STRUCTURE, A DIMER OF
DIMERS.

11. • THE FIRST 110 OR SO AMINO ACIDS OF THE AMINO-TERMINAL
REGION OF A LIGHT OR HEAVY CHAIN VARIES GREATLY AMONG
ANTIBODIES OF DIFFERENT SPECIFICITY.
• THESE SEGMENTS OF HIGHLY VARIABLE SEQUENCE ARE CALLED
V REGIONS.
• VL IN LIGHT CHAINS AND VH IN HEAVY.
• MOST OF THE DIFFERENCES AMONG ANTIBODIES FALL WITHIN
AREAS OF THE V REGIONS CALLED COMPLEMENTARITY-
DETERMINING REGIONS (CDRS), AND IT IS THESE CDRS, ON
BOTH LIGHT AND HEAVY CHAINS, THAT CONSTITUTE THE
ANTIGEN BINDING SITE OF THE ANTIBODY MOLECULE.

12. • THE REGIONS OF RELATIVELY CONSTANT SEQUENCE BEYOND
THE VARIABLE REGIONS HAVE BEEN DUBBED C REGIONS.
• CL ON THE LIGHT CHAIN AND CH ON THE HEAVY CHAIN.
• THE SITES OF ATTACHMENT FOR CARBOHYDRATES ARE
RESTRICTED TO THE CONSTANT REGION

13. • DIGESTION OF IGG WITH THE ENZYME PAPAIN PRODUCED THREE
FRAGMENTS, TWO OF WHICH WERE IDENTICAL FRAGMENTS AND
A THIRD THAT WAS QUITE DIFFERENT.
• EACH WITH A MW OF 45,000, HAD ANTIGEN-BINDING ACTIVITY
AND WERE CALLED FAB FRAGMENTS (“FRAGMENT, ANTIGEN
BINDING”).
• THE OTHER FRAGMENT (MW OF 50,000) HAD NO ANTIGEN
BINDING ACTIVITY AT ALL. BECAUSE IT WAS FOUND TO
CRYSTALLIZE DURING COLD STORAGE, IT WAS CALLED THE FC
FRAGMENT (“FRAGMENT, CRYSTALLIZABLE”).

15. • REPEATING DOMAINS OF ~110 A/A
• INTRACHAIN DISULFIDE BONDS WITHIN EACH DOMAIN
• HEAVY CHAINS
• 1 VH AND EITHER 3 OR 4 CH (CH1, CH2, CH3, CH4)
• LIGHT CHAINS
• 1 VL AND 1 CL
• HINGE REGION
• RICH IN PROLINE RESIDUES (FLEXIBLE)
• HINGE FOUND IN IGG, IGA AND IGD
• PROLINE RESIDUES ARE TARGET FOR PROTEOLYTIC DIGESTION (PAPAIN AND
PEPSIN)
• RICH IN CYSTEINE RESIDUES (DISULFIDE BONDS)
• IGM AND IGE LACK HINGE REGION
• THEY INSTEAD HAVE EXTRA CH4 DOMAIN

16. • THE HEAVY CHAINS OF A GIVEN ANTIBODY MOLECULE
DETERMINE THE CLASS OF THAT ANTIBODY: IGM(),
IGG(), IGA(), IGD(), OR IGE().
• EACH CLASS CAN HAVE EITHER K OR L LIGHT CHAINS .
• A SINGLE ANTIBODY MOLECULE HAS TWO IDENTICAL
HEAVY CHAINS AND TWO IDENTICAL LIGHT
CHAINS,H2L2, OR A MULTIPLE (H2L2)N OF THIS BASIC
FOUR-CHAIN STRUCTURE.
• MINOR DIFFERENCES IN THE AMINO ACID SEQUENCES
OF THE AND HEAVY CHAINS LED TO FURTHER
CLASSIFICATION OF THE HEAVY CHAINS INTO SUB
ISOTYPES THAT DETERMINE THE SUBCLASS OF
ANTIBODY MOLECULES.

17. • THE AMINO-TERMINAL HALF OF THE CHAIN, CONSISTING OF 100-110 AMINO
ACIDS, WAS FOUND TO VARY AND WERE CALLED VARIABLE (V) REGION.
• THE CARBOXYL-TERMINAL HALF OF THE MOLECULE, CALLED THE CONSTANT
(C) REGION, HAD 2 BASIC AMINO ACID SEQUENCES.
• THIS LED TO THE RECOGNITION THAT THERE WERE 2 LIGHT CHAIN TYPES,
KAPPA (K) AND LAMBDA (ƛ).
• IN HUMANS, 60 % OF THE LIGHT CHAINS ARE KAPPA AND 40 % ARE LAMBDA.
• IN MICE, 95% OF THE LIGHT CHAINS ARE KAPPA AND ONLY 5% ARE LAMBDA.
• A SINGLE ANTIBODY MOLECULE CONTAINS ONLY ONE LIGHT CHAIN TYPE,
EITHER ĸ OR ƛ, NEVER BOTH.
• THE AMINO ACID SEQUENCES OF LIGHT CHAINS SHOW MINOR DIFFERENCES
THAT ARE USED TO CLASSIFY LIGHT CHAINS INTO SUBTYPES.

18. • THE IMMUNOGLOBULINS CAN BE DIVIDED INTO FIVE DIFFERENT
CLASSES,
• 1. IGG - GAMMA HEAVY CHAINS
• 2. IGM - MU HEAVY CHAINS
• 3. IGA - ALPHA HEAVY CHAINS
• 4. IGD - DELTA HEAVY CHAINS
• 5. IGE - EPSILON HEAVY CHAINS

21. IGG• IGG IS THE MOST ABUNDANT ANTIBODY ISOTYPE IN
THE BLOOD (PLASMA), ACCOUNTING FOR 70-75%
OF HUMAN IMMUNOGLOBULINS (ANTIBODIES).
• IGG DETOXIFIES HARMFUL SUBSTANCES AND IS
IMPORTANT IN THE RECOGNITION OF ANTIGEN-
ANTIBODY COMPLEXES BY LEUKOCYTES AND
MACROPHAGES.
• IGG IS TRANSFERRED TO THE FETUS THROUGH THE
PLACENTA AND PROTECTS THE INFANT UNTIL ITS
OWN IMMUNE SYSTEM IS FUNCTIONAL.

22. IGM
IGM USUALLY CIRCULATES IN THE BLOOD, ACCOUNTING FOR
ABOUT 10% OF HUMAN IMMUNOGLOBULINS.
IGM HAS A PENTAMERIC STRUCTURE IN WHICH FIVE BASIC Y-
SHAPED MOLECULES ARE LINKED TOGETHER. B CELLS PRODUCE
IGM FIRST IN RESPONSE TO MICROBIAL INFECTION/ANTIGEN
INVASION.
ALTHOUGH IGM HAS A LOWER AFFINITY FOR ANTIGENS THAN
IGG, IT HAS HIGHER AVIDITY FOR ANTIGENS BECAUSE OF ITS
PENTAMERIC/HEXAMERIC STRUCTURE. IGM, BY BINDING TO THE
CELL SURFACE RECEPTOR, ALSO ACTIVATES CELL SIGNALING
PATHWAYS.

23. IGA• IGA IS ABUNDANT IN SERUM, NASAL MUCUS, SALIVA, BREAST
MILK, AND INTESTINAL FLUID, ACCOUNTING FOR 10-15% OF
HUMAN IMMUNOGLOBULINS.
• IGA FORMS DIMERS (I.E., TWO IGA MONOMERS JOINED
TOGETHER).
• IGA IN BREAST MILK PROTECTS THE GASTROINTESTINAL TRACT
OF NEONATES FROM PATHOGENS.

24. IGE
• IGE IS PRESENT IN MINUTE AMOUNTS, ACCOUNTING FOR NO MORE THAN
0.001% OF HUMAN IMMUNOGLOBULINS.
• ITS ORIGINAL ROLE IS TO PROTECT AGAINST PARASITES. IN REGIONS WHERE
PARASITIC INFECTION IS RARE, IGE IS PRIMARILY INVOLVED IN ALLERGY.

25. IGD
• IGD ACCOUNTS FOR LESS THAN 1% OF HUMAN IMMUNOGLOBULINS.
• IGD MAY BE INVOLVED IN THE INDUCTION OF ANTIBODY PRODUCTION IN B
CELLS, BUT ITS EXACT FUNCTION REMAINS UNKNOWN.