Antibody Structure and Function

Question 1

General antibody release mechanism

Answer 1

• a resting B cell with membrane bound Ig
• it then has an encounter with antigen like on a bacterium
• the stimulated B cell gives rise to anti-body secreting plasma cells

Question 2

General structure of the antibody

Answer 2

• It has a heavy chain, which is on the bottom and also internal on top
• the sides are held together by disulfide bonds
• the ends that bind antigen binding sites are the N-termini
• the antigen binding sites are the variable region, where the other parts are the constant region for that antibody type
• gamma globulin fraction contained most of the antibody

Question 3

Use of antibodies

Answer 3

• neutralization- such as with toxins or viruses

- opsonization by itself or combined with complement which causes the pathogen to be ingested and degraded by phagocyte

Question 4

Molecular weight and antibody type

Answer 4

IgGs -about 150 kDa

IgM- 970 kDa because there are multiple

Question 5

Functions of antibodies

Answer 5

• Neutralization- IgGs and IgA, IgM-a little
• Opsonization- IgG1 and 3 and IGg 4 and IgGA a little
• Activate NK cells- IgG1 and 3
• Activate Mast cells- IgG1 and 3 a little, IgE a lot
• Activation of complement- IgM, IgGs, IgA a little
• Transport across epithelium- IgA, a little IgM
• transport across placenta- IgG

Question 6

Mean serum level of antibody types

Answer 6

IgM- 1.5 (mg/ml)
IgD- 0.03
IgG1- 9
IgG2- 3
IgG3- 1
IgG4- 0.5
IgA- 2.5
IgE- 5 x 10^-5

Question 7

IgG

Answer 7

• MW 150,000
• predominant class in serum
• toxin neutralizing, agglutinating, opsonizing, bacteriolytic (with aid of complement system)
• subclasses IgG1, IgG2, IgG3, IgG4
• intrachain disulfide bond occurs between the same residues

Question 8

Phagocytosis via antibody use

Answer 8

• antibody binds to bacterium
• antibody coated bacterium binds to Fc receptors on cell surface
• macrophage membrane surrounds bacterium
• macrophage membranes fuse, creating a membrane-bounded vesicles, the phagosome
• lysosomes fuse with the phagosome, creating the phagolysosome

Question 9

Immunoglobin M

Answer 9

• MW 900,000- because usually pentameric
• predominant class in primary immune response
• antigen receptor on B lymphocyte. With the aid of the complement system it can be opsonizing and bacteriolytic. It cannot aid in opsonization without complement as cells do not have FcM receptors
• 5-10% of serum Igs
• 4 heavy chain domains, no hinge region
• J chain (15,000 MW)

Question 10

IgA

Answer 10

• MW 160,000- usually dimeric
• predominant class in secretions
• found in monomer, dimers, and trimers
• 15% of serum Igs
• secretory piece (T piece)- resists acid hydrolysis
• J chain
• agglutinating, opsonizing

Question 11

IgD

Answer 11

• MW 180,000
• located on the surface of human immature B-lymphocytes
• associated with some tumor cells
• 0.2% of serum Igs
• co-expressed with IgM on surface of B- lymphocytes
• may function as an antigen receptor

Question 12

IgE

Answer 12

• MW 200,000
• immediate hypersensitivity
• fixes to mast cells- antigen cross links IgE antibody bound at the mast-cell surface causing release of granule contents
• 0.1% of serum Igs
• mediate changes in vascular permeability
• may be involved in host defense against parasitic infection

Question 13

Proteolytic degradation of immunoglobins

Answer 13

• papain digests the hinge region
• one fragment can be crystalized- contain most of the IgG specific antigenic determinants and is called Fc
• other two regions can still bind antigen and are not crystalizable (more variabe)- Fab
• pepsin can make F(ab’)2 fragments

Question 14

Domains in antibodies

Answer 14

• light chain C domain- constant
• light chain V domain- variable
• also have three hypervariable regions- very specific antigen binding site