Antigen Antibody Reaction Questions and Answers

Q: Antigen binding sites are present in

Correct answer: Fab regions of an antibodyEach Fab fragment carries one antigen-binding site, formed jointly by the variable domains of the heavy and light chains - so neither chain alone constitutes the site. The Fc fragment carries no antigen specificity at all; it is the part that binds Fc receptors and complement and determines what the antibody does once bound.

Q: Precipitation reaction is relatively less sensitive for the detection of

Correct answer: antibodiesPrecipitation uses soluble antigen, so a large amount of antibody must be present before enough antigen-antibody lattice forms to come out of solution and be seen. Agglutination, with the antigen already particulate, gives a visible result from far less antibody. This difference in sensitivity is the reason soluble antigens are commonly adsorbed onto carrier particles before testing.

Q: All of the following are true of antigen EXCEPT which one of the following?

Correct answer: They contain paratopesA paratope is the antigen-binding site of an antibody, so it belongs to the antibody, not to the antigen. The antigen carries epitopes, also called antigenic determinants - the two terms are synonyms, which is why two of the other options say the same thing - and it is those that the paratope binds.

Q: Oral polio drops contain

Correct answer: attenuated pathogensThe oral polio vaccine contains live virus attenuated by repeated passage so that it replicates in the gut and provokes immunity without causing disease. Replication in the intestinal mucosa is the point - it induces secretory IgA and so interrupts transmission, which the killed injected vaccine does much less well.

Q: Which of the following antibodies is predominantly present in tears, saliva and mucous

Correct answer: IgASecretory IgA is the predominant antibody at every mucosal surface - tears, saliva, respiratory and intestinal secretions, and milk. It is secreted as a dimer with the J chain and acquires the secretory component as it crosses the epithelium, which protects it against the proteases it will meet on the other side.

Q: Quellung reaction is used for typing of

Correct answer: Both (a) and (b)The Quellung or capsular swelling reaction is used for any capsulated organism whose capsule can be typed by specific antiserum. Both Streptococcus pneumoniae and Klebsiella pneumoniae qualify. When antibody binds the capsule its refractive index changes, so under the microscope the capsule appears swollen and sharply outlined - a direct visual identification of the serotype.

Q: The antibody class found at highest concentrations in serum is

Correct answer: IgGIgG makes up about 75 to 80 per cent of serum immunoglobulin, at roughly 8 to 16 g per litre. IgA follows, then IgM; IgD and IgE are present in trace amounts, IgE at only a few hundred nanograms per millilitre. That predominance, together with its long half-life and placental transfer, is why IgG carries most of the durable humoral protection.

Q: Globulins of the blood plasma are responsible for

Correct answer: defence mechanismsThe globulin fraction of plasma contains the immunoglobulins, and the gamma globulins in particular are the antibodies. Albumin, not globulin, maintains the colloid osmotic pressure; clotting depends chiefly on fibrinogen; and oxygen is carried by haemoglobin inside the red cells rather than by any plasma protein.

Q: VDRL test is an example of

Correct answer: Slide testThe VDRL test is performed by mixing a drop of heat-inactivated serum with cardiolipin antigen on a glass slide and rotating it, the flocculation being read under the microscope. It is a slide flocculation test, which is what makes it quick and cheap enough for the mass screening it was designed for. Being non-treponemal it detects reagin rather than antibody to the organism, so a reactive result needs confirming.

31.

Antigen binding sites are present in

A.Fab regions of an antibody
B.Fc region of an antibody
C.only in the light chain
D.only in the heavy chain

Answer: A

Correct answer: Fab regions of an antibody

Each Fab fragment carries one antigen-binding site, formed jointly by the variable domains of the heavy and light chains - so neither chain alone constitutes the site. The Fc fragment carries no antigen specificity at all; it is the part that binds Fc receptors and complement and determines what the antibody does once bound.

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32.

Weil-Felix reaction is based on sharing of antigens between

A.sheep RBCs and EB virus
B.mycoplasma and human O group RBCs
C.rickettsial antigens and antigens of certain strains of Proteus
D.none of these

Answer: C

Correct answer: rickettsial antigens and antigens of certain strains of Proteus

The Weil-Felix reaction works by heterophile agglutination: certain Proteus strains, OX-19, OX-2 and OX-K, happen to share carbohydrate antigens with the rickettsiae, so serum from a patient with typhus agglutinates them. The pattern of which strain is agglutinated points to the rickettsial group. It is convenient rather than accurate, and specific serology has largely replaced it.

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33.

Precipitation reaction is relatively less sensitive for the detection of

A.antigens
B.antigen-antibody complexes
C.antibodies
D.complement

Answer: C

Correct answer: antibodies

Precipitation uses soluble antigen, so a large amount of antibody must be present before enough antigen-antibody lattice forms to come out of solution and be seen. Agglutination, with the antigen already particulate, gives a visible result from far less antibody. This difference in sensitivity is the reason soluble antigens are commonly adsorbed onto carrier particles before testing.

Discuss in Forum

34.

All of the following are true of antigen EXCEPT which one of the following?

A.They contain epitopes.
B.They will react with antibodies
C. They contain antigenic determinants
D.They contain paratopes

Answer: D

Correct answer: They contain paratopes

A paratope is the antigen-binding site of an antibody, so it belongs to the antibody, not to the antigen. The antigen carries epitopes, also called antigenic determinants - the two terms are synonyms, which is why two of the other options say the same thing - and it is those that the paratope binds.

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35.

Oral polio drops contain

A.harvested antibodies
B.activated pathogens
C.attenuated pathogens
D.gamma globulins

Answer: C

Correct answer: attenuated pathogens

The oral polio vaccine contains live virus attenuated by repeated passage so that it replicates in the gut and provokes immunity without causing disease. Replication in the intestinal mucosa is the point - it induces secretory IgA and so interrupts transmission, which the killed injected vaccine does much less well.

Discuss in Forum

36.

Which of the following antibodies is predominantly present in tears, saliva and mucous

A.IgM
B.IgG
C.IgE
D.IgA

Answer: D

Correct answer: IgA

Secretory IgA is the predominant antibody at every mucosal surface - tears, saliva, respiratory and intestinal secretions, and milk. It is secreted as a dimer with the J chain and acquires the secretory component as it crosses the epithelium, which protects it against the proteases it will meet on the other side.

Discuss in Forum

37.

Quellung reaction is used for typing of

A.Klebsiella pneumoniae
B.Streptococcus pneumoniae
C.Both (a) and (b)
D.None of these

Answer: C

Correct answer: Both (a) and (b)

The Quellung or capsular swelling reaction is used for any capsulated organism whose capsule can be typed by specific antiserum. Both Streptococcus pneumoniae and Klebsiella pneumoniae qualify. When antibody binds the capsule its refractive index changes, so under the microscope the capsule appears swollen and sharply outlined - a direct visual identification of the serotype.

Discuss in Forum

38.

The antibody class found at highest concentrations in serum is

A.IgE
B.IgM
C.IgD
D.IgG

Answer: D

Correct answer: IgG

IgG makes up about 75 to 80 per cent of serum immunoglobulin, at roughly 8 to 16 g per litre. IgA follows, then IgM; IgD and IgE are present in trace amounts, IgE at only a few hundred nanograms per millilitre. That predominance, together with its long half-life and placental transfer, is why IgG carries most of the durable humoral protection.

Discuss in Forum

39.

Globulins of the blood plasma are responsible for

A.defence mechanisms
B.blood clotting
C.oxygen transport
D.osmotic balance

Answer: A

Correct answer: defence mechanisms

The globulin fraction of plasma contains the immunoglobulins, and the gamma globulins in particular are the antibodies. Albumin, not globulin, maintains the colloid osmotic pressure; clotting depends chiefly on fibrinogen; and oxygen is carried by haemoglobin inside the red cells rather than by any plasma protein.

Discuss in Forum

40.

VDRL test is an example of

A.Tube test
B.Ring test
C.Slide test
D.none of these

Answer: C

Correct answer: Slide test

The VDRL test is performed by mixing a drop of heat-inactivated serum with cardiolipin antigen on a glass slide and rotating it, the flocculation being read under the microscope. It is a slide flocculation test, which is what makes it quick and cheap enough for the mass screening it was designed for. Being non-treponemal it detects reagin rather than antibody to the organism, so a reactive result needs confirming.

Discuss in Forum

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