Antigen Antibody Reaction Questions and Answers

Answer: D

Correct answer: Opsonisation for increased uptake by phagocytic cells

Antibody bound to a bacterial surface is recognised by Fc receptors on neutrophils and macrophages, so the organism is taken up far more readily than it would be unopsonised. Complement fixed by the same antibody adds C3b, a second opsonin. Cell-mediated cytotoxicity and lysis of infected host cells matter chiefly against viruses and parasites rather than against extracellular bacteria.

Answer: D

Correct answer: All of these

Counter-immunoelectrophoresis drives antigen and antibody towards each other electrophoretically, so a precipitin line forms in minutes instead of the many hours diffusion alone would take. That speed is why it was adopted for urgent diagnoses: meningococcal antigen in cerebrospinal fluid, hepatitis B surface antigen, and alpha-fetoprotein are all standard applications.

Answer: D

Correct answer: all of the above

The ring test simply layers antigen over antiserum in a narrow tube so that a ring of precipitate forms at the interface. All three named applications use it: the C-reactive protein test, Ascoli’s thermoprecipitin test for anthrax in tissue extracts, and the grouping of streptococci and pneumococci by their capsular or cell wall antigens.

Answer: A

Correct answer: IgG

IgG accounts for about three quarters of the serum immunoglobulin, at roughly 8 to 16 g per litre, well ahead of IgA and IgM. Its long half-life of about three weeks, longer than any other class, is maintained by the neonatal Fc receptor, which recycles it rather than letting it be degraded.

Answer: C

Correct answer: IgE

Type I hypersensitivity is IgE-mediated. IgE binds through its Fc to high affinity receptors on mast cells and basophils, and when antigen cross-links two adjacent molecules the cell degranulates, releasing histamine and other mediators within minutes. This is the mechanism of hay fever, allergic asthma and anaphylaxis.

Answer: C

Correct answer: IgA

Secretory IgA predominates in milk, as in all external secretions. In colostrum the concentration is especially high, and because the antibody is not absorbed it works locally in the infant’s gut, coating organisms and blocking their attachment to the mucosa - protection at the surface where the newborn is most exposed.

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.

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.

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.

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.