​A researcher raised antibodies against sheep red blood cells (SRBCs) and purified the IgG fraction. Some of the IgG antibodies were then subjected to enzymatic digestion to have Fab, Fc and F(ab')₂ fractions. He placed each preparation in a separate tube (1 to 3), labeled the three tubes to indicate their contents, and incubated them on ice. After a while he noticed that the label on two of the tubes (1 and 2) had gotten erased. He did a test for tube 1 and found that the preparation in the tube agglutinated SRBCs but did not lyse them in presence of complement. Which preparation was in tube 1?​

Correct option is C

Explanation-

Tube 1 agglutinated SRBCs → This means the antibody preparation could cross-link the antigens on the surface of red blood cells.
But did not lyse them in the presence of complement → This suggests the preparation lacks the Fc region, which is essential for complement-mediated lysis.

F(ab')₂ is Correct for Tube 1:
Agglutination → Requires at least two antigen-binding sites → F(ab')₂ has two Fab regions (so it cross-links and agglutinates SRBCs).
No complement lysis → Lack of Fc region → Exactly true for F(ab')₂ fragment.

Incorrect Options-
Option a - Fab → Monovalent → Cannot agglutinate.
Option b - Fc → No antigen-binding site → Cannot agglutinate.
Option d - IgG → Would agglutinate and lyse via complement → But lysis did not occur here.

Correct Answer:
Option c: F(ab')₂