How many polypeptide chains are present in each antibody molecule?

Passive acquired immunity can be obtained through the introduction of which of the following?

Antibodies defend the body from the invading antigen by:

Which of the following is a primary lymphoid organ?

Assertion $A$: Innate immunity consists of various barriers. Reason $R$: Exposure to a pathogen is required for the stimulation of innate immunity. Which option is correct for Assertion $A$ and Reason $R$?

Which of the following is a characteristic feature of acquired immunity?