Which of the following cells are involved in the defence mechanism of the body?

Assertion $A$: Vaccination protects an individual against infection by specific pathogens.
Reason $R$: $B$ and $T$ memory cells produce antibodies in large quantities to neutralize the pathogen attack.
Which option is correct for Assertion $A$ and Reason $R$?

What is true about $T-lymphocytes$ in mammals?

Humoral immunity is due to

In active acquired immunity,.........

In the structure of an antibody,the heavy chains and light chains are held together by which type of bond?