Which of the following is responsible for the rejection of organ transplants?

Which of the following statements are correct regarding the structure and types of antibodies in humans?
$i$. Antibodies are also called Immunoglobulins that are produced as an immune response to antigenic stimulation.
$ii$. They are produced by all types of $T$-lymphocytes and $B$-lymphocytes.
$iii$. An antibody is a $Y$-shaped molecule formed of four polypeptide chains. Antibodies present in plasma determine many blood group systems like $ABO$,$Rh$,$P$,$MNS$ etc.
$iv$. An antibody molecule consists of two antigen-binding sites called paratopes.

Which is an incorrect pair based on its role in the immune system?

Acid in the stomach,saliva in the mouth,and tears from the eyes all prevent microbial growth. These belong to which of the following barriers?

Memory cells are responsible for:

Interferons curb the infection of: