State the number of polypeptide chains in $H_{2}L_{2}$ (an antibody molecule).

Interferons are ...........

Which of the following provides the microenvironment for the maturation of $T$-lymphocytes?

Read the following statements and choose the correct set of statements $:$
$(a)$ Cell-mediated immune response is responsible for the graft rejection.
$(b)$ Grafts from just any source $-$ an animal,another primate,or any human beings can be made since the grafts would be accepted sooner or later.
$(c)$ The organs cannot be taken from just anybody for transplant.
$(d)$ Tissue matching,blood group matching are essential before undertaking any graft/transplant.
Choose the correct answer from the options given below $:$

How many types of barriers are included in innate immunity?

Some people who have suffered from a disease may not be affected again during their lifetime; such immunity is called