The inability to distinguish between self cells and non-self cells may lead to

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.

Long-term immunity against pathogens is provided by which of the following?

Match the columns:

Column-$I$	Column-$II$
$(a)$ Main lymphoid organ	$(1)$ Thymus
$(b)$ $MALT$	$(2)$ Spleen
$(c)$ Lobe-like organ arranged near heart	$(3)$ Bone marrow
$(d)$ Large bean-shaped organ	$(4)$ It constitutes $50\%$ of lymphoid tissue

The toxic substances produced by foreign bodies that trigger an immune response are known as:

The principle of vaccination is based on which property of the immune system?