Except for the skin,other physical barriers that also help to prevent the entry of microorganisms are:
$I.$ Mucus coating of the epithelium lining the respiratory tract
$II.$ Gastrointestinal tract
$III.$ Urogenital tract
Choose the correct option.

Which lymphoid organ decreases in size with increasing age?

Which types of cells are produced during the clonal selection of $B$-cells?

Identify the cells that form a phagolysosome.

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 of the following is $NOT$ included in primary lymphoid organs?