Given below are two statements :

Statement $I$ : Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.

Statement $II$ : Both bone marrow and thymus provide micro environments for the development and maturation of T-lymphocytes.

In the light of above statements, choose the most appropriate answer from the options given below :

$A$ – The vaccines generate memory – $B$ and $T$ – cells. $R$ – In vaccination activated pathogens are introduced into the body.

$R.B.C.$ in adult are formed

Give scientific reasons : Virus infected cells provide innate immunity to other cells.

Lacking of $B$-cells and $T$-cells is known as