Non-specific host defence that exists prior to the exposure to an antigen is called

The $SCID$ patient has a defective gene for the enzyme Adenosine Deaminase $(ADA)$. He/She lacks functional ...... and therefore,fails to fight the infecting pathogens.

Interferons curb the infection of:

For an organ transplant,it is an advantage to have an identical twin. Why?

When is the immune response most intense?

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