What causes immunodeficiency?

$A$ substance (low molecular weight protein) produced by host cells in response to viral infection that protects other cells against further viral infection is:

How many heavy polypeptide chains are present in each antibody molecule?

Person '$A$' cannot step out of his house. He has to spend his entire life in sterile isolation; otherwise, he would quickly contract a fatal infection. This person has almost no effective immune system. This disease is also called '$baby$ $in$ $a$ $bubble$ $syndrome$'. Identify the disease this person '$A$' is suffering from.

Which of the following reasons is mainly responsible for graft rejection in transplantation of organs?

What is produced in innate immunity to prevent the entry of foreign agents into the body?