What is an interferon?

Antibodies fight against

When children without $T-$ cells and $B-$ cells are kept in germ-free isolation suits,the disease is known as:

Which of the following is an autoimmune disease?

If you suspect a major deficiency of antibodies in a person,to which of the following would you look for confirmatory evidence?

Define the following terms:
$(a)$ Immunity
$(b)$ Retrovirus
$(c)$ Allergy