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

(N/A) Immunity: The overall ability of the host to fight the disease-causing organisms,conferred by the immune system,is called immunity.
$(b)$ Retrovirus: $A$ group of viruses that possess $RNA$ as their genetic material,which is enclosed within a protein coat and an envelope. They use the enzyme reverse transcriptase to convert their $RNA$ into $DNA$ inside the host cell.
$(c)$ Allergy: The exaggerated response of the immune system to certain antigens present in the environment is called allergy.