CBSE Class 12-science Answered

(i) Allergens are substances which act as weak antigens and cause inappropriate and excessive immune response. Allergens initiates harmful antibody formation which in turn binds to the receptors on the mast cell. The mast cell releases chemicals sucha as histamines, which in turn causes inflammation of the mucous membranes.

(ii) Principle of vaccination is based on the property of memory of immune system. In vaccination, a preparation of antigenic proteins of pathogens but weakened pathogens is introduced into the body. The antigens generate primary immune response by producing antibodies along with B-cells and T-cells. When the vaccinated person is attacked by the same pathogens, the existing memory B- cells and T-cells recognize the antigen and put a strong effect on the invaders with massive production of lymphocytes and antibodies.

Hepatitis B vaccine is produced from yeast.