The protection against small pox by previous infection with cowpox represents-

Antigenic cross reactivity- Sometimes single antigen stimulates production of variety of antibodies that more or less resemble the correct fit for the antigen. Each of these antibodies may attack different antigens that more or less resemble one another. Cross Reactivity:The ability of an individual AB combining site to react with more than one antigenic determinant. The ability of a population of AB molecules to react with more than one Ag Innate immunity is the resistance that an individual possesses by bih. Passive immunity-When immunity is conferred by transfer of serum or lymphocytes from a specifically immunized individual, it is known as passive immunity. Antigen specificity:- gain only protection against the encountersd disease.