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When the body encounters an antigen, it may produce an immune response against it. The first time the body encounters an antigen, this may take some time. The immune response may, however, include the development of "immune memory". When there is immune memory the immune system is "primed", and able rapidly to mount an immune response generate antibodies if the same antigens and other aspects are encountered in the future.
Active immunisation may give stimulate the development of immune memory; although not all vaccines do this (particularly unconjugated polysaccharide vaccines). Passive immunity (when immunoglobulins are given) does not generate immune memory.