Immunization
. When the body is first exposed to an antigen, several days pass before the adaptive immune response becomes active. Immune activity then rises, levels off, and falls. During following exposures to the same antigen, the immune system responds much more quickly and reaches higher levels. Because the first, or primary, immune response is slow, it cannot prevent disease, although it may help in recovery. In contrast, subsequent, or secondary, immune responses usually can prevent disease because the pathogen is detected, attacked, and destroyed before symptoms appear. This complete resistance to disease is called immunity and may be achieved through either active or passive immunization . A. Active Immunization Active immunization occurs when a person’s own immune system is activated and generates a primary immune response. Active immunization can be triggered in two ways, either by natural immunization or by vaccination. In natural immunization, the body contracts a disease and recovers