Immunization

The Immune Response

Immunization protects against infection by generating adaptive immunity, also called acquired [Page 1112]immunity. Four features separate adaptive from innate immunity. First, the adaptive immune system must be exposed to the infectious agent before it can mount an immune response. Second, unlike the innate immune system, which can only recognize a genetically preprogrammed set of features common to infectious agents, the adaptive immune system can theoretically recognize any infection. Third, the adaptive immune system creates memories of the specific agent and will mount a quicker and more effective response if exposed again. Lastly, during the initial exposure, the innate system can be activated in a matter of hours, while the adaptive immunity takes days. A person’s infection and vaccination history generates a collection of acquired immune responses, which collectively make up the memory of their adaptive immune system. For example, if an individual has never been exposed to the measles virus or the measles vaccine, they will lack that particular immunity and can contract measles. However, if a person contracts measles (and recovers) or receives the measles vaccine, the adaptive immune system will possess an acquired immunity/memory of measles and, in most cases, prevents future measles infections.

Immunologic memory and protection begins during the initial infection. In a typical infection, the infectious agent (microorganisms such as bacteria or viruses) enters the body and begins to replicate. If the infectious agent causes disease, it is termed a pathogen. Disease results from the proliferation and/or products of the pathogen, and the body’s attempt to fight the infection. The immune system recognizes pieces of the pathogen, known as antigens, as foreign. It adapts to the pathogen by producing antibodies, proteins targeted to those specific antigens. B cells make the antibodies, which are then secreted into the blood and mucous membranes and neutralize and/or clear the pathogenic toxin and/or organism. As an immune response progresses, more effective and greater numbers of antibodies are produced, eventually clearing the pathogen. Immune cells and antibodies will circulate in the person’s body from that point forward and reinforce that particular adaptive immune response. If the person is reinfected, effective antibodies can be generated more quickly and prevent disease. Some pathogens (e.g., HIV, malaria) can evade the immune response by either never being effectively cleared in the first place (HIV) or changing their presenting antigens to appear novel to the body (malaria).

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