More than a century ago, experiments showed that guinea pigs that were infected with bacteria that cause diphtheria produced a chemical in their blood that can protect other guinea pigs from infection. This observation led to the concept of adaptive immunity and to the understanding that factors promoting immunity are found in blood.
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The humoral and cellular immune responses work simultaneously and cooperatively.
Memory cells, the key to immunological memory, retain the capacity to divide to produce effector and more memory cells.
Vaccinations initiate a primary immune response, generating memory cells without causing illness.
In this section we outline the main features of the adaptive immune system. We will consider the two major types of adaptive responses: the humoral immune response, which produces antibodies; and the cellular immune response, which destroys infected cells.