The essence of the immune system is built on two separate foundation pillars: one is specific or adaptive immunity which is characterized by relatively slow response kinetics and the ability to remember. The other is nonspecific or innate immunity exhibiting rapid response kinetics but lacking memory. Lymphocytes are the key players of the adaptive immune system. Each lymphocyte expresses antigen receptors of unique specificity. Upon recognizing an antigen via the receptor, lymphocytes proliferate and develop effector function. Few lymphocytes exhibit specificity for a given antigen or pathogen and massive proliferation is usually required before an effector response can be measured. Hence, the slow kinetics of the adaptive immune system. Because a significant proportion of the expanded lymphocytes survive and may maintain some effector function after elimination of the antigen, the adaptive immune system reacts faster when encountering the antigen...

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