The body has a variety of defense mechanisms to protect it against invasion of potentially harmful agents such as viruses, bacteria, and parasites. The first line of defense Is a more physical barrier e.g. skin and mucosa. When organisms succeed in passing this first line of defense and enter the body, the immune system is activated and the ensuing immune response generally leads to elimination of the invaders and often results in resistance to the organism. The immune response involves the specific recognition of the organism which leads to humoral and/or cellular immunity and memory. Through the formation of memory cells the immune system is capable of responding to the same organism more rapid and effective upon subsequent encounter. In the humoral immune response, antibody producing B cells playa central role. Antibodies are secreted in body fluids and can bind to organisms with high specificity. Cellular immunity is primarily exerted by T cells, consisting of cytotoxic T cells, which kill virus-infected and tumor cells, or helper T cells which regulate the function of other cells via secreted cytokines. For different reasons, immune responses can be evoked in humans or animals via injection of an antigen (= substance to which an immune response is induced).

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TNO, National Institute of Public Healthand the Environment (RIVM), EMC Rotterdam,
H.J.H.M. Claassen (Eric)
Erasmus University Rotterdam
Erasmus MC: University Medical Center Rotterdam

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