From HomoExcelsior

1. A physiological reaction of an organism used in self-protection, as against infection. 2. Any of a variety of mental devices, usually unconscious, that an individual may use to protect the ego from shame, anxiety, conflict, loss of self-esteem, or other unacceptable feelings or thoughts. Defense mechanisms, commonly used in coping with problems, include behaviors such as repression, projection, denial, and rationalization.

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