It is one of the two main immunity strategies found in vertebrates (the other being the innate immune system). Adaptive immunity creates immunological memory after an initial response to a specific pathogen, leads to an enhanced response to subsequent encounters with that pathogen.
active immunity. that which follows exposure to an antigen; acquired immunity attributable to the presence of antibody or of immune lymphoid cells formed in response to antigenic stimulus. Called also adaptive immunity. Immunity obtained either from the development of antibodies in response to exposure to an antigen, as from vaccination or an attack of an infectious disease, or from the transmission of antibodies, as from mother to fetus through the placenta or the injection of antiserum. The adaptive immune or specific immune response consists of antibody responses and cell-mediated responses, which are carried out by different lymphocyte cells, B cells and T cells, respectively. A key feature of the adaptive immune system is memory, the development of immunological memory, in which each pathogen is remembered by a signature antibody. Part of activated B cells and T cells can develop to memory cells. Adaptive Immunity Definition. Adaptive immunity is an important part of the immune system. It is protection from an infectious disease agent that is mediated by B-and T-lymphocytes following exposure to specific antigen, and characterized by immunological memory. A key feature of the adaptive immune system is memory, the development of immunological memory, in which each pathogen is remembered by a signature antibody. Part of activated B cells and T cells can develop to memory cells. The component of the immune system in vertebrates involving cells that are modified to attack the specific antigens they encounter, as distinguished from the more nonspecific innate immune system. The adaptive immune system comprises lymphocytes (B cells and T cells) having a small number of genetically encoded proteins that can combine in such a way as to produce an enormous variety of proteins, having the potential to fight any antigen encountered. Adaptive immunity refers to antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a memory that makes future responses against a specific antigen more efficient adaptive immunity. The component of immunity that is pathogen-specific and creates memory. It consists of the mechanisms of cell-mediated and antibody-mediated immunity.  The adaptive immune system is one of the two main immunity strategies found in vertebrates (the other being the innate immune system). Adaptive immunity creates immunological memory after an initial response to a specific pathogen, leads to an enhanced response to subsequent encounters with that pathogen. This process of acquired immunity is the basis of vaccination. Like the innate system, the adaptive system includes both humoral immunity components and cell-mediated immunity components. Unlike the innate immune system, the adaptive immune system is highly specific to a specific pathogen. The adaptive system response destroys invading pathogens and any toxic molecules they produce. Sometimes the adaptive system is unable to distinguish foreign molecules, the effects of this may be hayfever, asthma or any other allergies. Antigens are any substances that elicit the adaptive immune response. The cells that carry out the adaptive immune response are white blood cells known as lymphocytes The cells of the adaptive immune system are lymphocytes – B cells and T cells. B cells, which are derived from the bone marrow, become the cells that produce antibodies. T cells, which mature in the thymus, differentiate into cells that either participate in lymphocyte maturation, or kill virus-infected cells. The immune response to viral infection comprises innate and adaptive defenses. The innate response, which we have discussed previously, functions continuously in a normal host without exposure to any virus. Most viral infections are controlled by the innate immune system. Features of an Immune Response. An immune response is generally divided into innate and adaptive immunity. Innate immunity occurs immediately, when circulating innate cells recognize a problem. Adaptive immunity occurs later, as it relies on the coordination and expansion of specific adaptive immune cells. 
http://medical-dictionary.thefreedictionary.com/Adaptive+immune+system http://www.sinobiological.com/Adaptive-Immunity-a-747.html http://www.sinobiological.com/Adaptive-Immunity-a-747.html http://www.thefreedictionary.com/Adaptive+immune+system http://www.biology.arizona.edu/immunology/tutorials/immunology/page3.html http://medical-dictionary.thefreedictionary.com/Adaptive+Immunity https://en.wikipedia.org/wiki/Adaptive_immune_system http://www.virology.ws/2009/07/03/adaptive-immune-defenses/ http://www.niaid.nih.gov/topics/immuneSystem/Pages/features.aspx
define adaptive immunity