The adaptive immune system, also known as the acquired immune system, is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system is one of the two main immunity strategies found in vertebrates (the other being the innate immune system). Like the innate system, the adaptive immune system includes both humoral immunity components and cell-mediated immunity components and destroys invading pathogens. Unlike the innate immune system, which is pre-programmed to react to common broad categories of pathogen, the adaptive immune system is highly specific to each particular pathogen the body has encountered. Adaptive immunity creates immunological memory after an initial response to a specific pathogen, and leads to an enhanced response to future encounters with that pathogen. Antibodies are a critical part of the adaptive immune system. Adaptive immunity can provide long-lasting protection, sometimes for the person's entire lifetime. For example, someone who recovers from measles is now protected against measles for their lifetime; in other cases it does not provide lifetime protection, as with chickenpox. This process of adaptive immunity is the basis of vaccination. The cells that carry out the adaptive immune response are white blood cells known as lymphocytes. B cells and T cells, two different types of lymphocytes, carry out the main activities: antibody responses, and cell-mediated immune response. In antibody responses, B cells are activated to secrete antibodies, which are proteins also known as immunoglobulins. Antibodies travel through the bloodstream and bind to the foreign antigen causing it to inactivate, which does not allow the antigen to bind to the host. Antigens are any substances that elicit the adaptive immune response. Sometimes the adaptive system is unable to distinguish harmful from harmless foreign molecules; the effects of this may be hayfever, asthma, or any other allergy. In adaptive immunity, pathogen-specific receptors are "acquired" during the lifetime of the organism (whereas in innate immunity pathogen-specific receptors are already encoded in the genome). This acquired response is called "adaptive" because it prepares the body's immune system for future challenges (though it can actually also be maladaptive when it results in allergies or autoimmunity). The system is highly adaptable because of two factors. First, somatic hypermutation is a process of accelerated random genetic mutations in the antibody-coding genes, which allows antibodies with novel specificity to be created. Second, V(D)J recombination randomly selects