A scanning electron microscope image of a single human lymphocyte The<unk> , 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). Google Ngram of "acquired immunity " vs.<unk>  immunity". The peak for<unk> " in the 1960s reflects its introduction to immunology by Robert A. Good and use by colleagues; the explosive increase in the 1990s was correlated with the use of the phrase "innate immunity". Like the innate system, the<unk>  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<unk>  is highly specific to each particular pathogen the body has encountered.<unk>  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<unk> .<unk>  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<unk>  immunity is the basis of vaccination. The cells that carry out the<unk>  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<unk>  immune response. Sometimes the<unk>  system is unable to distinguish harmful from harmless foreign molecules; the effects of this may be hayfever, asthma, or any other allergy. In<unk>  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<unk> " 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 one variable (V), one diversity (D), and one joining (J) region for genetic recombination and discards the rest, which produces a highly unique combination of antigen-receptor gene segments in each lymphocyte. This mechanism allows a small number of genetic segments to generate a vast number of different antigen receptors, which are then uniquely expressed on each individual lymphocyte. Since the gene rearrangement leads to an irreversible change in the DNA of each cell, all progeny (offspring) of that cell inherit genes that encode the same receptor specificity, including the memory B cells and memory T cells that are the keys to long-lived specific immunity.
Adaptive immune system
Immune system
Mononuclear phagocyte system
Immune cycle
Muscular defense
Lymphopoiesis
Immune response
Visual processing