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antibiotics
chemicals that either kill bacteria or slow their growth by interfering with the function of essential bacterial cell structures.
binary fission
a type of asexual reproduction in which one parental cell divides into two.
population
a group of organisms of the same species living together in the same geographic area.
evolution
change in allele frequencies in a population over time.
fitness
the relative ability of an organism to survive and reproduce in a particular environment.
natural selection
differential survival and reproduction of individuals in response to environmental pressure that leads to change in allele frequencies in a population over time.
adaptation
the response of a population to environmental pressure, so that advantageous traits become more common in the population over time.
directional selection
a type of natural selection in which organisms with phenotypes at one end of the spectrum are favored by the environment.
stabilizing selection
a type of natural selection in which organisms near the middle of the phenotypic range of variation are favored.
diversifying selection
a type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment.
virus
an acellular infectious particle consisting of nucleic acid surrounded by a protein shell.
immune system
a system of cells and tissues that acts to defend the body against foreign cells and infectious agents.
immunity
the resistance to a given pathogen conferred by the activity of the immune system.
pathogen
infectious agents including certain viruses, bacteria, fungi, and parasites.
innate immunity
nonspecific defenses, such as physical and chemical barriers and phagocytic cells, that are present from birth and are always active.
adaptive immunity
a protective response, mediated by lymphocytes, that confers long-lasting immunity against specific pathogens.
lymphocyte
a specialized white blood cell of the immune system.
histamine
a molecule released by damaged tissue and during allergic reactions.
inflammatory response
an innate defense that is activated by local tissue damage.
phagocyte
a type of white blood cell that engulfs and ingests damaged cells and pathogens.
macrophage
a phagocytic cell that resides in tissues and plays an important role in the inflammatory response.
neutrophil
a phagocytic cell in the circulation that plays an important role in the inflammatory response.
natural killer cell
a type of white blood cell that acts during the innate immune response to find and destroy virally infected cells and tumor cells.
interferon
antiviral proteins produced by virally infected cells to help protect adjacent cells from becoming infected.
complement proteins
proteins in blood that help destory pathogens by coating or puncturing them.
B cells
white blood cells that mature in the bone marrow and produce antibodies during the adaptive immune response.
thymus
the organ which T cells mature.
T cells
white blood cells that mature in the thymus and can destory infected cells or stimulate B cells to produce antibodies, depending on the type
lymph nodes
small organs in the lymphatic system where B and T cells may encounter pathogens.
lymphatic system
the organ system that works with the immune system, allowing B and T cells to respond to pathogens.
humoral immunity
the type of adaptive immunity that fights infections and other foreign substances in the circulation and lymph fluid.
helper T cells
a type of T cell that helps activate B cells during humoral responses.
antigen
a specific molecule (or part of a molecule) to which specific antibodies can bind, and against which an adaptive response is mounted.
plasma cell
an activated B cell that divides rapidly and secretes an abundance of antibodies.
antibody
a protein produced by B cells that bind to antigens and either neutralizes them or flags other cells to destroy pathogens.
cell-mediated immunity
the type of adaptive immunity that rids the body of altered (that is infected or foreign) cells.
cytotoxic T cells
a type of T cell that destroys altered cells, including virally infected cells.
allergy
a misdirected immune response against environmental substances such as dust, pollen, and foods that causes discomfort in the form of physical symptoms.
autoimmune disease
a misdirected immune response in which the immune system mistakenly attacks healthy cells.
primary response
the adaptive response mounted the first time a particular antigen is encountered by the immune system.
memory cell
a long-lived B or T cell that is produced during the primary response and that is rapidly activated in the secondary response.
secondary response
the rapid and strong response mounted when a particular antigen is encountered by the immune system subsequent to the first encounter.
vaccine
a preparation of killed or weakened microorganisms or viruses that is given to people or animals to generate a memory immune response.
antigen drift
changes in viral antigens caused by genetic mutation during normal viral replication.
antigen shift
changes in antigens that occur when viruses exchange genetic material with other strains.

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