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Cells: The smallest structural units of living matter capable of functioning independently.

Germ Cells: Collective term for cells in the reproductive organs of multicellular organisms that divide by meiosis to produce gametes.

Guard Cells: Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange and water loss.

Helper T Cells: A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present.

Cytoxic T Cells: T cells that destroy body cells infected by viruses or bacteria; also attack bacteria, fungi, parasites, and cancer cells and will kill cells of transplanted organs if they are recognized as . . . View Full Definition

Companion Cells: Specialized cells in the phloem that load sugars into the sieve elements and help maintain a functional plasma membrane in the sieve elements.

B Memory Cells: Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if . . . View Full Definition

Embryonic Stem (ES) Cells: An embryonic cell that can replicate indefinitely, transform into other types of cells, and serve as a continuous source of new cells.

Langerhans Cells: Epidermal cells that participate in the infiammatory response by engulfing microorganisms and releasing chemicals that mobilize immune system cells.

Mast Cells: Cells that synthesize and release histamine, as during an allergic response; found most often in connective tissue surrounding blood vessels.