Unbroken skin creates a physical barrier that cannot normally be penetrated by bacteria or viruses.
	Surface secretions from sebaceous and sweat glands give the skin an acidic pH that is unfavorable for bacterial colonization.
	Tears, saliva, and mucous secretions contain lysozyme, an enzyme that digests bacterial cell walls.
	Mucus traps microbes and other particles that come in contact with it.
	All of the above.

	They are antigen-presenting cells that originate from neutrophils.
	They have short life spans because they self-destruct after engulfing foreign invaders.
	They are phagocytic cells that originate from monocytes, which leave the circulation and enter the tissues.
	They are most effective against parasites.
	They do not phagocytize microorganisms; instead, they induce lysis of virus-infected body cells.

	An antigen is a protein molecule that helps defend the body against disease.
	An antigen induces development of white blood cells in the bone marrow.
	An antigen could be an invading virus or bacterium.
	An antigen is a foreign molecule that evokes a specific response by a lymphocyte.
	An antigen is a protein attacked by an invading microorganism.

	Memory cells are more numerous.
	Memory cells are responsible for the primary immune response.
	Memory cells combat antigens; effector cells do not.
	Memory cells live longer.
	Memory cells are capable of producing antibodies.

	body cells that have been infected
	circulating antibodies
	circulating proteins
	extracellular viruses and bacteria
	complement proteins

	cell-mediated immunity is longer lasting
	a subsequent secondary immune response can occur in humoral immunity
	clonal selection occurs in cell-mediated immunity
	they respond differently to invaders
	a humoral response is mounted more quickly

	a cell-mediated immune response
	innate immunity
	the complement system
	a group of phagocytic white blood cells
	all of the above

	antibodies from B cells
	cytotoxic T cells
	complement proteins
	helper T cells
	antigens

	T cells
	antibodies
	MHC (major histocompatibility complex) proteins
	histamines
	B cells

	cytotoxic T cells
	natural killer cells
	helper T cells
	plasma cells
	memory cells