Chapter 43

• Active immunity develops naturally when a pathogen invades the body and elicits a primary or secondary immune response
• Passive immunity provides immediate, short-term protection
• It is conferred naturally when IgG antibodies cross the placenta from mother to fetus or when IgA antibodies pass from mother to infant in breast milk
• Cells transferred from one person to another can be attacked by immune defenses
• To minimize rejection of a transplant or graft, surgeons use donor tissue with MHC molecules as similar as possible to those of the recipient
• The recipient takes medicines that suppress immune responses
• Antigens on red blood cells determine whether a person has blood type A (A antigen), B (B antigen), AB (both A and B), or O (neither)
• Antibodies to nonself blood types exist in the body
• Some pathogens have evolved ways to diminish the effectiveness of host immune responses
• When allergic, autoimmune, or immunodeficiency disorders disrupt the delicate balance of immune responses, the effects can be severe
• Allergies are exaggerated (hypersensitive) responses to antigens called allergens
• In localized allergies such as hay fever, IgE antibodies produced after first exposure to an allergen (such as pollen grains) attach to receptors on mast cells
• Mast cells release histamine and other inflammatory chemicals that bring about typical allergy symptoms
• An acute allergic response can lead to anaphylactic shock, a life-threatening reaction, within seconds of allergen exposure
• An injection of epinephrine can rapidly counteract the allergic response
• In individuals with autoimmune diseases, the immune system loses tolerance for self and turns against certain molecules of the body
• Autoimmune diseases include systemic lupus erythematosus, rheumatoid arthritis, insulindependent diabetes mellitus, and multiple sclerosis
• Inborn immunodeficiency (such as severe combined immunodeficiency (SCID)) results from a genetic or developmental defect in the innate or adaptive defenses, or both
• Acquired immunodeficiency develops later in life due to exposure to chemical and biological agents
• Pathogens have evolved mechanisms to thwart immune responses
• Through antigenic variation, some pathogens are able to change epitope expression and prevent recognition
• The human influenza virus mutates rapidly, and new flu vaccines must be made each year
• Human viruses occasionally exchange genes with the viruses of domesticated animals
• Some viruses may remain in a host in an inactive state called latency
• Herpes simplex viruses can be present in a human host without causing symptoms