Animal and Plant Defense Responses: Animal Defense Responses

Defenses of the Human Body

• The human body has developed defense mechanisms to cope with constant attacks from microorganisms.
• There are three lines of defense: physical barriers, defensive cells and proteins, and the immune system.

First Line of Defense

• Physical barriers:
  • Skin: cells filled with keratin make it impenetrable, waterproof, and resistant to toxins.
  • Dead skin cells are shed and replaced, taking microbes with them.
• Mucous membranes:
  • Line internal surfaces of respiratory, digestive, urinary, and reproductive systems.
  • Produce mucous that traps microbes.
  • Hair in the nose acts as a coarse filter.
• Chemical barriers:
  • Sweat: washes away microbes and slows bacterial growth due to acidity.
  • Mucous membranes: produce sticky mucous that traps microbes.
  • Saliva and tears: contain lysozyme, an enzyme that kills bacteria by rupturing cell walls.
  • Cerumen (ear wax): traps dirt and dust particles in the ear canal.

Second Line of Defense

• Defensive cells and proteins:
  • Phagocytes (e.g., neutrophils, macrophages) engulf pathogens, damaged tissues, or dead cells.
  • Eosinophils discharge destructive enzymes to destroy pathogens too big for phagocytes.
  • Natural killer cells seek out and destroy abnormal cells (e.g., cancer cells).
• Inflammation and fever: nonspecific responses to infection.

Immunity

• Types of immunity:
  • Inborn immunity: genetic predisposition received from parents and ancestors.
  • Acquired immunity: obtained through exposure to antigens, either naturally or artificially.

Acquired Immunity

• Active acquired immunity: obtained once a person comes into direct contact with a disease, providing permanent immunity.
• Passive acquired immunity:
  • Naturally acquired: occurs when a person is exposed to a disease, providing temporary immunity.
  • Artificially acquired: introduced through a vaccine, which stimulates antibody production.

Antibodies (Immunoglobulins)

• Types of immunoglobulins:
  • IgG: most abundant in blood, promotes opsonization, neutralization, and cross-linking of antigens.
  • IgA: found in secretions (e.g., tears, saliva, mucus, breast milk), provides localized defense of mucous membranes.
  • IgM: first antibody produced after initial exposure to an antigen, effective in complement system activation.
  • IgE: triggers release of histamine and other chemicals that cause allergic reactions.
  • IgD: acts as an antigen receptor in B cells, stimulating proliferation and differentiation.