11.1 - Body Defence

Introduction to Human Immunity

Key Definitions

• Pathogen: A microorganism that causes diseases. Examples include bacteria, viruses and parasites.
• Antigen: A foreign particle that enters the body and stimulates the immune response.
• Antibody: A protein found on the lymphocyte surface or released by lymphocytes into the blood plasma.
• Immune response: The interaction between antibodies and antigens that causes antigens to be destroyed.
• Immunity: The body’s ability to fight infections caused by pathogens or other foreign objects through specific attacks on the pathogens.

Three Lines of Defence in Humans

• First line: Non-specific physical and chemical defences that prevent pathogens from entering the body.
• Second line: Non-specific fever, phagocytosis and inflammation.
• Third line: Specific immune response of lymphocytes through antibodies and memory cells.

First Line of Defence

• Skin:
  • Tough and difficult for microorganisms to penetrate.
  • Continuous shedding of dead skin cells makes microorganism growth difficult.
  • Sebum contains oil and acid that prevent microorganism growth.
  • Sweat contains lysozyme that breaks down the cell wall of some bacteria.
• Lysozyme: An antimicrobial protein in tears, nasal mucus and saliva that dissolves and destroys some bacteria.
• Mucous membrane: Lines the respiratory tract and secretes mucus containing lysozyme, which destroys bacteria in inhaled air.
• Blood clotting mechanism: Prevents bacteria from entering through wounds.
• Hydrochloric acid: Destroys bacteria in food and drinks in the stomach.

Second Line of Defence

• Fever: Increases phagocytic activity and fights microorganisms that infect the body.
• Phagocytes: Leucocytes that carry out phagocytosis; neutrophils and monocytes are phagocytes.
• Phagocytosis: The trapping and digestion of microorganisms or other particles, such as dead cells, by phagocytes.

• Stages of phagocytosis:
  1. The phagocyte extends its pseudopodium towards a bacterium and envelops it.
  2. Bacterial ingestion forms a phagosome.
  3. The phagosome combines with a lysosome, which secretes lysozyme into the phagosome.
  4. Lysozyme destroys the bacterium in the phagosome.
  5. The phagocyte expels the remains of the digested microorganism.
• Inflammation: An immediate response that destroys and neutralises the harmful actions of microorganisms and toxins at the early stage of infection. The area swells, turns red and feels painful.
• Inflammatory response:
  1. Damaged tissue releases histamine, stimulating an immediate inflammatory response.
  2. Histamine expands blood capillaries, increases blood flow and increases capillary permeability to phagocytes; phagocytes and clotting factors accumulate at the infected area.
  3. The blood clotting mechanism is triggered.
  4. Phagocytes carry out phagocytosis.

Third Line of Defence

• Activated when pathogens overcome the second line of defence.
• Lymphocytes formed in lymph nodes produce antibodies.
• Antibody action is specific: each antibody combines with only one type of antigen.
• Lymphocytes accumulate in lymph nodes to destroy antigens and foreign particles, causing the lymph nodes to swell.
• Macrophages in lymph nodes destroy bacteria, dead tissues and foreign particles through phagocytosis.
• Lymphocytes are divided into T lymphocytes and B lymphocytes.
• T lymphocytes attack cells infected by pathogens and stimulate B lymphocytes to produce memory cells.
• If the same pathogen attacks again, memory cells are stimulated to produce antibodies immediately.