Innate Immunity

Innate Immunity: 1st & 2nd Lines of Defence

Mechanisms of Transmission

• Direct contact: Disease transfer from an infected to a susceptible person via physical contact.

  • Person-to-person: Direct touch or exchange of bodily fluids.

• Indirect contact: Transfer to a susceptible person without physical contact.

  • Airborne transmission.

  • Contaminated objects (fomites).

  • Food and drinking water.

  • Animal-to-person contact.

  • Vectors.

  • Environmental reservoirs.

• Person-to-person: Mucous, closeness.

• Droplet transmission: Sneezing, coughing.

• Airborne transmission: Pathogen survives longer and travels more than 1-2 metres.

• Transmission through contaminated objects (fomites): Objects or materials carrying pathogens.

• Transmission through food and drinking water.

• Animal-to-person: Helminths in the soil, parasites.

• Vectors: Mosquitoes, ticks, fleas, and lice are common vectors.

• Environmental reservoirs: Soil, water, and vegetation.

Vectors

• A vector is a carrier of a disease-causing agent from an infected to a non-infected individual or its food/environment.

• Example: Mosquitoes carrying malaria parasites.

• A vector is a living organism.

Why We Need Defence

• The human body is constantly challenged by pathogens (disease-causing agents).

• Infection is a battle between invading pathogens and the host.

• Our bodies are equipped to fight off invading pathogens that may cause disease.

Pathogens

Bacteria

• Prokaryotic cell with a cell wall.

• Size: 0.5-5 micrometres.

• Divide by binary fission; reproduce quickly.

• Direct contact transmission.

• Transmission: Ingested, bodily fluids.

• Examples: Salmonella, E. coli (food poisoning), ear and eye infections, cholera, diarrhoea.

• Treatment: Antibiotics.

Virus

• Acellular, no cell wall, but with a protein capsid.

• Size: 0.01 to 0.3 micrometres.

• Needs a host to reproduce; mutates, evolves, recombines quickly.

• Direct contact transmission.

• Transmission: Ingested, bodily fluids.

• Examples: Influenza, HIV/AIDS, smallpox, measles, rhinovirus (common cold), herpes, Ebola.

• Treatment: Antiviral (if accessible) or pain-relieving medicine.

Lines of Defence

• First line of defence: Physical barriers and the immune system defend the body against pathogens.

• Second line of defence: Innate immunity.

• Third line of defence: Specific immunity.

1st Line – Physical and Chemical Barriers

• Non-specific: Stops microbes from entering the body.

• Physical barrier: Skin and mucous membranes prevent penetration by microbes.

First Line of Defence Examples

• Lysozyme in tears and other secretions.

• Removal of particles by cilia in the nasopharynx.

• Mucus lining the trachea.

• Skin surface (physical barrier), fatty acids, normal flora.

• Rapid pH change.

• Stomach (pH 2).

• Normal flora.

• Flushing of the urinary tract.

Second Line of Defence

• Activated if microbes enter the body.

• Non-specific: Stops any type of microbe.

• Attacking cells and molecules target pathogens breaching the first line.

• Innate responses are non-specific, rapid, present in all animals, are fixed responses, and do not create immunological ‘memory’.

• Innate immune response functions as the second line of defence against infection. Non-specific

• Adaptive immune response develops slower but manifests as increased antigenic specificity and memory. This is the third line of defence.

Second Line of Defence – Inflammation, Phagocytosis, Fever

Inflammation

• A pathogen stimulates increased blood flow to an infected area.

• Blood vessels in the area expand.

• White blood cells leak into the tissue from the vessels to invade the infected tissue.

• WBCs (phagocytes) engulf and destroy bacteria.

• This causes a red, swollen, painful inflammatory response.

Phagocytosis

• Cellular process for ingesting and eliminating particles larger than 0.5 \,\text{μm} in diameter, including microorganisms, foreign substances, and apoptotic cells.

• Initial contact occurs between phagocytes and pathogens.

• Phagocyte engulfs the pathogen.

• Pathogen is enclosed within the cytoplasm in the phagocyte.

• A lysosome (filled with hydrolytic enzymes) fuses with the engulfed pathogen.

• The enzymes digest the pathogen.

• The digested parts are released.

• The antigen is presented on the plasma membrane.

Fever

• Increased temperatures enhance a variety of immune cell functions.

• Includes phagocytosis and reactive oxygen species production by neutrophils.

• Enhanced function of natural killer cells, T-helper cells, and antibody-producing cells.

• Fever can induce heat-shock proteins in both pathogens and host cells.