Barriers and Cells of the ainnate immune system

Overview of the Immune System

• The immune system provides resistance to disease.

• It is a functional system rather than a structural one.

• The lymphatic system is considered the structural component with:

  • Lymphatic vessels

  • Lymph nodes

  • MALT tissues (mucosa-associated lymphoid tissue)

  • Spleen

  • Other supporting structures

• The immune system actively fights against or prevents disease.

Two Intrinsic Systems of the Immune System

Innate Defense System

• Also known as the non-specific defense system.

• Present at birth: provides immediate protection against pathogens.

• Constitutes the first and second lines of defense:

  • First line of defense:

  • External body membranes:

    • Skin

    • Mucous membranes

  • Second line of defense:

  • Non-specific antimicrobial proteins

  • Phagocytic cells

  • Other cells that inhibit pathogen spread

  • Inflammatory responses

  • Fever

Adaptive Defense System

• Also known as the specific defense system.

• Functions as the third line of defense:

  • Targets specific foreign substances.

  • Becomes primed during the initial encounter with a pathogen.

  • Maintains specificity towards that pathogen throughout its lifespan.

Interaction Between Innate and Adaptive Immunity

• Both the innate and adaptive systems can be activated simultaneously.

• They release and recognize many of the same defensive molecules.

• Innate defenses have specific pathways but are mainly non-specific.

• Innate responses alert adaptive responses through the release of proteins.

• Continuous communication enhances overall immune response.

Details of Innate Defenses

First Line of Defense: Surface Barriers

• Skin:

  • Contains keratin, resistant to:

  • Weak acids and bases

  • Bacterial enzymes and toxins

  • Functions as a barrier against pathogens and their toxins.

  • Acidic secretions (acid mantle) inhibit microbial growth:

  • Present on skin, mucous membranes, including the mouth and vaginal canal.

• Mucous Membranes:

  • Serve as mechanical barriers in internal body areas.

  • Produce protective chemicals to destroy microorganisms:

  • Lysozyme in saliva acts as an antimicrobial enzyme.

  • Mucin traps microorganisms in digestive and respiratory tracts.

  • Lactoferrin and stomach acids kill many microorganisms.

Second Line of Defense: Internal Defenses

• Activated when surface barriers are breached:

  • Phagocytic cells (e.g., neutrophils, macrophages):

  • Engulf and digest foreign invaders.

  • Neutrophils are the most abundant; often die after combatting infections.

  • Monocytes convert to macrophages, which become highly phagocytic over time.

  • Natural killer cells:

  • Target and kill virus-infected and cancer cells via apoptosis (programmed cell death).

  • Inflammation:

  • Triggers increased blood flow, bringing in more immune cells.

  • Antimicrobial proteins:

  • Attack and destroy pathogens.

  • Fever:

  • Elevates body temperature to enhance immune response and inhibit pathogen growth.

Phagocytosis Process

• Phagocytosis is the process where phagocytic cells ingest and digest pathogens: white blood cellssuch as macrophages and neutrophils play a key role in identifying, engulfing, and breaking down foreign invaders, utilizing enzymes and reactive oxygen species to effectively eliminate them. Neutrophils usually die off during the process.Macrophage starts as monocytes tend to show up after neutrophils.

  • Starts with recognition of pathogen's carbohydrate signature (antigens).

  • Phagocyte adheres to the pathogen, extending pseudopods to engulf it in a vesicle called a phagosome.

  • The phagosome fuses with a lysosome, forming a phagolysosome, where digestive enzymes break down the pathogen.

  • Indigestible materials are exocytosed out of the cell.

Opsonization is the process by which pathogens are marked for destruction by antibodies or complement proteins, enhancing their visibility to phagocytes and facilitating a more efficient immune response.

• Process that enhances phagocytosis:

  • Involves coating pathogens with antibodies or proteins, making them easier for phagocytes to grab.

  • Acts as a handle for phagocytic cells, improving capture efficiency.

Natural Killer Cells : A type of lymphocyte that plays a crucial role in the innate immune response by identifying and destroying infected or cancerous cells without prior sensitization.

• Large granulocytes that seek out and destroy non-self cells (cancer or virus-infected).

• Do not use phagocytosis; induce target cells to undergo apoptosis.

• Release chemicals that can also enhance inflammation.

• Apoptosis is a programmed cell death that plays a crucial role in maintaining homeostasis and preventing the spread of infections within the body.

• Infflamation triggered whenever body tissues are injured.

Summary of Immune System Components

• The immune system consists of integrated components that work defense against pathogens:

  • Innate defenses are first responders.

  • Adaptive defenses provide long-lasting immunity and specialization against specific pathogens.

• Misidentification or evasion by pathogens may occur, requiring flexible immune strategies.