Overview of Physiological Systems and Immune System

Overview of Physiological Systems and the Immune System

• Physiological systems are circular, without a clear beginning or end, maintaining a balance called homeostasis
• The immune system is complex and interconnected, requiring a breakdown of its parts to understand its function
• Understanding the immune system is crucial to understanding its responses and functions

Key Questions About the Immune System

• How do we know we have an immune system?
  • When injured, the area swells, turns red, and heals
  • Allergies demonstrate the system's ability to react to substances
  • The immune system learns from infections and avoids repeating the same illness
• How do we know we need an immune system?
  • Some children are born with no immune system, and without treatment, they cannot survive infections.
  • Experiments with animals show the immune system is necessary for survival.

What is the purpose of the immune system?

• Defends against invaders (bacteria, viruses)
• Distinguishes self from non-self to avoid attacking the body's own tissues
• Detects and targets threats to the body
• Maintains harmony with harmless microorganisms that live in the body (good bacteria)

Components of the Immune System

• Barriers (today's focus)
• Cells (e.g., white blood cells)
• Molecules (e.g., antibodies)

Barriers: The First Line of Defense

• Mechanical Defenses: physical actions or structures that prevent harmful things from entering the body
  • Epithelial cells joined by tight junctions: prevent invaders like bacteria or viruses from entering.
  • Longitudinal flow of fluids (saliva, urine, etc.): washes away harmful substances
  • Movement of mucus by cilia: removes dirt and germs from the respiratory system
  • Tears and nasal cilia: removes harmful particles
• Chemical Defenses: substances the body produces to kill or stop harmful invaders
  • Fatty acids: create an environment where bacteria struggle to survive
  • Low pH (acidic environment): kills most bacteria in food
  • Enzymes (pepsin): breaks down harmful substances or kills bacteria
  • Pulmonary surfactant: helps keep harmful germs away
  • Molecules that directly attack invaders (ß-defensins, a-defensins, RegIII, cathelicidins, and histatins): kill bacteria, fungi, or other harmful organisms
• Microbiological Defenses: normal microbiota living on the skin and inside the body
  • Takes up space and nutrients, preventing harmful bacteria from growing
• Nutritional Barriers: body tightly controls important nutrients, like iron, to prevent bacteria from thriving

Innate Immunity

• Present at birth and inherited from parents
• Fast but non-specific (reacts the same way to all threats)
• Encoded by genes that don't change much over generations (found in all living organisms)
• No "true memory"

Adaptive Immunity

• Develops over time as we encounter antigens.
• Slower initially but highly specific and improves with repeated exposure.
• Encoded by genes that recombine to create unique immune responses.
• Found mainly in advanced animals (humans)
• Remembers past infections and responds faster and stronger next time (immunological memory)

Cells of the Immune System

• Adaptive Immunity Cells
  • T cells: attack infected cells directly
  • B cells: produce antibodies to fight specific germs
  • Natural Killer (NK) cells: kill harmful cells without needing prior instructions
• Innate Immunity Cells
  • Neutrophils: first responders to infection or injury, eat germs (phagocytosis)
  • Macrophages: eat germs and clean up dead cells
  • Dendritic Cells: act as messengers between innate and adaptive immunity, capture germs
  • Mast Cells: found in tissues; release chemicals to help fight infections

How the Immune System Recognizes Pathogens

• Immune cells have sensors (Pattern Recognition Receptors (PRRs)) to detect germs
• These receptors are located on cell surfaces, inside the cell (cytoplasm), or inside cellular compartments (endosomes)
• There are different families of PRR families including TLRs, CLRs, NLRs, and RLRs
• Pathogen-Associated Molecular Patterns (PAMPs) are unique features that PRRs recognize

Phagocytosis

• Immune cells engulf and destroy germs forming a phagolysosome (phagosome + lysosome).
• The phagolysosome uses low pH, toxic molecules (hydrogen peroxide, bleach-like chemicals), and enzymes.

Blood Composition

• Plasma: liquid part of blood, containing hormones, antibodies, and nutrients.
• Cells (45%): red blood cells (carry oxygen), white blood cells (fight infections), platelets (help blood clot).

Blood Cell Formation

• Blood cells are made in the bone marrow from stem cells
• Stem cells produce myeloid cells (neutrophils, macrophages, dendritic cells) and lymphoid cells (T cells, B cells, NK cells).

Founders of Immunology

• Paul Ehrlich: discovered antibodies
• Elie Metchnikoff: discovered phagocytosis

Overview: Anatomy of the Immune Response

• Three main stages
  • Barrier Immunity: Physical barriers prevent germs from entering (skin, mucus, etc.)
  • Innate Immunity: First line of defense (immediate but general)
  • Adaptive Immunity: Second line of defense – slower but highly specific

Immune System Organization

• Circulates throughout the body
• Made of cells and molecules working together

Primary Organs

• Bone Marrow: produces immune cells
• Thymus: matures T cells
• Secondary Organs
  • Lymph Nodes: hubs for immune cells
  • Spleen: recycles old red blood cells, houses immune cells to fight infections

The Lymphatic System

• Network of vessels and nodes parallel to the circulatory system
• Transports immune cells and germs to lymph nodes
• Returns immune cells to infection sites
• Contains different compartments:
  • Cortex (B-cell production)
  • Paracortex (T-cell function/production)
  • Medulla (macrophage and plasma cells complete the immune response)