Host Defenses I: Overview and Innate Immunity (Chapter 15 Study Notes)

Host Defenses: An Overview

Introduction to Host Defenses

The human body is constantly exposed to a variety of potential pathogens. To protect itself, it relies on a complex system of defenses that can be broadly categorized as innate (nonspecific) and acquired (specific) immunity. These defenses work together to prevent infection and maintain health.

• Innate Immunity: Present from birth, provides immediate but nonspecific protection against pathogens.

• Acquired Immunity: Develops after exposure to specific antigens and provides targeted, long-lasting protection.

Major Components of Host Defenses

Host defenses are organized into three main lines:

• First Line of Defense (Innate): Physical and physiological barriers that block entry of pathogens (e.g., skin, mucous membranes, microbiota, chemical barriers).

• Second Line of Defense (Innate): Cellular and chemical responses that act if pathogens bypass the first line (e.g., phagocytosis, inflammation, fever, antimicrobial products).

• Third Line of Defense (Acquired): Specific immune responses involving lymphocytes (B cells and T cells) and the production of antibodies.

Pathogen Recognition

Self vs. Nonself Recognition

White blood cells (WBCs) patrol the body, distinguishing between self (body's own cells) and nonself (foreign invaders such as bacteria and viruses). This recognition is crucial for effective immune responses and for preventing autoimmune reactions.

• Antigens (Markers): Molecules on cell surfaces, composed of proteins and/or sugars, that are evaluated by immune cells to determine if a cell is self or nonself.

Proper Functioning of the Immune System

• Foreign substances must be recognized as threats and dealt with appropriately.

• Self cells and harmless substances (e.g., food, commensal microbes) must not be attacked.

• Autoimmune Disorders: Occur when the immune system mistakenly attacks the body's own tissues (e.g., Rheumatoid arthritis).

Molecular Patterns in Immunity

Pathogen-Associated Molecular Patterns (PAMPs)

PAMPs are molecular structures commonly found on many microbes but not on host cells. They serve as universal signals for the immune system to recognize potential pathogens.

• Examples: Peptidoglycan, lipopolysaccharide, double-stranded RNA (in viruses).

Pattern Recognition Receptors (PRRs)

PRRs are receptors on immune cells (such as phagocytes and dendritic cells) that detect PAMPs and trigger innate immune responses.

• Found on the surface of immune cells and in soluble form (e.g., collectins).

• Enable rapid recognition and response to a wide range of pathogens.

Immune Tolerance

• The immune system is trained not to react against non-harmful antigens, such as those from food or commensal (beneficial) microorganisms.

Summary Table: Major Components of Host Defenses

Line of Defense	Type	Main Components	Examples
First	Innate	Physical, chemical, and microbiota barriers	Skin, mucous membranes, normal flora, lysozyme
Second	Innate	Cellular and chemical defenses	Phagocytes, inflammation, fever, complement proteins
Third	Acquired	Specific immune responses	B cells, T cells, antibodies

Key Terms and Definitions

• Antigen: Any substance that can be recognized by the immune system and provoke an immune response.

• PAMP (Pathogen-Associated Molecular Pattern): Molecular structures found on pathogens but not on host cells, recognized by PRRs.

• PRR (Pattern Recognition Receptor): Receptors on immune cells that detect PAMPs and initiate immune responses.

• Autoimmune Disease: A condition in which the immune system attacks the body's own tissues.

Example: Autoimmune Disease

Rheumatoid Arthritis: An autoimmune disorder where the immune system attacks the joints, leading to pain, swelling, and loss of function.

Additional info: The notes above are based on the provided slides and expanded with standard academic context for clarity and completeness.