Host Defenses I: Overview and Innate Immunity – Microbiology Study Notes

Host Defenses: An Overview

Introduction to Host Defenses

The human body is equipped with a complex system of defenses to protect against infection and disease. These defenses are categorized into innate (nonspecific) and acquired (specific) immunity, each with distinct mechanisms and components.

• Innate Immunity: Provides immediate, nonspecific protection against pathogens.

• Acquired Immunity: Develops more slowly and targets specific pathogens through specialized immune cells.

Major Components of Host Defenses

Host defenses are organized into three main lines:

• First Line of Defense: Physical and physiological barriers that prevent pathogens from entering body compartments.

• Second Line of Defense: Cellular and chemical systems that act if infectious agents bypass surface defenses.

• Third Line of Defense: Specific host defenses that target and destroy pathogens using specialized white blood cells (B cells, T cells).

Defense Line	Type	Main Components
First	Innate	Physical barriers, Microbiota barrier, Chemical barriers
Second	Innate	Phagocytosis, Inflammation, Fever, Antimicrobial products
Third	Acquired	B cells, T cells

Pathogen Recognition

Self vs. Nonself Recognition

White blood cells continuously patrol the body, distinguishing between self (body cells) and nonself (pathogens such as bacteria and viruses).

• Antigens (Markers): Molecules on cell surfaces, composed of proteins and/or sugars, evaluated by immune cells.

• Immune System Function: Must recognize and respond to potential threats while avoiding attack on self tissues.

Immune Disorders

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

• Example: In rheumatoid arthritis, the immune system attacks joint tissues, causing pain and loss of function.

Molecular Patterns in Immunity

Pathogen-Associated Molecular Patterns (PAMPs)

PAMPs are molecular markers shared by many microbes, recognized by the host's innate immune system.

• PAMPs: Include structures like peptidoglycan, lipopolysaccharide, and double-stranded RNA.

Pattern Recognition Receptors (PRRs)

• PRRs: Receptors on immune cells (phagocytes, dendritic cells, etc.) that bind to PAMPs and initiate immune responses.

• Collectins: A class of soluble PRRs that mark PAMPs for destruction.

The immune system is also signaled to tolerate non-harmful antigens, such as those from food and commensal microorganisms.

Summary Table: Key Terms

Term	Definition
Innate Immunity	Immediate, nonspecific defense against pathogens
Acquired Immunity	Specific, adaptive defense involving B and T cells
Antigen	Molecule on cell surface recognized by immune system
PAMP	Pathogen-associated molecular pattern, common to microbes
PRR	Pattern recognition receptor, binds PAMPs

Additional info:

• Autoimmune diseases result from failure to distinguish self from nonself.

• PAMPs and PRRs are central to the activation of innate immunity.