Immune System and Defense Mechanisms: Chapter 43 Summary (2025/2026)

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Pathogen
- an organism or virus that causes disease
- Humans are excellent hosts for these

Macrophage
- Immune cell
- Engulfed rod-shaped bacteria

Lymphocytes
- White blood cells
- Recognize and respond to specific types of pathogens

Immune System
- An organism's systems of defenses against agents that cause disease
- 1st line of defense
- Prevent pathogens from gaining entrance to body
- Secretion
- Traps or kills pathogens guard the body's entrance and exits
- Fight infections
- Must detect foreign particles and cells within body
- Immune cell produce receptor molecules that bind specifically to molecules from
foreign cells or viruses and activate defense responses
Molecular recognition
- Innate immunity
- Common to all animals
- Form of defense
- Active immediately upon exposure to a pathogen and that is the same whether or
not the pathogen has been encountered previously
- Adaptive immunity
- Found in only vertebrates
- Defense
- Mediated by B cells and T cells
- Exhibit specificity, memory, and self-nonself recognition
- “Acquired immunity”




Innate Immunity