20.1: Physical Defenses Overview

Learning Objectives

• Describe the various physical barriers and mechanical defenses that protect the human body against infection and disease
• Describe the role of microbiota as a first-line defense against infection and disease

Nonspecific innate immunity can be characterized as a multifaceted system of defenses that targets invading pathogens in a nonspecific manner. In this chapter, we have divided the numerous defenses that make up this system into three categories: physical defenses, chemical defenses, and cellular defenses. However, it is important to keep in mind that these defenses do not function independently, and the categories often overlap. Table \(\PageIndex{1}\) provides an overview of the nonspecific defenses discussed in this chapter.

Table \(\PageIndex{1}\): Overview of Nonspecific Innate Immune Defenses

Overview of Nonspecific Innate Immune Defenses
Physical defenses	Physical barriers
	Mechanical defenses
	Microbiome
Chemical defenses	Chemicals and enzymes in body fluids
	Antimicrobial peptides
	Plasma protein mediators
	Cytokines
	Inflammation-eliciting mediators
Cellular defenses	Granulocytes
	Agranulocytes

Physical defenses provide the body’s most basic form of nonspecific defense. They include physical barriers to microbes, such as the skin and mucous membranes, as well as mechanical defenses that physically remove microbes and debris from areas of the body where they might cause harm or infection. In addition, the microbiome provides a measure of physical protection against disease, as microbes of the normal microbiota compete with pathogens for nutrients and cellular binding sites necessary to cause infection.

Query \(\PageIndex{1}\)

 

Interactive Element

Case Study Preview: “When Breathing Isn’t Easy: A Hidden Genetic Threat”

Angela arrives at the ER gasping for air, her throat swelling and her voice nearly gone. She has no known allergies, no asthma history - just a haunting family memory: her father died suddenly of similar symptoms at age 27. With no clear allergic trigger and no microbial infection, doctors dig deeper. What they uncover is a rare but dangerous condition: type I hereditary angioedema, caused by a deficiency in C1 esterase inhibitors that leads to uncontrolled inflammation.

In this case, you’ll follow Angela’s journey from misdiagnosis to lifesaving insight, learning how complement proteins, genetic inheritance, and the body’s inflammatory responses can sometimes become their own worst enemies.

It wasn’t an allergy. It was her immune system missing a critical off switch.

Chapter 17 Case Study - When Breathing Isn’t Easy: A Hidden Genetic Threat

Key Concepts and Summary

• Nonspecific innate immunity provides a first line of defense against infection by nonspecifically blocking entry of microbes and targeting them for destruction or removal from the body.
• The physical defenses of innate immunity include physical barriers, mechanical actions that remove microbes and debris, and the microbiome, which competes with and inhibits the growth of pathogens.
• The skin, mucous membranes, and endothelia throughout the body serve as physical barriers that prevent microbes from reaching potential sites of infection. Tight cell junctions in these tissues prevent microbes from passing through.
• Microbes trapped in dead skin cells or mucus are removed from the body by mechanical actions such as shedding of skin cells, mucociliary sweeping, coughing, peristalsis, and flushing of bodily fluids (e.g., urination, tears)
• The resident microbiota provide a physical defense by occupying available cellular binding sites and competing with pathogens for available nutrients.