18. Innate Nonspecific Host Defenses
18.1 Physical Defences
Learning Objectives
- Describe the various physical barriers and mechanical defences that protect the human body against infection and disease
- Describe the role of microbiota as a first-line defence against infection and disease
Angela, a 25-year-old female patient in the emergency department, is having some trouble communicating verbally because of shortness of breath. A nurse observes constriction and swelling of the airway and labored breathing. The nurse asks Angela if she has a history of asthma or allergies. With some difficulty, she explains that her father died suddenly at age 27, when she was just a little girl, of a similar respiratory attack. The underlying cause had never been identified.
- What are some possible causes of constriction and swelling of the airway?
- What causes swelling of body tissues in general?
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Nonspecific innate immunity can be characterized as a multifaceted system of defences that targets invading pathogens in a nonspecific manner. In this chapter, we have divided the numerous defences that make up this system into three categories: physical defences, chemical defences, and cellular defences. However, it is important to keep in mind that these defences do not function independently, and the categories often overlap. Table 18.1 provides an overview of the nonspecific defences discussed in this chapter.
Table 18.1. Overview of Nonspecific Innate Immune Defences
| Overview of Nonspecific Innate Immune Defences | |
|---|---|
| Physical defences | Physical barriers |
| Mechanical defences | |
| Microbiome | |
| Chemical defences | Chemicals and enzymes in body fluids |
| Antimicrobial peptides | |
| Plasma protein mediators | |
| Cytokines | |
| Inflammation-eliciting mediators | |
| Cellular defences | Granulocytes |
| Agranulocytes |
Physical defences provide the body’s most basic form of nonspecific defence. They include physical barriers to microbes, such as the skin and mucous membranes, as well as mechanical defences 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.
Physical Barriers
Physical barriers play an important role in preventing microbes from reaching tissues that are susceptible to infection. At the cellular level, barriers consist of cells that are tightly joined to prevent invaders from crossing through to deeper tissue. For example, the endothelial cells that line blood vessels have very tight cell-to-cell junctions, blocking microbes from gaining access to the bloodstream. Cell junctions are generally composed of cell membrane proteins that may connect with the extracellular matrix or with complementary proteins from neighbouring