23 Chapter 23: Immune System
Learning Objectives
After studying this chapter, you should be able to:
- Demonstrate your understanding of the defensive mechanisms of the human body by recognizing (or naming) and describing the components of the three lines of defense and distinguishing the functions of each.
- Understand the range of diseases that affect humans by identifying problems that result from defects in your immune system (i.e., allergy, immunodeficiency, autoimmune disease).
23.1 Introduction to the Immune System
The environment contains numerous pathogens, which are microorganisms that cause diseases in their hosts. A host is the organism that is invaded and often harmed by a pathogen. Pathogens include viruses, bacteria, protists, fungi, and other infectious organisms. We are constantly exposed to pathogens in food and water, on surfaces, and in the air. Mammalian immune systems evolved for protection from such pathogens; they are composed of an extremely diverse array of specialized cells and soluble molecules that coordinate a rapid and flexible defense system capable of providing protection from a majority of these disease agents.
Components of the immune system constantly search the body for signs of pathogens. When pathogens are found, immune factors are mobilized to the site of an infection. The immune factors identify the nature of the pathogen, strengthen the corresponding cells and molecules to combat it efficiently, and then halt the immune response after the infection is cleared to avoid unnecessary host cell damage. The immune system can remember pathogens to which it has been exposed to create a more efficient response upon re-exposure. This memory can last several decades. Features of the immune system, such as pathogen identification, specific response, amplification, retreat, and remembrance are essential for survival against pathogens. The immune response can be classified as either innate or adaptive. The innate immune response is always present and attempts to defend against all pathogens rather than focusing on specific ones. Conversely, the adaptive immune response remembers past infections and mounts pathogen-specific defenses.
23.2 Innate Immunity
The human immune system is a complex, multilayered system for defending against external and internal threats to the integrity of the body. The system can be divided into two types of defense systems: the innate immune system, which is nonspecific toward a particular kind of pathogen, and the adaptive immune system, which is specific (Figure 23.2). Innate immunity is not caused by an infection or vaccination and depends initially on physical and chemical barriers that work on all pathogens, sometimes called the first line of defense. The second line of defense of the innate system includes chemical signals that produce inflammation and fever responses as well as mobilizing protective cells and other chemical defenses. The adaptive immune system mounts a highly specific