Points to Consider
- How do you think pathogens can be recognized?
- Why do you think the body needs specific defenses as well as general ones?
21.4 Lesson 21.4: Immune System Defenses
Lesson Objectives
- Describe the immune system.
- Explain how lymphocytes respond to pathogens.
- Define immunity and vaccination.
Check Your Understanding
- What are the first two lines of defense?
- Give examples of pathogens.
Introduction
If pathogens manage to get through the body’s first two lines of defense, a third line of
defense takes over. This third line of defense involves the immune system. It is called an
immune response. The immune system has a special response for each type of pathogen.
What Is the Immune System?
Theimmune systemis also called the lymphatic system. It is named forlymphocytes,
which are the type of white blood cells involved in an immune response. The parts of the
immune system are shown inFigure 21.20. They include several lymph organs, lymph
vessels, lymph, and lymph nodes (Figure21.21).
Lymph Organs
The lymph organs are the red bone marrow, thymus gland, spleen, and tonsils. Each organ
has a different function in the immune system. They are described inFigure21.24.
Organs of the Immune System