Learning Objectives
By the end of this section, you will be able to:
- Discuss the transmission of an infective antigen into the host
- Describe the key components of the immune system’s response to infection
- Explain the important roles played by the lymphatic system in infection protection
Have you ever wondered how nurses can be exposed to patients with communicable diseases day after day but not become ill? Many factors affect the body’s ability to defend against infection, and some individuals are at greater risk of developing an infection. When an infection does occur, early recognition is important to prevent it from spreading within the individual, as well as to others. Protecting people from developing an infection, as well as preventing the spread of infection, are major concerns for nurses. This section discusses the anatomy and physiology of the immune and lymphatic systems and their roles in fighting infection.
The immune system is the complex collection of cells and organs that destroy or neutralize pathogens that would otherwise cause disease or death. The immune system is associated with the lymphatic system to such a degree that the two are virtually indistinguishable. However, they are separate body systems with distinct components and functions. The lymphatic system is the system of vessels, cells, and organs that carry excess fluids to the bloodstream and filter pathogens from the blood. The swelling of lymph nodes during an infection and the transport of lymphocytes via the lymphatic vessels are but two examples of the many connections between these critical organ systems.
Infection
An infectious disease is caused by a pathogen—an agent such as a bacterium, virus, parasite, or fungus—that enters a human host, multiplies, and causes infection that can spread via direct or indirect transmission (CDC, 2022a). Two models—the epidemiologic triad and the chain of infection—are vital to understanding transmission of infectious diseases. The epidemiologic triad includes the host, agent, and environment, and describes the who, what, and where of the infectious process. The chain of infection builds on this triad; a disease is considered communicable or contagious (capable of spreading from one person to another) if the infected host has a portal of exit, a means of transmission, and a portal of entry into another susceptible host. Some infectious diseases can be spread by germs in the air, food, water, or soil and by vectors or by animals to humans. Although these diseases are still considered infectious, because they are caused by bacteria, viruses, parasites, or fungi, they are not considered communicable from person to person.
The agent’s pathogenicity (the potential ability to cause disease in a susceptible host) depends on the agent’s infectivity and its ability to invade the host, destroy host body cells, and produce toxins that result in the virulence, or severity, of the infectious disease. The human immune system has evolved over thousands of years to be able to control many pathogens, but the pathogens themselves have also evolved ways to evade the immune response. The first step in the immune response is for the body to recognize the presence of an antigen, or a molecule on the cell surface of a pathogen that is recognized as an invader by the immune system. However, pathogens mutate constantly, and some mutations change the chemical structure of an antigen sufficiently, so the immune response no longer recognizes it. This explains why a new strain of bacteria can evade the immune response of a body that has already been exposed to an older strain: the new strain is characterized by a structurally distinct antigen. This also explains why a new flu vaccine must be developed each year: each flu season, a new variant of the influenza virus, with a unique antigen, may have evolved.
The susceptibility of a host is a key component in the epidemiologic triad and chain of infection (CDC, 2022a). Susceptibility depends on many factors, such as age, sex, physical health, and immune status. Portals of entry and exit can include the skin, conjunctiva, respiratory tract, gastrointestinal (GI) tract, genital tract, and vertical transmission during the birthing process (CDC, 2022a). The environment is a component common to both the epidemiologic triad and the chain of infection. In this context, the environment refers to reservoirs of infectious agents, which may be humans, animals, plants, insects, water, and soil. The environment and any environmental changes can have a significant impact on the transmission of waterborne, foodborne, and vector-borne agents. Transmission of infectious agents includes airborne, direct contact, indirect contact, and droplet transmission. If an environment is favorable for the survival of an infectious agent and there is an opportunity for the host to be exposed to the agent, infection and disease will ensue. Thankfully, there are many pharmacologic treatments available for most infections, but, as you will see later in the chapter, in some cases, the established treatments are becoming ineffective, making it difficult to completely eradicate the pathogen.
Life-Stage Context
Older Adults and Susceptibility to Infection
Older adults are at an increased risk for infections for many reasons. First, with aging, the effectiveness of the immune system decreases, making it harder for the body to fight off infections. Additionally, many older adults reside in facilities that put them in regular, close contact with a variety of people. This puts them at higher risk for developing infections that are passed from person to person. Although older adults are more susceptible to most infections, the most common ones seen in this population include pneumococcal disease and influenza, which are often acquired while receiving care for other conditions. Infections acquired in this manner are called nosocomial (health care–associated) infections (Cristina at al., 2021).
Functions of the Immune System
The immune system is a collection of barriers, cells, and soluble proteins that interact and communicate with each other in extraordinarily complex ways (CDC, 2022a). The modern model of immune function is organized into three phases based on the timing of their effects:
- A barrier defense, such as the skin and mucous membranes, acts instantaneously to prevent pathogenic invasion into body tissues.
- The rapid but nonspecific innate immune response consists of a variety of specialized cells and soluble factors, such as vascular endothelial growth factor or certain hormones, that can bind to receptors as part of the immune response.
- The slower but more specific and effective adaptive immune response involves many cell types and soluble factors. However, it is primarily controlled by white blood cells (WBCs) known as lymphocytes, which help control immune responses.
For the purpose of this chapter, we will focus on the respective functions of barrier defenses and WBCs.
Link to Learning
Review how the immune system works with this video.
Barrier Defenses
Physical barriers are defense mechanisms that prevent pathogens from entering the body, destroy them after they enter, or flush them out before they can establish themselves as an infection. These barriers are among the body’s most basic defense mechanisms. They are not themselves a response to infection, but they are continuously working to protect against a broad range of pathogens.
The different modes of barrier defenses are associated with the external surfaces of the body, where pathogens may try to enter. The primary barrier is the skin. The outer skin consists of a layer of cells that are too dry for bacteria to grow in; furthermore, these outer cells are continuously sloughed off, carrying with them bacteria and other pathogens. Skin secretions such as sweat physically wash microbes away; they also contain lipids and other chemicals that contribute to a toxic environment for pathogens—for example, by lowering pH, creating an acidic environment in which the pathogen can no longer survive. The acidic environment of the stomach, which is fatal to many pathogens, is also a barrier. Additionally, the mucus layers of the gastrointestinal tract, respiratory tract, reproductive tract, eyes, ears, and nose trap microbes and a wide range of debris and facilitate their removal.
Leukocytes
Another name for a WBC is a leukocyte. They are the primary cells of the immune system, tasked with fighting off pathogens to prevent infection (Tigner et al., 2022; University of Rochester Medical Center, 2024). There are several specific types of leukocytes, which are listed in Table 22.1.
| Type of Leukocyte (normal percentage of all WBCs) | Description |
|---|---|
| Monocyte (2%–8%) |
|
| Lymphocyte (20%–40%) |
|
| Neutrophil (40%–60%) |
|
| Basophil (0.5%–1%) |
|
| Eosinophil (1%–4%) |
|
Functions of the Lymphatic System
A major function of the lymphatic system is to drain body fluids and return them to the bloodstream. These processes happen via a series of vessels, trunks, and ducts. The term lymph is used to describe interstitial fluid once it has entered the lymphatic system. When the lymphatic system is damaged in some way (e.g., blocked by cancer cells or destroyed by injury), protein-rich interstitial fluid accumulates in the tissue spaces. This inappropriate accumulation of fluid, or lymphedema, may lead to serious medical consequences, such as pain and limited mobility. The main organs and other components of the lymphatic system include the lymph nodes, spleen, bone marrow, and thymus (Figure 22.2).
Lymph Nodes
A lymph node is a small, bean-shaped organ that removes debris and pathogens from the lymph; lymph nodes are sometimes referred to as the filters of the lymph (Figure 22.3). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within the nodes internalize and kill many of the pathogens that pass through, thereby removing them from the body. (This is the reason swollen lymph nodes may be a symptom of an infection.) The lymph nodes are also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system.
Spleen
The spleen is another important lymphoid organ (Figure 22.4). It is about 12 cm (5 in.) long and attached to the lateral border of the stomach. The spleen also is sometimes called the filter of the blood because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the bloodstream. The spleen is also the location of immune responses to blood-borne pathogens. Because of its role in immune function, patients who have had their spleen surgically removed for medical reasons are at higher risk of infection.
Bone Marrow
The soft, jelly-like tissue located inside bones is called bone marrow (Figure 22.5). Its main function is to produce blood cells and release them into the bloodstream once they are fully matured (Lucas, 2021). The importance of bone marrow cannot be overstated. It produces and matures red blood cells (including platelets) in addition to WBCs. Without optimal functioning of the bone marrow, the body would be unable to effectively fight infection or stop bleeding, problems that could very quickly result in a life-threatening condition such as hemorrhagic shock or sepsis (Lucas, 2021).
Thymus
The thymus is lymphatic organ whose main function is the production of a group of hormones that contribute to the development and differentiation of T lymphocytes, immune cells that contain specific receptors for antigens and play a large role in active immunity. The thymus is located in the chest cavity, between the lungs and behind the sternum (Figure 22.6).