4 Treatment and Prevention

Reading 3

On Guard: The Immune System

The purpose of this reading is to introduce students to the processes by which the immune system provides protection from viral and bacterial infections. Students read a short overview about the components of the immune system and how these components interact in their response to infection. Students then watch a video that clearly and concisely demonstrates this response.

In 1971 a baby boy was born and within 20 seconds after birth he was placed in a sterile plastic bubble where he was destined to live until his death at the age of 12. David Vetter was born with a rare disease called severe combined immunodeficiency (SCID) that occurs only in boys (see Figure 1). SCID patients lack components of their immune system that fight off infections by viruses, bacteria, and fungi. Left untreated, patients usually die before their first birthday from persistent infections.

Figure 1. David in his sterile bubble (left) and in a suit designed by NASA that allowed him to leave his bubble (right).

Courtesy of Baylor College of Medicine Archives and the Yovisto Blog

The world around us, and we ourselves, teems with microbial life. Most of these microorganisms will not harm us but a small number can do serious damage if allowed to enter and grow in the human body. As evidenced by patients with SCID, it is the immune system that protects and defends against these pathogens.

The body’s cells have markers on their surface that identify the cells as part of that body or “self”. The immune system is a complex collection of specialized cells, proteins, tissues, and organs designed to seek out and destroy anything unrecognizable as “self”. This includes not only viruses and bacteria but also toxins, pollutants, pollen and any other foreign substances that are potentially harmful.

First Line of Defense

Even before a pathogen comes within striking range of the immune response it must first gain entry into the body. The first barrier is skin. The skin not only acts as a physical barrier to keeping pathogens out but it also wages chemical warfare by secreting oils and sweat that acidifies the surface of the skin, making it difficult for microorganisms to attach and grow. Where openings in this barrier exist, such as the mouth or nostrils, mucous membranes that line them can trap microorganisms, denying them entry and secrete enzymes that can destroy them.

Second Line of Defense

If foreign substances make it through the first line of defense, the second line of defense, the immune system, is activated.

Antigens
Immune Cells
Lymph
Proteins

The immune system reacts to foreign substances by recognizing markers on the substances called antigens. These markers indicate that the substance is foreign or non-self.

The organs of the immune system produce different cell types involved in the immune response and are involved in moving these cells around the body (see Figure 2).

Bone marrow, the tissue inside bones, produces small white blood cells (lymphocytes) that mature in the thymus to form T-cells.

The bone marrow also produces another type of specialized cell called B-cells. B-cells produce antibodies, which are proteins that react specifically with different antigens. A third cell type involved in the immune response is the macrophage whose role is to engulf foreign substances and break them down.

Figure 2: Organs of the Immune System.

Source: National Cancer Institute

Lymphocytes travel throughout the body through blood vessels and also a network of lymphatic vessels containing a clear fluid called lymph that monitors the body for foreign substances.

Lymph nodes, made up of lymphatic vessels, are positioned around the body in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells encounter antigens. The spleen similarly provides a meeting place for antigens and cells of the immune system.

Figure 2: Organs of the Immune System.

Source: National Cancer Institute

Proteins play a central role in the immune response. Antigens on the surface of bacteria and viruses are made of protein and identify them as foreign or non-self to the immune system.

Proteins synthesized by specialized immune cells circulate throughout the body through the blood and lymphatic systems, interacting directly with antigens or acting as a call to arms for other cells of the immune system to initiate actions that help defend against infection.

Defending Against Infection

The immune system is very complex and involved in many different activities to keep a body healthy. For the purposes of learning about infectious diseases, the focus here is on how the immune system responds to viruses and bacteria.

Macrophages
T-Cells
B-Cells

The first cell that viruses or bacteria encounter after entering the body is the macrophage.

Large, amorphous and amoeba-like, the macrophage engulfs the invader, digests it, and inserts some of the pathogen’s antigens into its own surface membranes. These viral or bacterial antigens on the macrophage surface serve as a red flag, signaling to the rest of the immune system that a foreign substance has entered.

A chain of events begins with the activation of two types of T-cells, helper T-cells and killer (cytotoxic) T-cells. The response involving these cells is called a cell-mediated response.

A second kind of immune response known as the humoral response occurs when B-cells are stimulated to divide and to produce proteins called antibodies that bind to antigens and destroy the infectious agents. Another kind of B-cell, memory B-cells, linger long after an infection has cleared, producing antibodies that keep an infection from occurring again.

This video entitled The Immune System Explained I – Bacteria Infection by Kurzgesagt – In a Nutshell will show you how these components of the immune system interact to protect you against infection. As you watch, pause the video and take notes or make drawings about each step of the response: the cells and proteins involved, how they interact, and how their action leads to the next step of the response. You will be using this information in the next activity.

Agents of the Immune System

The table below provides a more detailed description of the major players in the immune response to viruses and bacteria and how they act.

Table 1. The Immune System's Response to Infection.

Immune System Component	Action	Interacts with
Macrophage	Circulates in the blood and lymph. Engulfs viruses, bacteria, infected cells that display the antigen on the surface. Inserts engulfed viral antigen (protein) on their own surface membrane. Signals to other immune cells (T-cells and B-cells). Stimulates helper T cells to bind to the viral antigen on the macrophage surface causing the macrophage to release a protein called interleukin-1. Interleukin-1 stimulates helper T cells to divide. Engulfs viruses, bacteria, or infected cells coated in the antibody.	viruses bacteria an infected host cell displaying viral antigen on its surface helper T cells viruses, bacteria, or infected cells with antibodies bound to antigens
Helper T Cell	Originates in bone marrow, matures in thymus gland (thus the “T” in T cell). Circulates in blood and lymph. Recognizes bacteria, and viruses by the proteins found on their surfaces. Recognizes viral and bacterial antigens on macrophage’s surface. Binds to viral and bacterial antigens on a macrophage, causing macrophage to release a chemical substance interleukin-1, which stimulates helper T cells to divide. Releases activating factor, interleukin-2, which stimulates other T cells and B cells to divide. Part of cell-mediated response.	macrophages killer T cells B cells
Killer (Cytotoxic) Cell	Originates in bone marrow, matures in thymus gland (thus the “T” in T cell). Circulates in blood and lymph. Arrives at infection when macrophages send out signal. Activated by interleukin released from helper T cells. Destroys virus infected cells that display viral antigens on their surface by injecting toxic chemicals into them. Part of cell-mediated response.	infected host cell displaying viral antigen on its surface
B Cell	Originates in bone marrow (thus the “B” in B cell). Circulates in blood and lymph. Binds to viruses or bacteria through antibodies on B-cell surface. Binds to viral fragments found on infected cell surface. Secretes antibodies that recognize viral and bacterial antigens. Stimulated to divide by T-cell interleukin. Host cell with viral antigens displayed on its surface.	viruses bacteria
Antibody	Produced by B cells. Recognizes specific antigen. Binds to viruses, preventing viruses from infecting cells. Binds to viral antigens on the surface of infected host cells and tags those cells for destruction. Stimulates macrophages to engulf viruses, bacteria, and cells with antigens that have antibodies bound to them.	viruses bacteria antigens on infected cells
Memory B Cell	Formed by B cell activated by interactions with antigens. Remains after infection. Responds rapidly when encountering the same antigen again. If identical viruses or bacteria infect the body again, the antibody on the memory B cell binds to the viruses or bacteria and marks them for destruction. The pathogen is destroyed before infection is established.	viruses and bacteria that have infected the body before

Additional Resources for Module 4 Reading 3

The Howard Hughes Medical Institute (HHMI) has developed videos, texts, images, lectures, and a virtual lab relating to the immune system:

Cells of the Immune System, HHMI

provides graphics of the components of the immune system and their functions in the immune response.

Immunology Virtual Lab, HHMI

Immunology Virtual Lab provides an interactive experience describing the ELISA assay for detecting antibodies. The lab uses to detect lupus, an autoimmune disease, but can be modified for infectious diseases.

Infectious Disease, HHMI

Infectious Disease provides a collection of resources related to infectious diseases and the immune system.