Immune System, HIV, and Cancer

Lesson 3 Overview

In this lesson, you'll learn the basic components, functions, and diseases of the immune system. The immune system is comprised of white blood cells, enzymes, hormones, skin, bone marrow, and the lymphatic system. The immune system is your body’s defense mechanism to protect itself against various infections that can be caused by pathogens such as bacteria and viruses. 

In addition, you'll learn in detail about the development and presentation of the most debilitating diseases—HIV infection and cancer. HIV is a virus that can destroy the immune system and expose your body to various infections. Cancer is a disease of uncontrolled cell division and growth that can spread to your entire body, taking over the normal bodily functions.

You'll gain knowledge of various treatment options for immune system disorders, HIV infection, and cancer. While discussing treatment for these diseases, you will learn about the medications used to treat each disease. Medications are listed throughout this unit by their generic names, with brand names provided in parenthesis.

 

Lesson Objectives

• Illustrate immune system components and functions
• Describe the common immune system diseases and treatments
• Explain cancer and chemotherapy medications used to treat cancer

 

Introduction

The immune system is your body's defensive network against any disease-causing microorganisms. It's a complex system composed of various organs, cells, enzymes, and hormones that provide protection against infection-causing microorganisms. Your body has two levels of protection against bacteria, viruses, parasites, and cancer. The integumentary system and certain types of blood cells are part of the first line of defense. Hair and the fluids lining the mucous membranes of the body openings provide physical and chemical barriers against pathogen invasion. Apart from producing an immediate immune response against pathogens, your body has a complex mechanism to produce antibodies against particular pathogens that are successful in invading the physical barrier. These antibodies then protect against future invasion of the same pathogen by initiating a full-blown immune response at the first trigger. 

 

Functions of the Immune System

The main function of the immune system is to prevent or decrease the risk of various infections. Individuals with weaker immune systems due to hereditary factors or concomitant diseases such as cancer and human immunodeficiency virus (HIV) infection are at higher risk of contracting life-threatening bacterial, viral, fungal, and parasitic infections. Microorganisms that a normal body is able to fight off easily can overtake patients with compromised immune systems. 

Pregnant women have weaker immune systems as a protecting measure against the developing baby in the womb. The body may recognize new cells of the baby as foreign and may initiate an immune-mediated response to reject the pregnancy. Decreased effectiveness of the immune system in pregnant women is crucial, but puts women at risk of infections. 

The immune system has a mechanism that differentiates between normal, healthy cells and unhealthy cells. The unhealthy cells may have been invaded by foreign microorganisms such as viruses, bacteria, and fungi, or they may have been damaged by noninfectious conditions such as sunburn and cancer. When the immune system first recognizes specific signals from these unhealthy cells, it immediately activates immune processes to attack these cells. If your immune system is unable to recognize and get activated when there's an invasion of pathogens, serious infections may occur. However, if the immune system gets activated without the invasion of disease-causing organisms, allergic reactions and autoimmune diseases occur.

*The immune system is made up of a complex network of numerous cell types that either circulate throughout the body or reside in a particular tissue. Each cell type plays an important role in recognizing pathogens, communicating the message of danger to other cells, and working together to destroy the disease-causing pathogens. 

 

Types of Immunity

Innate immunity is the general and nonspecific immunity that your body possesses to fight various diseases. It comprises defense barriers such as the skin, hair, sebum, saliva, neutrophils, monocytes, and other enzymes that immediately detect and destroy the pathogens. It's a rapid-response system that acts as the first line of defense against infections. You're born with the components of innate immunity, and it can't be learned, created, or produced—hence the name "innate." Since bacteria and viruses multiply quickly, innate immunity is crucial in providing protection until the second line of defense, or adaptive immunity, starts involving in the process. In addition to providing the first line of defense against disease-causing microorganisms, the innate immune system activates, directs, and instructs components of the adaptive immunity. It also regulates inflammation and maintains equilibrium among various immune system processes. 

Adaptive immunity, also referred to as acquired immunity, comes into play when innate immunity is breached. Adaptive immunity isn't very helpful in protecting your body when a specific bacteria, virus, or any other microorganism enters your body for the first time. Its components function by learning the pathogen and then developing immunity against it. 

 

The adaptive immune system develops antibodies against the invading pathogen after the first entry. If the same or similar pathogen enters the bloodstream again, your body will have a defense ready to destroy the pathogen. The mechanism of adaptive immunity is essentially concerned with your body's capability to recognize and destroy substances which it interprets as foreign and harmful and is more complex than the innate immunity. Adaptive immune responses are continually refined and adjusted throughout your lifetime. As microorganism or your body's own defective cells trigger an immune response, the adaptive immune system continues to develop antibodies against these cells. The foreign substance is recognized by your body in the form of an outside protein called an antigen. 

An antigen triggers the production of antibodies, which attack the antigen to destroy it. A specific type of white blood cell (WBC) called the lymphocyte is a major cell of the adaptive immune system. Lymphocytes are further classified as T lymphocytes (T cells) and B lymphocytes (B cells). Vaccines are manufactured based on the adaptive immune system function. An antigen derived from a particular disease-causing bacteria or virus is manufactured, then introduced into the body in the form of a vaccine. Vaccines are administered to healthy individuals to trigger the adaptive immune response and produce antibodies against the antigen without causing the disease.

 

Components of the Immune System

Unlike the digestive, respiratory, and musculoskeletal systems, the immune system isn't an organized system of organs working together. It's essentially a functional system that depends on various cells, enzymes, hormones, skin, bone marrow, and the lymphatic system. Immune cells are produced in the bone marrow and are circulated to organs, where they mature and develop to become a part of the body's defense system. The lymphatic system helps to carry, circulate, and store the immune cells.  Many enzymes, proteins, and hormones released by various cells and glands also play a crucial role in the immune system's ability to carry out its functions of fighting infections.

 

The bone marrow produces white blood cells (WBCs), which play a major role in your body's defense mechanism on a cellular level. These cells are also known as leukocytes (leuko- means white and -cyte means cell). The main function of WBCs is to defend against infection and foreign invaders. An increase in the number of WBCs usually indicates infection. The bone marrow is the precursor to innate immune cells. It produces granulocytes, which are WBCs that contain secretory granules in their cytoplasm. There are three types of granulocytes: neutrophils, eosinophils, and basophils. 

Neutrophils are the most prevalent type of WBCs. They respond to bacterial infections, such as pneumonia or urinary tract infections, by engulfing the bacteria through the process of phagocytosis and are often referred as phagocytes. If the infection is moderate or severe, the bone marrow will produce extra neutrophils. Neutrophils are observed during the early stages of acute inflammation. 

Eosinophils are predominantly involved in allergic reactions. Their concentration may be elevated after allergic reactions to foods or drugs. Eosinophils also help fight parasitic infections. 

Basophils, like the other granulocytes, are involved in the process of inflammation. They're the rarest of leucocytes. 

Basophils help combat allergic reactions by releasing two chemicals:

1. Histamine

2. Heparin

 

The bone marrow, which is the precursor to innate immune cells, also produces agranulocytes, which are WBCs that don't contain secretory granules in their cytoplasm.

There are two types of agranulocytes:

1. Lymphocytes

2. Monocytes

- They're part of innate immunity and are the important first responders to infection. The bloodstream helps by continuously circulating immune cells throughout the body.

Lymphocytes produce antibodies against foreign particles, such as microorganisms, drugs, cancerous cells, and cells from transplanted organs. Lymphocytes are further classified as T lymphocytes (T cells) and B lymphocytes ( B cells). B cells produce antibodies and T cells participate in cell-mediated immune responses against bacterial and viral infections. Lymphocytes primarily function as part of adaptive immunity by producing antibodies against the invader cells. If the same type of cell attacks again, your body's defense force, in the form of antibodies, is ready to attack.

Monocytes, like neutrophils, are phagocytic cells, and their main function is to engulf and digest the damaged cells and pathogens. Monocytes function as part of innate immunity. 

 

Antibodies, or immunoglobulins, bind with pathogens to deactivate them or tag them to be destroyed by specific WBCs. Producing antibodies against a known antigen is your body's way of providing future protection from infection by the same pathogen.

Cytokines are proteins produced by various cells, including the immune cells. Cytokines play an important role in the immune system by sending and receiving signals to carry out various immune system processes. Cytokines communicate with immune cells to help regulate, direct, and amplify the immune response. 

The thymus is a small endocrine gland located in the upper chest area. The main function of the thymus gland is to develop a newborn's immune response; in adults, the thymus continues to be important in the maturation of lymphocytes, specifically T lymphocytes. While the bone marrow is responsible for the production of lymphocytes from stem cells, only B cells mature there. T cells, on the other hand, mature in the thymus. First, the bone marrow sends some immature lymphocytes out to the blood to be deposited in the thymus gland. Here, the immature lymphocytes become thymocytes and further mature to become T lymphocytes or the T cells. T cells are responsible for the cell-mediated immunity. T cells don't produce antibodies. They simply attack atypical or abnormal cells, causing death of the diseased cell by activating microphages or by inducing a series of chemical reactions that encourage the cell to die (a process known as apoptosis). The thymus gland is considered a part of the lymphatic system.

The lymphatic system is a network of vessels and tissues that carry lymph throughout your body. This system is also composed of the thymus gland and lymph nodes. Similar to blood, lymph is an extracellular fluid that continuously circulates throughout your body. Immune cells are carried through the lymphatic system and stored in lymph nodes, which are found throughout your body. The network of lymphatic capillaries, vessels, ducts, glands, and nodes perform a number of important jobs. This network transports excess fluid from the tissues back to the circulatory system, filters bacteria, acts as an emergency reservoir of blood, destroys and removes old red blood cells, and produces monocytes and T lymphocytes for the immune system. The lymphatic system also absorbs fats from the small intestine and delivers them into the bloodstream. Lymph nodes serve as spots where stored immune cells may detect a foreign pathogen and initiate the immune response against the pathogen. Thus, swollen lymph nodes are usually indicative of your body's active immune response.

The spleen is a soft, dark red organ located on the left side of the abdomen, just below the diaphragm. Even though the spleen isn't directly connected to the lymphatic system, it plays an important part in your body's immune response. The spleen also stores immune cells and activates these cells to respond to disease-causing pathogens. It stores lymphocytes and other blood cells, and it filters out bacteria and viruses from the arterial blood circulation. As the blood passes through the capillary network in the spleen, it comes into contact with lymphocytes that clear the blood of any infected cells that may be there. T cells, as you've learned, work to destroy atypical, infected, or defective cells. B cells produce antibodies that will work to destroy future infections when they're recognized again. In a fetus, one of the main functions of the spleen is to produce lymphocytes and red blood cells. Before the long bones are fully formed and able to produce cells, the spleen is where blood cell formation occurs. Once the bones take over production, the spleen focuses on storing and filtering blood and fighting infection.

The tonsils are masses of lymph tissue located in the rear of the throat. Their location as "guards" of the mouth often exposes them to microorganism invading the body through food and air. Like all lymphoid tissue, they store lymphocytes and release them to fight infections as soon as microorganisms enter the mouth. 

 

Key Points

• The immune system is your body's defense against foreign pathogens; it prevents or decreases the risk of various infections.
• Your body produces two types of immune response, innate and adaptive.
• Innate immunity is general and nonspecific immunity against any pathogen or allergen trying to enter your body.
• Adaptive immunity is specific and learned immunity against a specific pathogen or allergen.
• Major components of the immune system include skin, hair, mucus, blood, WBCs, bone marrow, antibodies, cytokines, enzymes, and the lymphatic system.
• The lymphatic system includes a network of vessels, the thymus gland, spleen, lymph nodes, and tonsils.