Linda Crampton is a writer and teacher with a first-class honors degree in biology. She often writes about the scientific basis of disease.
Our bodies are constantly exposed to microorganisms, unless we are in a sterilized environment. The organisms enter the body through any opening that they encounter. Some of the invaders can make us sick. Fortunately, our immune system generally serves us well. It can prevent us from getting an infection, weaken the infection if it develops, and help us recover from the illness. The system consists of two divisions: the innate system and the acquired one. Lymphocytes are important components of each division.
The immune system produces leukocytes (white blood cells) and chemicals that attack invaders. Lymphocytes are a type of leukocyte and exist in three forms—natural killer or NK cells, T cells or T lymphocytes, and B cells or B lymphocytes. Lymphocytes and the rest of the immune system play a vital role in keeping us healthy.
The Innate or Nonspecific Immune System
Humans are born with a nonspecific immune system. Components of this system respond quickly to pathogens (microbes that cause disease) without having had previous exposure to them. The innate system attacks or inhibits many different pathogens regardless of their antigens. An “antigen” is a specific molecule on the surface of a cell or particle that triggers an attack by the acquired immune system.
The innate immune system consists of the following components:
- physical barriers that prevent pathogen entry to the body, such as the skin and the lining of the digestive tract
- secretions such as sweat, saliva in the mouth, mucus in the nose, and hydrochloric acid in the stomach
- specific proteins
- cells that destroy or help to remove the invaders
As the quote below says, the cells in the innate immune system can only recognize general indicators that an entity that they've encountered could be a problem. They can't recognize specific types of bacteria, viruses, or fungi. The innate system is beneficial, however, because it starts working very soon after we are exposed to a pathogen and before the acquired system is ready to help us.
Innate immune cells express genetically encoded receptors, called Toll-like receptors (TLRs)... Collectively, these receptors can broadly recognize viruses, bacteria, fungi, and even non-infectious problems. However, they cannot distinguish between specific strains of bacteria or viruses.
— NIAID (National Institute of Allergy and Infectious Diseases)
Cells in the Innate Immune System
Cells in both the innate and the acquired immune system are made in the red bone marrow. Some of our bones contain red marrow in the center while others contain yellow marrow.
- Lymphocytes, monocytes, macrophages, eosinophils, neutrophils, basophils, and mast cells are classified as leukocytes, or white blood cells. The first term comes from the Greek "leukos," which means white, and "kytos," which means cell. The cells are said to be white because they lack the red hemoglobin found in red blood cells, or erythrocytes.
- Natural killer cells are classified as lymphocytes. Research suggests that their behavior is more complex than that of other cells in the innate system.
- Though B and T lymphocytes belong to the leukocyte group, they are part of the acquired immune system, not the innate one.
- Macrophages are derived from monocytes, as shown in the illustration above. The origin of dendritic cells (which are not shown in the illustration) is still being studied. In at least some cases, they are derived from monocytes.
Macrophages and dendritic cells influence one type of T lymphocyte. They provide a link between the innate and the acquired immune system.
The Acquired or Adaptive Immune System
The acquired, adaptive, or specific immune system develops during our life as we are exposed to pathogens or after we receive vaccinations. The components of this system are more specialized than the components of the innate system. They take longer to react to a pathogen and are antigen-specific.
The acquired system is able to identify specific fungi, bacteria, viruses, and other potentially harmful items. It also has a memory component. This allows the body to quickly attack a pathogen when it's exposed to the invader for a second or subsequent time after the initial exposure.
The combination of the rapid but generalized innate system and the slower but specialized acquired system is very often an effective way to protect the body from infection or to help the recovery from one.
Natural Killer or NK Cells
Natural killer or NK cells are unusual lymphocytes because they contain noticeable granules. They are larger than B and T cells. NK cells attack cancer cells and ones that are infected by a virus. They attack immediately without going through an activation process, which is why they are called “natural” killers. Their activity at least in part involves a special kind of plasma membrane protein called an MHC protein. The plasma or cell membrane is the outer covering of a human cell.
Facts About MHC Proteins
- All cells in our bodies that contain a nucleus also contain proteins in their plasma membranes called MHC (major histocompatibility complex) proteins.
- Everybody has a different set of MHC proteins.
- Natural killer cells use MHC proteins to distinguish “self” (cells that belong in the body) from “non-self” (those that do not belong in the body).
- The major histocompatibility complex proteins that NK cells detect are classified as MHC class l proteins.
Natural Killer Cell Activity
Natural killer cells "recognize" the correct MHC proteins in a membrane by binding to them. The NK cells are inhibited and no attack occurs. If the NK cells are unable to find normal MHC proteins, or if these proteins are present at a very low level, they attack and destroy the abnormal cell. Cancer cells and ones infected by a virus often have a low number of normal MHC proteins.
During its attack, the NK cell first releases an enzyme called perforin, which creates a pore in the membrane of the infected cell. It then sends other enzymes called granzymes through the pore. These enzymes kill the cell via stimulation of a process called apoptosis, or self-destruction.
The animation above shows natural killer cells at work. In the last scene of the animation, human NK cells are depicted killing sheep erythrocytes. Natural killer cells in our bodies don't kill our own erythrocytes, even though mature ones don't contain a nucleus and don't have surface MHC class l proteins.
Understanding the Activity of NK Cells
Researchers have discovered that natural killer cells have Toll-like receptors on their cell membrane, which means that they may have more than one way to detect harmful invaders in our body. (The word "Toll" is generally capitalized.) In addition, scientists have found that different types of natural killer cells with different properties exist. Some appear to "remember" a pathogen that they've previously classified as dangerous.
NK cells are sometimes said to have features of both the innate and the acquired immune system. Though they are generally classified in the innate immune system, some scientists think that this classification is inaccurate. Discovering and understanding the structure and behavior of the cells is an important area of research.
Rearchers have shown in mouse models that NK cells, like adaptive cells, can be retained as memory cells and respond to subsequent infections by the same pathogen.
B cells or B lymphocytes are an important part of the acquired immune system. Like other blood cells, they are made in the red bone marrow. They also mature there. They are known as B lymphocytes because they were discovered in the bursa of Fabricius, an organ found only in birds.
Young B lymphocytes released from the bone marrow are said to be "naive" because they haven't been activated by an antigen. An antigen is a substance that triggers a cell to produce antibodies, which attack the antigen. Pathogens bear chemicals on their surface that act as antigens for B lymphocytes.
During the activation process, receptors on the surface of a B lymphocyte that have a particular shape join to a specific type of antigen found on the surface of a pathogen. The receptors are sometimes referred to as membrane-bound antibodies. Once a B lymphocyte has bound to the pathogen, the lymphocyte is activated. It divides to produce two types of cells—a plasma or effector one and a memory B one.
Plasma or effector cells are considered to be mature B cells. They are made in large numbers. Instead of bearing the antibodies for a particular pathogen on their surface, they secrete antibodies that leave the cell. These chemicals attack the same pathogen as the one recognized by the parent cell.
Antibodies destroy invaders by various methods. Some coat or mark pathogens, making it easier for phagocytes to identify and engulf them. Others cause pathogens to stick together or immobilize motile pathogens. Specific antibodies can neutralize toxins.
Memory B Cells
Memory B cells live for a long time. They have receptors on their surface that can bind to the same pathogen as their parent and siblings, but they don't secrete antibodies. Some survive for many years after the initial infection has disappeared.
Memory B cells can produce plasma cells when necessary. They enable the acquired immune system to attack a specific pathogen more efficiently on the second and subsequent exposure to the entity.
After T cells are created in the red bone marrow, they migrate to the thymus gland in the chest, where they mature. The "T" in their name stands for thymus. Multiple types of T cells exist, including helper, cytotoxic, regulatory, and memory types. These varieties are described in more detail below.
The thymus decreases in size as we age, starting at puberty. This means that fewer mature T lymphocytes are produced as we grow older. Fortunately, some of the lymphocytes live for a long time. In addition, researchers are discovering ways in which T lymphocytes located outside the thymus can reproduce.
Helping Other Lymphocytes
Helper T cells are unable to kill pathogens, but they stimulate other lymphocytes to do this job. They are sometimes known as CD4+ cells because they have a protein known as CD4 on their plasma membrane. Unfortunately, they are destroyed by the HIV (human immunodeficiency virus) that causes AIDS.
Helper T cells must be activated before they can carry out their function. The activation process requires the presence of other components of the immune system, such as macrophages and dendritic cells. These cells are phagocytes—they surround pathogens and then engulf and digest them. The phagocytes display a fragment from the digested pathogen on their surface membrane attached to an MHC class ll protein. The phagocytes are then known as antigen-presenting cells.
Helper T Cell Activation
A helper T cell is activated when the receptor on its surface joins with an antigen on a presenting cell. The receptor and antigen must match in order for a union to occur. The body has a large variety of helper T cells, resulting in many receptor variations that can join with many different antigens. Activated T cells trigger the activity of cytotoxic T cells and B lymphocytes.
Actions of Cytotoxic T Cells
Cytotoxic T cells are also known as killer T cells, cytotoxic T lymphocytes, and CTLs. They have a CD8 protein on their surface. They kill tumor cells and ones infected by viruses.
CTLs have three ways to attack. Two of them resemble methods used by NK cells. They release specific cytokines that can destroy cancer cells and viruses. Cytokines are small proteins that act as signaling molecules, or ones that transmit "messages" controlling cell behavior.
Perforin and Granzymes
CTLs also release granules containing perforin and granzymes. Perforin creates pores in the cell targeted for attack. Granzymes enter the target cell through the pores and then break up proteins. This triggers apoptosis. The lymphocyte can then move to another target cell and repeat the process of destruction by perforin and granzymes.
Fas and FasL Proteins
CTLs have a protein called FasL on their plasma membrane. This binds to a protein receptor called Fas on the target cell. The binding causes the structure of the Fas molecule to change and a signaling molecule to be produced. The signaling molecule triggers a process called the caspase cascade inside the target cell. Caspases are enzymes involved in programmed cell death. The cascade causes apoptosis.
Interestingly, CTLs also have the Fas receptor. This enables T cells to kill each other. This process sometimes happens at the end of the immune response once the lymphocytes have done their job.
Regulation and Memory
Regulatory or suppressor T cells suppress the activity of the immune system after a pathogen has been destroyed. They are important because they help reduce the probability of an autoimmune reaction. In this type of reaction, the immune system attacks normal tissue in the body. Multiple types of regulatory T cells exist.
Like memory B cells, memory T ones live for a long time. They are exposed to an antigen during an infection. During a subsequent infection with the same antigen, the T cells enable the immune system to attack the infection more rapidly than it did the first time. As in the case of regulatory cells, multiple types of memory T cells exist.
A Complex and Very Helpful System
We are bombarded by potentially dangerous pathogens ever day. The immune system does a wonderful job in protecting most of us for most of the time. Without the system, even apparently minor threats to our health could be dangerous, and ones that require medical treatment might be more dangerous than they are at the moment.
The human immune system is complex. The information in this article describes some important behavior of lymphocytes, but scientists are discovering that the cells also behave in other ways. Some of them appear to protect us by multiple mechanisms. There seems to be a lot to learn about them.
Studying the immune system and its components is very important. The knowledge that researchers gain may help us to prevent or at least reduce infections and may even be used to save lives. Those are very worthy goals.
- Overview of the immune system from the National Institute of Allergy and Infection Diseases (NIAID)
- NK cell facts from the British Society for Immunology
- NK cells in health and disease from Science Direct
- Toll-like receptors in natural killer cells (abstract) from the National Library of Medicine
- Information about acquired immunity (including B and T lymphocytes) from the Merck Manual
- Facts about CD8+ T lymphocytes from the British Society for Immunology (This site also contains information about other aspects of the immune system.)
- Histocompatability complex and proteins from the NIH (National Institutes of Heath)
- Information and news about the immune system from Immunopaedia.org
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2010 Linda Crampton
Linda Crampton (author) from British Columbia, Canada on August 11, 2013:
Thank you, FlourishAnyway. I appreciate your visit and votes!
FlourishAnyway from USA on August 11, 2013:
Well researched and a nice description. Thanks for educating us! Voted up and more.
Linda Crampton (author) from British Columbia, Canada on October 17, 2012:
Hi, Joanna. The normal level of lymphocytes varies, but in adults it's generally between 1000 and 4800 lymphocytes per microliter (or cubic millimeter) of blood.
Joanna on October 17, 2012:
What should be level of Lymphocyte count in the human body?
Linda Crampton (author) from British Columbia, Canada on September 20, 2012:
Thank you, Mary. I appreciate the comment, the vote and the share very much!!
Mary Hyatt from Florida on September 20, 2012:
You really did a lot of reseach on this Hub. As a former medical technologist, I found the Hub very interesting and informative.
I voted this Hub up, etc. and will share.