Lymphocytes in the Immune System: NK, T, and B Cells
Our bodies are constantly being exposed to infectious organisms, which enter the body through any opening that they encounter. Many of these invaders can make us sick. Fortunately, our immune system is constantly on guard to protect us. It can either prevent us from getting an infection or help us to recover from one.
The immune system produces proteins and white blood cells that attack invaders. One type of white blood cell is the lymphocyte, which exists in several different forms. These include natural killer cells, T cells, and B cells. The cells work in different ways to fight disease. Lymphocytes play a vital role in keeping us healthy and in saving our lives when we develop a potentially serious infection.
Our immune system consists of two major components: the innate system and the acquired system.
The Innate or Nonspecific Immune System
Humans are born with an innate or nonspecific immune system. Components of this system respond quickly to pathogens (microbes that cause disease) without having had previous exposure to them. In addition, the innate system attacks 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.
Barriers that prevent pathogen entry to the body, such as skin, are part of the innate immune system, and so are substances such as sweat, mucus in the nose, and acid in the stomach. Natural killer cells are also part of the system.
The Acquired or Specific 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 also take longer to react to a pathogen and are antigen-specific.
The acquired system has a memory component, allowing the body to efficiently attack a pathogen when the body is exposed to the invader for a second or subsequent time after the initial exposure. T cells and B cells are part of the acquired immune system.
An Overview of Immunity
There are three main types of lymphocytes—natural killer cells, T cells, and B cells.
Natural Killer or NK Cells
Natural killer or NK cells are unusual lymphocytes because they contain granules. They are also larger than B and T cells. NK cells attack cancer cells and those that are infected by a virus. Their activity often involves a special kind of cell membrane protein called an MHC protein.
All cells in our bodies that contain a nucleus also contain proteins in their cell 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” (cells that do not belong in the body). These identifying MHC proteins are classified as MHC Class One proteins.
Cells attach fragments of proteins that they have made to their membrane MHC proteins. Cells infected by a virus attach fragments of virus protein to the MHC proteins, producing abnormal MHC molecules. Cancer cells often have a low number of normal MHC proteins.
Natural Killer Cell Activity
Natural killer cells "recognize" the correct MHC proteins in a cell membrane by binding to them. If the NK cells are unable to find normal MHC proteins, or if these proteins are present at a very low level, the NK cells attack and destroy the infected cell. There are other ways in which natural killer cells recognize their target, however. They can bind to more than one type of membrane receptor and can be either activated or inhibited. Their behaviour is complex and not yet fully understood.
One Method of NK Cell Action
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 into the cell through the pore. These enzymes kill the infected cell via a process called apoptosis.
The animation above shows natural killer cells at work. In the last scene of the animation human NK cells are depicted killing sheep red blood cells. Natural killer cells in our bodies don't kill our own red blood cells, even though mature human red blood cells don't contain a nucleus and don't have surface MHC Class One proteins.
Like all blood cells, T cells are created in the red bone marrow found inside certain bones. After they are made they migrate to the thymus gland in the chest to mature. The "T" in their name stands for thymus. There are several kinds of T cells, including helper ones, cytotoxic ones, which are sometimes called killer T cells, regulatory ones, also known as suppressor T cells, and memory ones.
Helping Other Lymphocytes
Helper T cells are unable to kill pathogens, but they stimulate other lymphocytes to do this job. Helper T cells are sometimes known as CD4+ cells because they have a protein known as CD4 on their cell membrane. Helper T cells are destroyed by HIV (Human Immunodeficiency Virus), the 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 types of immune system cells 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 Two 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. The activated T cell produces proteins called cytokines which, in combination with other factors, trigger the activity of cytotoxic T cells and of B cells.
Immune System Components
Since macrophages and dendritic cells are part of the innate immune system, by becoming antigen-presenting cells and displaying the antigens to helper T cells they are providing a link between our innate immune system and our acquired system.
Like natural killer cells, cytotoxic T cells kill tumor cells and ones infected by viruses. Also like natural killer cells, they release perforin and granzymes into the infected cell. However, NK and cytotoxic T cells detect the infected cells in different ways. Cytotoxic T cells are part of the acquired immune system and attack infected cells when they detect specific viral or tumor antigens attached to MHC proteins on the cell membrane. Natural killer cells are part of the innate immune system and attack cells when they don’t find normal MHC proteins.
Regulation and Memory
Regulatory (or suppressor) T cells suppress the activity of the immune system after a pathogen has been destroyed. They help reduce the probability of an autoimmune reaction. In this type of reaction, the immune system attacks normal tissue in the body.
Memory T cells 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.
Like other blood cells, B cells are made in the red bone marrow. They also mature here, which is the reason for their “B” designation. Mature B cells contain receptors on their cell membranes. Each receptor joins to a specific antigen on the surface of pathogens. Once the B cells have bound to the pathogens, they are activated and then divide to produce two types of cells—plasma cells and memory B cells.
Plasma cells are considered to be mature B cells. They are made in large numbers and produce proteins called antibodies. The antibodies join to the same antigen which was attached to the original B cell. Antibodies are also called immunoglobulins. Some antibodies coat pathogens, making it easier for phagocytes to engulf them. Others cause pathogens to stick together or immobilize motile pathogens. Certain antibodies can neutralize toxins.
Like memory T Cells, memory B cells live for a long time. They enable the acquired immune system to attack a pathogen more efficiently on the second and subsequent exposure to the pathogen.
Maintaining a Healthy Immune System
We are bombarded by potentially dangerous pathogens ever day. The immune system does a wonderful job of protecting us. Occasionally it needs some help in the form of a medication, however.
Following a healthy lifestyle helps to maintain the efficiency of our immune system and reduces the need for medication. Eating a nutritious diet, exercising regularly, and getting an adequate amount of sleep all help the system to function at its best. Using stress reduction techniques and being a non-smoker are also helpful.
The human immune system is very complex. Scientists are still learning more about how lymphocytes are activated and how they act. This research is extremely important because it has many medical applications. The knowledge that researchers gain will almost certainly improve our health and save lives.
© 2010 Linda Crampton