The immune system is a complex network of organs that contains cells that recognize foreign invaders in the body and destroys them. It protects the body against pathogens such as viruses, bacteria, fungi and other parasites. The immune system offers two kinds of immunity. One is called humoral innate immunity which involves a variety of substances found in the humors or body fluids. These substances interfere with the growth of pathogens and clump them together so they can be eliminated from the body. The second is called cellular innate immunity, which is accomplished by cells called phagocytes that ingest or eat the pathogens and by natural killer cells that destroy certain types of cancer cells. Innate immunity is nonspecific, meaning it is not directed against articular invaders but against any pathogens that attack the body.
An additional and more sophisticated system of defense is called adaptive immunity which has the ability to recognize and destroy specific pathogens. This type of defensive reaction is called the immune response. Any substance that is capable of generating this type of immune response is called an antigen or immunogen. Antigens are toxins or enzymes that the immune system considers foreign. Immune responses directed against antigens are called antigen specific. Specificity is one of the properties that distinguish adaptive immunity from innate immunity. Adaptive immunity works with innate immunity to provide the body with a heightened resistance to parasites and other intruders. Adaptive immunity is also responsible for allergic reactions and for rejecting transplanted tissue which it mistakes for a foreign invader.
Lymphocytes are a class of white blood cells that are the principal components of the adaptive immune system. The other components are antigen-presenting cells which trap antigens and brings them to the attention of the lymphocytes. Lymphocytes are different from other cells in the body because they have nearly 100,000 identical receptors on their cellular membrane that enables them to recognize one specific antigen. The receptors are proteins containing grooves that fit into patterns formed by the atoms of the antigen molecule , so that the lymphocyte can bind to the antigen. There are more than ten million different types of grooves in the lymphocytes of the body’s immune system.
Protects Against Damaging Antigens
When antigens invade the body, daughter cells are generated by lymphocytes that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes called a lymphocyte clone. The clone continues to grow after lymphocytes first encounter the antigen, so if the same type of antigen invades or attacks the body a second time, there will be plenty of lymphocytes to fight it.
Like all blood cells, lymphocytes are made from stem cells in the bone marrow. Lymphocytes then undergo a second stage of development in which they acquire their antigen-specific receptors. Some lymphocytes are created with receptors that happen to be specific to normal, healthy components in the body. A healthy immune system then purges these lymphocytes and leaves only lymphocytes that ignore normal body components and only react to foreign pathogens. If the purging process goes amiss or is not completely successful, the result is an autoimmune disease that is, in effect, the immune system attacking healthy cells, molecules or tissue.
Some lymphocytes are processed in the bone marrow and then migrate to other areas in the body, specifically to the lymphoid organs. These lymphocytes are called B cells. Other lymphocytes move from the bone marrow and are processed in the thymus, a pyramid-shaped organ located beneath the breastbone. These lymphocytes are called T lymphocytes or T cells (thymus cells).
These two types of lymphocytes –B cells and T cells–play different roles in the immune response, although they may act together. The part of the immune response that involves B cells is often called humoral immunity because it takes place in the bodily fluids. The part involving T cells is called cellular immunity because it takes place directly between the T cells and the antigens. All adaptive immune responses are cellular, or initiated by cells (lymphocytes) reacting to antigens. B cells may initiate an immune response, but the triggering antigens are actually eliminated by soluble products that the B cells release into the blood and other bodily fluids. These soluble products are called antibodies and belong to a special group of blood proteins called immune-globulins. When an antigen is encountered in the bodily fluids, it stimulates a B cell and it transforms, with the aid of a type of T cell called a helper T cell, into a larger cell called a blast cell.
The blast cell begins to divide rapidly, forming a clone of identical cells.
Some of these transform further into plasma cells which are antibody-producing factones. These plasma cells produce a single type of antigen-specific antibody at a rate of about 2,000 antibodies per second. The antibodies then circulate through the body fluids, attacking the triggering antigen.
Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading pathogens and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, called the complement system, which consists of at least 30 different components.
In these cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction can either cause the invader to burst or it can attract scavenger cells that ingest or eat the attacker.
Not all the cells from the clone formed from the original B cell transform into antibody-producing plasma cells . Some serve as memory cells. These resemble the original B cell, but can respond more quickly to a second invasion by the same antigen .
There are two major classes of T cells produced in the thymus : helper T cells and cytotoxic or killer T cells. Helper T cells secrete molecules called interleukins that promote the growth of B and T cells. The interleukins that are secreted by lymphocytes are also called lymphokines. The interleukins that are secreted by other kinds of blood cells ( monocytes and macrophages) are called monokines. (IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-5, IL-7, interferon, lymphotoxin, and tumor necrosis factor).
Each interleukin has complex biological effects.
Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancer cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune system is active only when necessary.
The receptors of T cells are different from those of B cells because they are ‘trained’ to recognize fragments of antigens that have been combined with a set of molecules found on the surface of all the body’s cells. These molecules are called MHC, or major histocompatibility complex. As T cells circulate through the body, they scan the surfaces of cells for the presence of foreign antigens that have been picked up by MHC molecules. This function is sometimes called immune surveillance.
When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by pre-formed antibodies that act together with the complement system.
Often the lymphocytes of the adaptive immune system are also brought into play.
The human immune system contains approximately one trillion T cells and one trillion B cells, located in the lymphoid organs and in the blood, plus approximately ten billion antigen-presenting cells located in the lymphoid organs. To maximize the chances of encountering antigens wherever they may invade the body, lymphocytes continually circulate between the blood and certain lymphoid tissues. A given lymphocyte spends an average of thirty minutes per day in the blood and re-circulates about fifty times per day between the blood and lymphoid tissues.
If lymphocytes encounter an antigen trapped by the antigen-presenting cells of the lymphoid organs, lymphocytes with receptors specific to that antigen stop their migration and assemble to mount an immune response locally. As these lymphocytes accumulate in the affected lymphoid tissue, it often becomes enlarged or swollen.
Antigen-presenting cells degrade antigens and often eliminate them without the help of lymphocytes. However, if there are too may antigens for them to handle alone, the antigen-presenting cells secrete IL-1 and display fragments of the antigens (combined with MHC molecules) to alert the helper T cells.
The IL-1 facilitates the responsiveness of T and B cells to antigens and, if released in large amounts as in the case of infection, can cause drowsiness and fever. Helper T cells that encounter IL-1 and fragments of antigens transform into cells called lymphoblasts, which then secrete a variety of interleukins that are essential to the success of the immune response. The IL-2 produced by helper T cells promotes the growth of cytotoxic T cells, which may be necessary to destroy tumor cells or cells infected with viruses. The IL-3 increases the production of blood cells in the bone marrow and, thus, helps maintain an adequate supply of the lymphocytes and lymphocyte products necessary to fight infections. Helper T cells also secrete interleukins that act on B cells, stimulating them to divide to transform into antibody-secreting plasma cells. The antibodies then perform their part of the immune function.
The process of inducing an immune response is called immunization. It may be either natural through infection of a pathogen, or artificial through the use of a serum or vaccine. The heightened resistance acquired when the body responds to infection is called active immunity. Passive immunity results when the antibodies from an actively immunized individual are transferred to a second, nonimmune subject. Active immunization, whether natural or artificial, is longer-lasting than passive immunization because it takes advantage of immunologic memory.
Scientists can now produce antibody-secreting cells in the laboratory by a method known as the hybridoma technique. Hybridomas are hybrid cells made by fusing a cancerous, or rapidly reproducing, plasma cell with a normal plasma cell obtained from an animal immunized with a particular antigen. The hybridoma cell can produce large amounts of identical antibodies, called monoclonal or hybridoma antibodies, which have widespread use in medicine and biology.