immune system | Description, Function, & Facts | ddttrh.info
The immune system, composed of special cells, proteins, tissues, and organs infection, they might order a blood test to see if a patient has an increased number of Innate immunity also includes the external barriers of the body, like the skin and For example, antibodies in a mother's breast milk give a baby temporary. were compared for maternal 25(OH)D by Mann–Whitney U-test. relationship between maternal plasma 25(OH)D and infant infections, .. fetus, whereby vitamin D regulates the development of innate and adaptive immune. Thus, the innate immune system of human milk is an important standards, over 20, mother/infant dyads were evaluated for the relation between . was more potent than the other when the isolated peptides were tested.
The same applies, with rare exceptions, to many other diseases, such as smallpoxchicken poxmeaslesand mumps. Yet having had measles does not prevent a child from contracting chicken pox or vice versa. The protection acquired by experiencing one of these infections is specific to that infection; in other words, it is due to specific, acquired immunity, also called adaptive immunity.
Acquired immunity depends on the activities of T and B lymphocytes T and B cells. One part of acquired immunity, humoral immunity, involves the production of antibodies by B cells.
The other part, cell-mediated immunity, involves the actions of T cells. When an antigen such as a bacterium enters the body, it is attacked and engulfed by macrophages, which process and display parts of it on their cell surface. A helper T cell, recognizing the antigen displayed, initiates maturation and proliferation of other T cells.
Cytotoxic killer T cells develop and attack foreign and infected cells.
Immune System (for Parents)
B cells stimulated by the presence of antigen are activated by helper T cells to divide and form antibody-producing cells plasma cells. Released antibody binds to antigen, marking the cell for destruction.
Helper T cells also induce the development of memory T and B cells needed to mount future immune responses on reinfection with the same pathogen. There are other infectious conditions, such as the common coldinfluenzapneumoniaand diarrheal diseases, that can be caught again and again; these seem to contradict the notion of specific immunity.
But the reason such illnesses can recur is that many different infectious agents produce similar symptoms and thus the same disease. For example, more than viruses can cause the cluster of symptoms known as the common cold. Consequently, even though infection with a particular agent does protect against reinfection by that same pathogen, it does not confer protection from other pathogens that have not been encountered.
Acquired immunity is dependent on the specialized white blood cells known as lymphocytes. This section describes the various ways in which lymphocytes operate to confer specific immunity.
Innate Immunity of Neonates and Infants
Although pioneer studies were begun in the late 19th century, most of the knowledge of specific immunity has been gained since the s, and new insights are continually being obtained.
Lymphocytes are mainly a dormant population, awaiting the appropriate signals to be stirred to action. The inactive lymphocytes are small, round cells filled largely by a nucleus.
Although they have only a small amount of cytoplasm compared with other cells, each lymphocyte has sufficient cytoplasmic organelles small functional units such as mitochondriathe endoplasmic reticulumand a Golgi apparatus to keep the cell alive.
Lymphocytes move only sluggishly on their own, but they can travel swiftly around the body when carried along in the blood or lymph. The majority are concentrated in various tissues scattered throughout the body, particularly the bone marrowspleenthymuslymph nodestonsilsand lining of the intestines, which make up the lymphatic system.
Organs or tissues containing such concentrations of lymphocytes are described as lymphoid. The lymphocytes in lymphoid structures are free to move, although they are not lying loose; rather, they are confined within a delicate network of lymph capillaries located in connective tissues that channel the lymphocytes so that they come into contact with other cells, especially macrophages, that line the meshes of the network.
This ensures that the lymphocytes interact with each other and with foreign materials trapped by the macrophages in an ordered manner. The human lymphatic system, showing the lymphatic vessels and lymphoid organs. T and B cells Lymphocytes originate from stem cells in the bone marrow ; these stem cells divide continuously, releasing immature lymphocytes into the bloodstream. Some of these cells travel to the thymuswhere they multiply and differentiate into T lymphocytes, or T cells.
The T stands for thymus-derived, referring to the fact that these cells mature in the thymus. Once they have left the thymus, T cells enter the bloodstream and circulate to and within the rest of the lymphoid organs, where they can multiply further in response to appropriate stimulation.
About half of all lymphocytes are T cells. NIAID Some lymphocytes remain in the bone marrow, where they differentiate and then pass directly to the lymphoid organs. They are termed B lymphocytes, or B cellsand they, like T cells, can mature and multiply further in the lymphoid organs when suitably stimulated.
Although it is appropriate to refer to them as B cells in humans and other mammals, because they are bone-marrow derived, the B actually stands for the bursa of Fabriciusa lymphoid organ found only in birds, the organisms in which B cells were first discovered. B and T cells both recognize and help eliminate foreign molecules antigenssuch as those that are part of invading organisms, but they do so in different ways.
B cells secrete antibodiesproteins that bind to antigens. Since antibodies circulate through the humours i. T cells, in contrast, do not produce antibodies but instead directly attack invaders. Because this second type of acquired immunity depends on the direct involvement of cells rather than antibodies, it is called cell-mediated immunity.
These two types of specific, acquired immunity, however, are not as distinct as might be inferred from this description, since T cells also play a major role in regulating the function of B cells. In many cases an immune response involves both humoral and cell-mediated assaults on the foreign substance.
Furthermore, both classes of lymphocytes can activate or enhance a variety of nonspecific immune responses. Ability to recognize foreign molecules Receptor molecules Lymphocytes are distinguished from other cells by their capacity to recognize foreign molecules. Recognition is accomplished by means of receptor molecules. A receptor molecule is a special protein whose shape is complementary to a portion of a foreign molecule.
This complementarity of shape allows the receptor and the foreign molecule to conform to each other in a fashion roughly analogous to the way a key fits into a lock. Receptor molecules are either attached to the surface of the lymphocyte or secreted into fluids of the body. B and T lymphocytes both have receptor molecules on their cell surfaces, but only B cells manufacture and secrete large numbers of unattached receptor molecules, called antibodies. Antibodies correspond in structure to the receptor molecules on the surface of the B cell.
Antigens Any foreign material—usually of a complex nature and often a protein—that binds specifically to a receptor molecule made by lymphocytes is called an antigen. Antigens include molecules found on invading microorganisms, such as virusesbacteriaprotozoansand fungias well as molecules located on the surface of foreign substances, such as pollendust, or transplanted tissue.
When an antigen binds to a receptor molecule, it may or may not evoke an immune response. Antigens that induce such a response are called immunogens. Thus, it can be said that all immunogens are antigens, but not all antigens are immunogens.
For example, a simple chemical group that can combine with a lymphocyte receptor i. Although haptens cannot evoke an immune response by themselves, they can become immunogenic when joined to a larger, more complex molecule such as a protein, a feature that is useful in the study of immune responses.
Many antigens have a variety of distinct three-dimensional patterns on different areas of their surfaces. Each pattern is called an antigenic determinant, or epitopeand each epitope is capable of reacting with a different lymphocyte receptor. Some antigenic determinants are better than others at effecting an immune response, presumably because a greater number of responsive lymphocytes are present. It is possible for two or more different substances to have an epitope in common.
In these cases, immune components induced by one antigen are able to react with all other antigens carrying the same epitope. Such antigens are known as cross-reacting antigens.
T cells and B cells differ in the form of the antigen they recognize, and this affects which antigens they can detect. B cells bind to antigen on invaders that are found in circulation outside the cells of the body, while T cells detect only invaders that have somehow entered the cells of the body.
Thus foreign materials that have been ingested by cells of the body or microorganisms such as viruses that penetrate cells and multiply within them are out of reach of antibodies but can be eliminated by T cells. Diversity of lymphocytes The specific immune system in other words, the sum total of all the lymphocytes can recognize virtually any complex molecule that nature or science has devised. This remarkable ability results from the trillions of different antigen receptors that are produced by the B and T lymphocytes.
Each lymphocyte produces its own specific receptor, which is structurally organized so that it responds to a different antigen.Animal Behavior - CrashCourse Biology #25
After a cell encounters an antigen that it recognizes, it is stimulated to multiply, and the population of lymphocytes bearing that particular receptor increases. How is it that the body has such an incredible diversity of receptors that are always ready to respond to invading molecules?
To understand this, a quick review of genes and proteins will be helpful. Antigen receptor molecules are proteinswhich are composed of a few polypeptide chains i. The sequence in which the amino acids are assembled to form a particular polypeptide chain is specified by a discrete region of DNAcalled a gene. But if every polypeptide region of every antigen receptor were encoded by a different gene, the human genome all the genetic information encoded in the DNA that is carried on the chromosomes of cells would need to devote trillions of genes to code just for these immune system proteins.
Since the entire human genome contains approximately 25, genes, individuals cannot inherit a gene for each particular antigen receptor component. Instead, a mechanism exists that generates an enormous variety of receptors from a limited number of genes. What is inherited is a pool of gene segments for each type of polypeptide chain. As each lymphocyte matures, these gene segments are pieced together to form one gene for each polypeptide that makes up a specific antigen receptor. This rearrangement of alternative gene segments occurs predominantly, though not entirely, at random, so that an enormous number of combinations can result.
Additional diversity is generated from the imprecise recombination of gene segments—a process called junctional diversification—through which the ends of the gene segments can be shortened or lengthened. The genetic rearrangement takes place at the stage when the lymphocytes generated from stem cells first become functional, so that each mature lymphocyte is able to make only one type of receptor.
Thus, from a pool of only hundreds of genes, an unlimited variety of diverse antigen receptors can be created. Still other mechanisms contribute to receptor diversity. Superimposed on the mechanism outlined in simplified terms above is another process, called somatic mutation.
Mutation is the spontaneous occurrence of small changes in the DNA during the process of cell division. Although somatic mutation can be a chance event in any body cell, it occurs regularly in the DNA that codes for antigen receptors in lymphocytes. Thus, when a lymphocyte is stimulated by an antigen to divide, new variants of its antigen receptor can be present on its descendant cells, and some of these variants may provide an even better fit for the antigen that was responsible for the original stimulation.
B-cell antigen receptors and antibodies The antigen receptors on B lymphocytes are identical to the binding sites of antibodies that these lymphocytes manufacture once stimulated, except that the receptor molecules have an extra tail that penetrates the cell membrane and anchors them to the cell surface. Thus, a description of the structure and properties of antibodies, which are well studied, will suffice for both.
Basic structure of the immunoglobulin molecule Antibodies belong to the class of proteins called globulins, so named for their globular structure. Collectively, antibodies are known as immunoglobulins abbreviated Ig. All immunoglobulins have the same basic molecular structure, consisting of four polypeptide chains. Two of the chains, which are identical in any given immunoglobulin molecule, are heavy H chains; the other two are identical light L chains.
The terms heavy and light simply mean larger and smaller. Each chain is manufactured separately and is encoded by different genes. The four chains are joined in the final immunoglobulin molecule to form a flexible Y shape, which is the simplest form an antibody can take. The four-chain structure of an antibody, or immunoglobulin, moleculeThe basic unit is composed of two identical light L chains and two identical heavy H chains, which are held together by disulfide bonds to form a flexible Y shape.
Each chain is composed of a variable V region and a constant C region. At the tip of each arm of the Y-shaped molecule is an area called the antigen-bindingor antibody-combining, site, which is formed by a portion of the heavy and light chains. Every immunoglobulin molecule has at least two of these sites, which are identical to one another. The antigen-binding site is what allows the antibody to recognize a specific part of the antigen the epitopeor antigenic determinant.
Chemical bonds called weak bonds then form to hold the antigen within the binding site. The heavy and light chains that make up each arm of the antibody are composed of two regions, called constant C and variable V. These regions are distinguished on the basis of amino acid similarity—that is, constant regions have essentially the same amino acid sequence in all antibody molecules of the same class IgG, IgM, IgA, IgD, or IgEbut the amino acid sequences of the variable regions differ quite a lot from antibody to antibody.
This makes sense, because the variable regions determine the unique shape of the antibody-binding site. The tail of the molecule, which does not bind to antigens, is composed entirely of the constant regions of heavy chains. The variable and constant regions of both the light and the heavy chains are structurally folded into functional units called domains. Each light chain consists of one variable domain VL and one constant domain CL. The tail of the antibody determines the fate of the antigen once it becomes bound to the antibody.
Variable V and constant C domains within the light L and heavy H chains of an antibody, or immunoglobulin, molecule. It provides the molecule with flexibility, which is very useful in binding antigens. This flexibility can actually improve the efficiency with which an antigen binds to the antibody.
It can also help in cross-linking antigens into a large lattice of antigen-antibody complexes, which are easily identified and destroyed by macrophages.
A The hinge region of an antibody molecule opens and closes to allow better binding between the antibody and antigenic determinants on the surface of an antigen. B Hinge flexibility also facilitates the cross-linking of antigens into large antigen-antibody complexes. Classes of immunoglobulins The term constant region is a bit misleading in that these segments are not identical in all immunoglobulins.
Rather, they are basically similar among broad groups. All immunoglobulins that have the same basic kinds of constant domains in their H chains are said to belong to the same class. Each class has its own properties and functions determined by the structural variations of the H chains.
In addition, there are two basic kinds of L chains, called lambda and kappa chains, either of which can be associated with any of the H chain classes, thereby increasing still further the enormous diversity of immunoglobulins. The five main classes of antibodies immunoglobulins: IgG IgG is the most common class of immunoglobulin. It is present in the largest amounts in blood and tissue fluids. Each IgG molecule consists of the basic four-chain immunoglobulin structure—two identical H chains and two identical L chains either kappa or lambda —and thus carries two identical antigen-binding sites.
There are four subclasses of IgG, each with minor differences in its H chains but with distinct biological properties. IgG is the only class of immunoglobulin capable of crossing the placenta ; consequently, it provides some degree of immune protection to the developing fetus. IgM IgM is the first class of immunoglobulin made by B cells as they mature, and it is the form most commonly present as the antigen receptor on the B-cell surface. When IgM is secreted from the cells, five of the basic Y-shaped units become joined together to make a large pentamer molecule with 10 antigen-binding sites.
This large antibody molecule is particularly effective at attaching to antigenic determinants present on the outer coats of bacteria. When this IgM attachment occurs, it causes microorganisms to agglutinate, or clump together. IgA IgA is the main class of antibody found in many body secretions, including tears, saliva, respiratory and intestinal secretions, and colostrum the first milk produced by lactating mothers.
Very little IgA is present in the serum. IgA is produced by B cells located in the mucous membranes of the body. Two molecules of IgA are joined together and associated with a special protein that enables the newly formed IgA molecule to be secreted across epithelial cells that line various ducts and organs. Although IgG is the most common class of immunoglobulin, more IgA is synthesized by the body daily than any other class of antibody.
However, IgA is not as stable as IgG, and therefore it is present in lower amounts at any given time. IgD IgD molecules are present on the surface of most, but not all, B cells early in their development, but little IgD is ever released into the circulation. It is not clear what function IgD performs, though it may play a role in determining whether antigens activate the B cells.
IgE IgE is made by a small proportion of B cells and is present in the blood in low concentrations. Each molecule of IgE consists of one four-chain unit and so has two antigen-binding sites, like the IgG molecule; however, each of its H chains has an extra constant domain CH4which confers on IgE the special property of binding to the surface of basophils and mast cells.
When antigens bind to these attached IgE molecules, the cell is stimulated to release chemicals, such as histamines, that are involved in allergic reactions see immune system disorder: IgE antibodies also help protect against parasitic infections. Normal production of antibody Most individuals have fairly constant amounts of immunoglobulin in their blood, which represent the balance between continuous breakdown of these proteins and their manufacture. Part of the normal production of immunoglobulin undoubtedly represents the response to antigenic stimulation that happens continually, but even animals raised in surroundings completely free from microbes and their products make substantial, though lesser, amounts of immunoglobulin.
It is therefore not surprising that extremely sensitive methods can detect traces of antibodies that react with antigenic determinants to which an animal has never been exposed but for which cells with receptors are present. All B cells have the potential to use any one of the constant-region classes to make up the immunoglobulin they secrete. As noted above, when first stimulated, most secrete IgM.
Memory B cells, which are specialized for responding to repeat infections by a given antigen, make IgG or IgA immediately. What determines the balance among the classes of antibodies is not fully understood. However, it is influenced by the nature and site of deposition of the antigen for example, parasites tend to elicit IgEand their production is clearly mediated by factors, called cytokineswhich are released locally by T cells.
T-cell antigen receptors Structure of the T-cell receptor T-cell antigen receptors are found only on the cell membrane. For this reason, T-cell receptors were difficult to isolate in the laboratory and were not identified until T-cell receptors consist of two polypeptide chains. The most common type of receptor is called alpha-beta because it is composed of two different chains, one called alpha and the other beta. A less common type is the gamma-delta receptor, which contains a different set of chains, one gamma and one delta.
A typical T cell may have as many as 20, receptor molecules on its membrane surface, all of either the alpha-beta or gamma-delta type. The basic structure of a typical T-cell antigen receptor. The T-cell receptor molecule is embedded in the membrane of the cell, and a portion of the molecule extends away from the cell surface into the area surrounding the cell. The chains each contain two folded domains, one constant and one variable, an arrangement similar to that of the chains of antibody molecules.
And, as is true of antibody structure, the variable domains of the chains form an antigen-binding site. However, the T-cell receptor has only one antigen-binding site, unlike the basic antibody molecule, which has two. Many similarities exist between the structures of antibodies and those of T-cell receptors. Therefore, it is not surprising that the organization of genes that encode the T-cell receptor chains is similar to that of immunoglobulin genes. Similarities also exist between the mechanisms B cells use to generate antibody diversity and those used by T cells to create T-cell diversity.
These commonalities suggest that both systems evolved from a more primitive and simpler recognition system. Function of the T-cell receptor Despite the structural similarities, the receptors on T cells function differently from those on B cells. The functional difference underlies the different roles played by B and T cells in the immune system.
B cells secrete antibodies to antigens in blood and other body fluids, but T cells cannot bind to free-floating antigens. Instead they bind to fragments of foreign proteins that are displayed on the surface of body cells.
Thus, once a virus succeeds in infecting a cell, it is removed from the reach of circulating antibodies only to become susceptible to the defense system of the T cell. Infantile immunity is a true elaborate system, simply because while it exits the friendly intra-uterine environment, it is entering the hostile microbe-laden external world. In many ways, the immune system we are born with is the product of the immune environment during pregnancy 14. It is crafted and built block by block and day by day, forged through continuous and never ending improvement during gestation.
While still responding to all allo-antigens, the maternal immune system must be tolerant to the fetus, even though it is haplo-mismatched, or semi-allogeneic half of the antigens being of paternal, and therefore of foreign origin. This results in immunomodulation during pregnancy and extends to early life.
Initially, the tolerogenic status of neonatal immunity was attributed to the immaturity and lack of memory within immune components during infancy.
This premise of immune immaturity was replaced by the notion of immunodeviant characteristic of neonatal immunity 15. However, in light of recent discoveries, mounting evidence supports the concept that infantile immunity is in fact a highly regulated, but intellect, orchestrated, functional, and dynamic network of competent molecular and cellular components.
Considering the flexibility of neonatal immune system in response to outstretched number of stimulants make it staggeringly simple to further adopt the idea of regarding the infantile immune system as a vigilant establishment rather than immature. This wakeful immune scheme plays pivotal roles in protecting the growing and developing infants from pathologic conditions e.