Introduction
One of the main functions of the system is that the production of soluble proteins that circulate freely and exhibit properties that contribute specifically to immunity and protection against foreign material. These soluble proteins are antibodies, which belong to the category of proteins called globulins due to their globular structure. Initially, due to their migratory properties in an electrophoretic field, they were called γ-globulins (in reference to the sooner migrating albumin, α-globulin, and β-globulin); today they're known collectively as immunoglobulins (Igs).
Immunoglobulins are often membrane-bound or secreted. The membrane-bound antibody is present on the surface of B cells where it is the antigen-specific receptor. The membrane-bound sort of antibody is related to a heterodimer called Igα/Igβ to make the B-cell receptor (BCR). The Igα/Igβ heterodimer mediates the intracellular signalling mechanisms related to B-cell activation. Secreted antibodies are produced by plasma cells—the terminally differentiated B cells that function as antibody factories that reside largely within the bone marrow.
The structure of immunoglobulins incorporates several features essential for his or her participation within the immune reaction. The 2 most vital of those features are specificity and biological activity. Specificity is attributed to an outlined region of the antibody molecule containing the hypervariable or complementarity-determining region (CDR). This restricts the antibody to mix only with those substances that contain a specific antigenic structure. The existence of a huge array of potential antigenic determinants, which also are referred to as epitopes, prompted the evolution of a system for producing a huge repertoire of antibody molecules, each of which is capable of mixing with a specific antigenic structure. Thus, antibodies collectively exhibit great diversity, in terms of the kinds of molecular structures with which they're capable of reacting, but individually they exhibit a high degree of specificity since each is in a position to react with just one particular antigenic structure.
Despite the massive numbers of antigen-specific antibodies, the biologic effects of antigen-antibody reactions are rather few in number. Counting on the character of the antigen to which the antibody is restricted, these include neutralization of toxins; immobilization of microorganisms; neutralization of viral activity; agglutination (clumping together) of microorganisms or of antigenic particles; or binding with soluble antigen, resulting in the formation of precipitates. The latter is an example of how the adaptive system collaborates with the innate system since precipitated antigens are readily phagocytized and destroyed by phagocytic cells. Other samples of this collaboration, which occurs once antibodies react with antigens, include activation of complement to facilitate the lysis of microorganisms, and complement-mediated opsonization, which also leads to phagocytosis and destruction of microbes. Still another important biologic function of antibodies is that the ability of certain classes of immunoglobulins to cross the placenta from the mother to the fetus.
The differences within the various biologic activities of antibodies are attributed to structural properties conferred by the germline-encoded portions of the Ig molecule. Thus, not all antibody molecules are equal within the performance of all of those biological tasks. In simplest terms, antibody molecules contain structural components that are shared with other antibodies within their class and an antigen-binding component that's unique to a given antibody.
Isolation and characterization of immunoglobulins
The serum is that the antibody-containing component of blood. It's the liquid portion left when blood has been withdrawn and allowed to clot. Unless measures are taken to stop clotting of blood within the vacutainer during which blood is collected (e.g., the addition of heparin), clotting factors are going to be activated and cause a cellular clot to make. When the serum component is subjected to electrophoresis (separation in an electrical field) at slightly alkaline pH (8.2), five major components can normally be visualized. The slowest, in terms of migration toward the anode, called γ-globulin, contains the immunoglobulins. This original demonstration entailed the straightforward comparison of the electrophoretic pattern of antiserum from a hyperimmune rabbit (one that had received multiple immunizations with an equivalent test antigen) before and after the test antigen-specific antibody had been removed by precipitation with the antigen.
From the broad electrophoretic peaks, it's clear that a heterogeneous collection of immunoglobulin molecules with slightly different charges is present. This heterogeneity was one of the first obstacles in attempts to work out the structure of antibodies since analytical chemistry requires homogeneous, crystallizable compounds as starting material. This problem was solved, in part, by the invention of myeloma proteins, which are homogeneous immunoglobulins produced by the progeny of one plasmacyte that has become neoplastic within the malignant disease called myeloma. This is often demonstrated by the γ-globulin spike within the electrophoretic pattern of serum proteins of a patient with myeloma. When it became clear that some myeloma proteins bound antigen, it also became apparent that they might be addressed as typical immunoglobulin molecules. Another aid to structural studies of antibodies was the invention of Bence Jones proteins within the urine. These homogeneous proteins, produced in large quantities by some patients with myeloma, are dimers of immunoglobulin κ or λ light chains. They were very useful within the determination of the structure of this portion of the immunoglobulin molecule. Today, the powerful technique of cell-cell hybridization (hybridomas), which allows for the in vitro immortalization of antibody-producing B-cells, permits the assembly of huge quantities of homogeneous preparations of an antibody of virtually any specificity.
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References :
1. Immunology 7th Edition by Richard Coico and Geoffery Sunshine.
2. The image is from freepik.com.
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