Blood contains antibodies which defend the body from infection. They constitute the third line of defense. Antibodies have already been defined as specific substances formed by the body in response to stimulation by specific foreign and protein like substances called antigens. Chemically, they too are proteins, composed principally of gamma globulins, although other immunoglobulins may also be present. IMMUNOGLOBULINS The discovery of the gamma globulins in human serum investigation has revealed that the gamma globulin fraction is not one but at least five distinct classes, all of which have antibody activity.
These globulins are identified as classes of immunoglobulins and designated IgG, IgM, IgA, IgD, and IgE, which is shorthand for immunoglobulin G, or γG, immunoglobulin M or γM, etc. there may even be more than one immunoglobulin in a class. IgG, for example, has four subclass designated by subscripts such as IgGα, etc. IgG. Some 80 percent of the serum antibodies found in normal adult serum is IgG. This is 700 to 1,500 mg of IgG per 100 ml of serum. Globulins of class IgG and its subclass have a molecular weight of 160,000. IgM. Of the total immunoglobulin in human and rabbit serum, 5 to 10 percent is IgM, which contains two subclasses. Antibodies specific for lipopolysaccharide antigens of gram-negative bacteria and other antibodies belong to this class. Their molecular weight is 900,000. IgA. IgA is present in normal serum in amounts ranging from 150 to 250 mg per 100 ml. this is about 10 percent of the total immunoglobulin present. These globulins are found in serum, while another subclass occurs in colostrums, tears, and other body secretions. These secretory globulins differ in molecular weight and in other ways. It is believed that the secretory IgA assists in protecting exterior cavities of the body and their mucous membranes against infection. IgD. Class IgD was isolated from myeloma proteins. About 3 percent of the total gamma globulin content of the human serum belongs to this class. Its function is not known. Its molecular weight is 160, 000. IgE. P-K antibodies (reagin) belong to the immunoglobulin class IgE. They are of clinical importance because they are responsible fro various allergies, some drug sensitivities, allergic asthma, and anaphylaxis. The molecular weight is 160, 000. Antibodies react against specific microorganisms, their toxic products, and other compounds. They can be used in the treatment of infection caused by the homologous microorganisms, and, more importantly, they prevent infection and disease caused by theses agents. Antibodies are designated by names that describe their reaction in vitro or in vivo, when they are allowed to act on certain types of antigens: (1) antitoxins, (2) agglutinins, (3) precipitins, (4) lysins, (5) complement-fixing antibodies, (6) opsonins, and (7) neutralizing antibodies. These antibodies are all produced as a result of antigenic stimulus and are present in blood serum. They are called humoral antibodies and can be differentiated as follows: (1) Antitoxins neutralize toxins, (2) Agglutinins cause clumping of the bacterial cells for which they are specific, (3) Precipitins cause precipitation or flocculation of extracts of bacterial cells or other soluble antigens, (4) Lysins cause dissolution of bacterial or other cells that specifically sensitive to their action, (5) Certain antigen-antibody reactions require that complement be present to bind the reactants together. In so doing the complement is “fixed”, or used up. To determine whether a reaction has taken place between an antigen and an antibody, a test is made for the presence or absence of complement, (6) Opsonins render microorganisms more susceptible to ingestion by phagocytes and (7) Neutralizing (protective) antibodies protect against infection by neutralizing the infectivity of the pathogen or by neutralization of toxin.
