Porth's Pathophysiology, 9e

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UNIT IV Infection, Inflammation, and Immunity

TABLE 13.4 CLASSES AND CHARACTERISTICS OF Igs

FIGURE

CLASS

PERCENTAGE OF TOTAL

CHARACTERISTICS

IgG

75.0

Displays antiviral, antitoxin, and antibacterial properties; only Ig that crosses the placenta; responsible for protection of newborn; activates complement and binds to macrophages

IgA

15.0

Predominant Ig in body secretions, such as saliva, nasal and respiratory secretions, and breast milk; protects mucous membranes Forms the natural antibodies such as those for ABO blood antigens; prominent in early immune responses; activates complement

IgM

10.0

IgD

0.2

Found on B lymphocytes; needed for maturation of B cells

IgE

0.004

Binds to mast cells and basophils; involved in parasitic infec- tions, allergic and hypersensitivity reactions

and is composed of two different kinds of polypeptide chain. IgG possesses antiviral, antibacterial, and antitoxin properties. It is present in all body fluids, readily enters the tissues, and is capable of crossing the placenta where it confers immu- nity upon the fetus. Intact IgG functioning requires the help of APCs. It binds to target cells as well as Fc receptors on NK cells and macrophages, leading to lysis of the target cell. There are four subclasses of IgG ( i.e., IgG1, IgG2, IgG3, and IgG4) with specificity for certain types of antigens. For exam- ple, IgG2 appears to be responsive to bacteria that are encap- sulated with a lipopolysaccharide layer, such as Streptococcus pneumoniae , Neisseria gonorrhoeae , and several strains of Salmonella . 30 IgA possesses a dimeric structure and is the second most common Ig found in serum accounting for approximately 15% of all antibodies. It is primarily a secretory Ig that is found in saliva, tears, colostrum ( i.e., first milk of a nursing mother), and bronchial, gastrointestinal, prostatic, and vagi- nal secretions. Because it is primarily found in secretions, its primary function is in local immunity on mucosal surfaces. IgA prevents the attachment of viruses and bacteria to epi- thelial cells.

IgM accounts for approximately 10% of all circulating antibodies. It normally exists as a pentamer with identical heavy chains and identical light chains. Because of its struc- ture, it is an efficient complement fixing Ig and is instrumental in the ultimate lysis of microorganisms. It also functions as an effective agglutinating antibody, capable of clumping organ- isms for eventual lysis and elimination. IgM is the first anti- body to be produced by the developing fetus and by immature B lymphocytes. IgD is a monomer found primarily on the cell membranes of B lymphocytes where it functions as a receptor for antigen. It circulates in the serum in extremely low levels where its function is essentially unknown. IgD on the surface of B lym- phocytes contains extra amino acids at C-terminal so that it can successfully anchor to the membrane. It also associates with the Ig-alpha and Ig-beta chains. IgE is the least common serum IgE because it binds very tightly to the Fc receptors on basophils and mast cells. It is involved in inflammation and allergic responses by causing mast cell degranulation and release of chemical mediators including histamine. IgE is also essential for combating para- sitic infections.