9.
Each immunoglobulin is composed of at least one basic unit or monomer
comprised of four polypeptide chains. This basic four-chain subunit consists of
two identical heavy chains (H) and two identical light chains (L). (para 2-4a)
10.
Based on structural differences in the constant regions, there are five classes of
heavy chains. The different forms of heavy chains are designated gamma, alpha,
mu, delta, and epsilon. The type of heavy chain determines the class of the
immunoglobulin. There are five classes of immunoglobulins, designated as IgG,
IgM, IgA, IgD, and IgE. Light chains can be classified as kappa and lambda on
the basis of multiple structural differences in the constant region. (para 2-4b)
11.
The immunoglobulins are divided into how many classes? 5 The most common of
these classes is IgG. In normal human adults, IgG constitutes approximately 75%
of total serum immunoglobulins. The molecule is made up of one basic structural
unit known as a monomer, consisting of 2 heavy chains and 2 light chains. IgG is
the only class of immunoglobulin that can cross the placenta, and it is responsible
for the protection of the newborn during the first few months of life. (para 2-5a)
12.
The third most common form of immunoglobulin in serum is IgM. It has a high
molecular weight. The macromolecule makes up about 10% of the normal serum
immmunoglobulins. It exists as a pentamer consisting of 10 heavy chains and 10
light chains joined together by a J chain. (para 2-5b)
13.
The second most common form of immunoglobulin in serum is IgA. It represents
approximately 15% of the total serum immunoglobulins. It normally exists in serum
both in monomeric and polymeric forms. The IgA dimer consists of two monomeric
units. It is the predominant immunoglobulin class found in body secretions. (para 2-5c)
14.
The IgD molecule is a monomer and is normally present in serum in trace
amounts. Its main function has not been determined. IgE comprises only 0.004%
of the total serum immunoglobulins. IgE normally exists in monomeric form.
(para 2-5d, e)
15.
Complement is a system of serum proteins that is the primary dissolved mediator
of antigen-antibody reactions. The complement system is one of the basic
defense mechanisms of the body. It is a complex series of enzymatic proteins
occurring in normal serum which interact to enhance the immune response. The
system involves the sequential activation and interaction of at least 14 serum
proteins and may occur via the classical pathway or the alternative pathway.
These two pathways are parallel but independent. The two pathways do become
identical at the point of the membrane attack complex. Natural inhibitors and
instability of the complexes act to balance the system so that uncontrolled
activation does not occur. Thus, total consumption of the components does not
occur every time there is activation. (para 2-6)
MD0838
2-18