• Antibodies

Humoral immunity is due to the production of antibodies, a special class of proteins that are soluble in the body fluids. An antibody is produced by a vertebrate host in response to the introduction of an antigen into the body, and it binds specifically with the antigen that stimulated its formation. Antigen-antibody reactions protect the host against many detrimental effects of intruding microbes or other foreign substances.

Antibodies are also called immunoglobulins because they participate in immune reactions and belong to a class of serum proteins called globulins Antibodies are monospecific molecules - they combine only with the single type of antigenic determinant that stimulate their formation. Most antibodies are also bivalent - they possess two identical reactive sites and can couple with two identical antigenic determinants. The reaction between antibody and antigen results in the formation of an antigen-antibody complex.

• Antibody structure. Although some structural variations exist among antibodies, the typical antibody molecule consists of four protein chains linked together by disulfide bonds in what is usually illustrated as a Y-shaped structure. The two shorter chains, called light (L) chains, are covalently linked to the branches of the longer heavy (H) chains. Each chain has variable and constant regions. Both H and L chains are devided into constant region domains (designated CH and CL) and variable region domains (designated VH and VL) (Fig. 2). The specificity of the antibody’s combining sites for antigen is determined by the amino acid sequence in the variable regions of both the H and L chains.

The amino acid sequence in the constant region determines other characteristic properties of the antibody, such as its ability to cross chain tissue barriers, to activate the complement system, or to adhere to phagocytic cells. The properties of the tail portion, called the Fc region, define the five major classes of immunoglobulins.

1. Immunoglobulin G (Ig G) is the most common class of antibodies, comprising 80 percent of the antibodies found in the blood. Ig G is typically Y-shaped and bivalent. It crosses the placenta, so that a mother’s Ig G antibodies help to protect her developing fetus. When coupled with antigen, Ig G activates the complement cascade.

2. Immunoglobulin M (Ig M) antibodies consist of five Y-shaped subunits linked together by disulfide bonds in the Fc region. They are the first antibodies to appear after initial exposure to antigen. They fix complement but do not cross the placenta.

3. Immunoglobulin A (Ig A) is a class of antibodies found in two forms. In the serum, Ig A structure resembles that of Ig G. The other form, called secretory Ig A, is the principal antibody found in saliva, mucus, tears, milk, and other external secretions. Secretory Ig A is a dimer composed of two Ig A molecules coupled in the tail region. It is readily secreted across mucous membranes, providing local protection on surfaces of such areas as the alimentary, respiratory, and genitourinary tracts.

4. Immunoglobulin E (Ig E) attaches by its Fc fragment to certain host cells (e.g., mast cells), leaving its antigen-combining sites available for binding with antigen. Ig E is responsible for one group of allergic reactions, the best known of which are hay fever and asthma. Its protective function is detected primarily against parasites.

5. Immunoglobulin D (Ig D) is found in very low concentrations in blood. It is bound to the surface of B lymphocytes and is believed to be necessary for the differentiation of immune cells.

Some properties of the five human immunoglobulin classes are shown in Table 12-1.

Table 12-1

The Main Properties of Human Immunoglobulins

Class of Ig	Structure	Molecular Weight (daltons)	Percent in Blood	Location	Crosses Placenta?	Fixes Complement?
IgG	Monomer	150,000	75-80	Blood and tissue fluids	Yes	Yes
IgA	Monomer, dimer or trimer	160,000- 318,000-380,000	15-21	Serum, saliva, mucus, and secretions	No	No
IgM	Pentamer	900,000	6-7	Blood and tissue fluids	No	Yes
IgD	Monomer	180,000	<1	Serum	No	No
IgE	Monomer	200,000	<1	Skin, respiratory tract, and tissue fluids	No	No

Reactions of antigens and antibodies are highly specific. An antigen will react only with antibody elicited by its own kind or by a closely related antigen. Because of this high specificity, reactions between an antigen and an antibody can be used to identify one by means of the other. This specificity is the basis of serologic reactions.

Antigen-antibody reactions are used to identify specific components in mixtures of either one. Microorganisms and other cells possess a variety of antigens and may thus react with many different antibodies. Countless in vitro diagnostic techniques detect specific reactions between antibodies and their corresponding antigens. Any diagnostic technique that depends on specific reactions between antigen and antibody is called immune or serological (= serologic) test. Immune tests are usually used in the following cases:

1. to identify an «unknown antigen» (bacteria, virus, toxin, etc. in a specimen) with the help of the «known antibody» (diagnostic antibody-containing antisera);

2. to determine an «unknown antibody» (in a patient’s blood serum) with the help of the «known antigen» (diagnosticum).

Thus, one component (ingredient) in serological tests should always be known.

Although most serological tests depend on the same theoretical principle, methods for detecting specific antibody-antigen reactions differ. Agglutination, precipitation, complement-fixation test, neutralization, IF-test, ELISA and RIA are the most important of them.