Antibodies are globulin proteins (immunoglobulins [Ig]) that react specifically with the antigen that stimulated their production. They make up about 20% of the protein in blood plasma. Blood contains three types of globulins, alpha, beta, and gamma, based on their electrophoretic migration rate. Antibodies are gamma globulins. There are five classes of antibodies: IgG, IgM, IgA, IgD, and IgE. Antibodies are subdivided into these five classes based on differences in their heavy chains.
The most important functions of antibodies are to neutralize toxins and viruses, to opsonize microbes so they are more easily phagocytosed, to activate complement, and to prevent the attachment of microbes to mucosal surfaces. The specific antibody classes that mediate these functions are described in Table 59–1. In addition to these functions, antibodies have a catalytic (enzymatic) capability that is described in a separate section at the end of this chapter.
TABLE 59–1Properties of Human Immunoglobulins |Favorite Table|Download (.pdf) TABLE 59–1 Properties of Human Immunoglobulins
|Property ||lgG ||lgA ||lgM ||lgD ||lgE |
|Percentage of total immunoglobulin in serum (approx) ||75 ||15 ||9 ||0.2 ||0.004 |
|Serum concentration (mg/dL) (approx) ||1000 ||200 ||120 ||3 ||0.05 |
|Sedimentation coefficient ||7S ||7S or 11S1 ||19S ||7S ||8S |
|Molecular weight (×1000) ||150 ||170 or 4001 ||900 ||180 ||190 |
|Structure ||Monomer ||Monomer or dimer ||Monomer or pentamer ||Monomer ||Monomer |
|H chain symbol ||γ ||α ||μ ||δ ||ε |
|Complement fixation ||++ ||– ||++ ||– ||– |
|Transplacental passage ||++ ||– ||– ||– ||– |
|Mediation of allergic responses ||– ||– ||– ||– ||++ |
|Found in secretions ||– ||++ ||– ||– ||– |
|Opsonization ||++ ||– ||–2 ||– ||– |
|Antigen receptor on B cell ||– ||– ||++ ||? ||– |
|Polymeric form contains J chain ||– ||++ ||++ ||– ||– |
Antibodies that arise in an animal in response to typical antigens are heterogeneous, because they are formed by several different clones of plasma cells (i.e., they are polyclonal). Antibodies that arise from a single clone of cells (e.g., in a plasma cell tumor [myeloma])1 are homogeneous (i.e., they are monoclonal).
Monoclonal antibodies also can be made in the laboratory by fusing a myeloma cell with an antibody-producing cell (Figure 59–1; also see box “Hybridomas & Monoclonal Antibodies”). Such hybridomas produce virtually unlimited quantities of monoclonal antibodies that are useful in diagnostic tests and in research (see box “Hybridomas & Monoclonal Antibodies”).
Production of monoclonal antibodies.