Manual of basic techniques for a health laboratory - libdoc.who.int

11. Immunological and serological techniques 329promote production of antibodies. In humans there are five major classes ofantibodies or immunoglobulins: IgG, IgM, IgA, IgE and IgD.These proteins differ in their electrophoretic mobility, relative molecular mass,antigenic structure, sedimentation coefficient, shape and other properties. Allthe immunoglobulins have a structure composed of four polypeptide chainswith two long or heavy (H) chains and two short or light (L) chains (Fig.11.1).Within these H and L chains there are regions known as constant regions,where amino acid sequences are very similar, and variable regions (usually atthe ends of the chains), where amino acid sequences are highly variable. Thevariable regions give the different antibodies their specificity.H chainFig. 11.1 Structure ofimmunoglobulinsL chain11.1.2 AntigensAntigens are molecules (usually proteins) that can elicit an immune response.Antigens have sites on them known as antigenic determinants which can be recognizedby antibodies. Antigens may have several determinants of differentconfiguration or several of the same configuration such that antibodies of the samekind or of several different kinds can bind to these sites.Several properties can influence the immunogenicity of an antigen; these are brieflydiscussed below.Recognition of the antigen as a foreign substanceThe most important property of an antigen is that which allows the body to recognizethe substance as foreign. An individual’s immune system is normally capableof distinguishing substances belonging to the body from those that do not (selffrom non-self). Certain conditions incapacitate the self-tolerance mechanism andthe system starts to react against itself.Relative molecular massThe relative molecular mass or size of an antigen also affects its immunogenicity.As a general rule, large molecules make good antigens (i.e. they are effective ingenerating an immune response), provided they are foreign to the body. Smallmolecules are usually poor antigens.ComplexityThe more complex the molecule, the better the immune response to the antigen.Complex proteins make “better” antigens than repeating polymers of lipids, carbohydratesand nucleic acids.StabilityIt is essential for the antigen to be stable for it to be recognized by the immunesystem.DegradabilityThe substance must be degradable. In order to initiate an immune response theimmune system must be able to process the substance.Route of antigen administration and dosageAntigens must be administered correctly. Some substances elicit an immune responseif administered subcutaneously, but not if administered intravenously. The