Immunology

Cross reactivity is often observed in polysaccharide antigens that contain similar oligosaccharide residue. A, B, O blood group antigens - These are glycoprotein expressed on RBC.

Antigen-antibody reactions are specific, but in some cases antibody elicited by one antigen can cross react with another antigen. This reaction is called as cross-reaction and the antigen that produces cross-reaction is called as cross-reactive antigen. Cross-reaction is due to the presence of two or more antigenic determinants on the related antigen.

The capacity of an antiserum containing various antibodies to combine with the whole antigen is called avidity. Thus, avidity is used to denote the overall capacity of an antibody to combine with multivalent antigen.

A multivalent antigen has many types of antigenic determinants, when this is injected into the blood each antigenic determinant stimulate the production of particular antibody.

The strength of binding of an antibody to a monovalent antigen or single antigenic determinant is called affinity of an antibody.

Temporary transfer of electrons from one molecule to another will result in the force of attraction between them. This is seen when the interacting molecules come close to each other.

Contribute up to 50% of the total strength of antigen- antibody interactions. These reactions are found when ever the side chains of non-polar amino acids of antigen-antibody come together.

Reversible hydrogen bonds are formed between hydrophilic groups such as hydroxyl, amino and carboxylic group. Although hydrogen bonds are relatively weak, they play an important role in interaction of antigen-antibody.

These are formed due to the attraction between opposite charged protein side chains.

The process that holds antigen-antibody together is called non-specific interactions. Inter molecular forces may be classified into four-

1. Electrostatic bonds

2. Hydrogen bonds

3. Hydrophobic interactions

4. Vander Val interactions

The portion of the antibody molecule that binds to the epitope is called as paratope. Epitope and paratope determine the specificity of immunological reactions.

The smallest unit of antigenesity is known as antigenic determinant or epitope.

The part of the antigen at which the antibody reacts is known as epitope or antigenic determinant.

It is a small area possessing specific chemical structure and stereo configuration on the antigen capable of sensitizing on immuno site and of reacting with its complimentary site on the specific antibody.

Antigen antibody reaction is specific and specificity is determined by special configuration of antigenic determine.

1. The reaction is specific and antigen combines only with its corresponding antibody and vice versa.

2. Entire molecules react but not the fragment.

3. There is no denaturation of antigen or antibody during the reaction.

4. The combination of antigen – antibody is firm but reversible. The firmness of the reaction is influenced by the affinity and avidity of the reaction.

5. Both antigens and antibodies participate in the formation of agglutination and precipitation reactions.

6. Antigens and antibodies can combine in various proportions unlike chemicals with fixed valancy.

Antigen-antibody interaction is similar to an enzyme substrate interaction. The reaction between antigen and antibody occurs in two stages. Primary stage is the initial interaction of antigen-antibody without any visible effect .The reaction is rapid and obeys the general law of thermodynamics and physical chemistry.

The primary stage is followed by the secondary stage leading to demonstrate events such as precipitation, lysis of cells, neutralization of toxins and fixation of compliments etc.

Combination of optical dosage and routes of administration will induce a peak immune response in a given animal. An insufficient dose will not stimulate an immune response. An excessive dose does not give a peak immune response because it causes a state of immunological unresponsiveness or non-response known as immunological tolerance.

To be antigenic macromolecules must come from foreign source. Antigens from related species are less antigenic than that of unrelated species. More distance the antigen source the better is the antigenesity.

Ex: Plant proteins are good antigens in animals, where as duck serum proteins are not good antigens for chick.

Most naturally, occurring antigens are proteins and polysaccharides. Lipids and nucleic acids are less antigenic. The antigenic property of these compounds is enhanced by combination with proteins.

Certain degree of structural complexity is required for antigenesity. Synthetic polymers are macromolecules in size are not antigenic because they lack structural complexity. Ex: Gelatin

Molecule size of an antigen has a direct relation to antigenesity. Very large molecules such as haemocyanin (6.7 million Daltons) and thymoglobulin (669 kd) are highly antigenic where as low molecular weight compounds whose molecular weight is less than 10,000 Daltons are poor antigenic. Ex: Insulin and histones

A number of factors have been identified which make a substance antigenic. They are -

► Size

► Chemical nature

► Susceptibility to tissue enzymes

► Foreignness

► Immunogen dose and route of administration

Small molecules such as DNP (Dinitro phenyl), M-amino benzene sulphonate by themselves are not immunogenic. However, when they conjugate with a protein such as Bovine serum Albumin (BSA), they can act as complete antigens.