[Biology Class Notes] on Antigen and Immunology Pdf

Antigen:

Antigen is a kind of substance that is capable of stimulating an immune response that mainly stimulates lymphocytes which fight against infection and this fighting cell known as white blood cells.Antigen themself stimulate immune response and they are both antigenic and immunogenic in nature. Mainly antigen is divided into two types: one is foreign antigen known as heteroantigens and other is antigen which originates within the body known as autoantigens, example:autoimmune disorder.

When antigen enters the body it provokes lymphocytes to produce antibodies and this process is known as immunogen.  An antigen that  induces  humoral and/or cell-mediated immune response,this ability is called immunogenicity. whereas antigens are any substance that binds specifically to an antibody or a T-cell receptor.

Thus we can say that all immunogen are antigen but all antigen are not immunogenic. Most of antigens are proteins but some are carbohydrates, lipids, and nucleic acids. Active regions of an immunogen that binds to an antigen-specific membrane receptor known as Epitope. This surface is also called as antigen-determinants.

Types of Antigen:

A. Types of Antigen on the Basis of Origin:

1. Exogenous Antigens: antigen which are produced outside the body and start circulating in the body fluid and trapped by antigen processing cells such as macrophages, dendritic cells and this process is phagocytic in nature. Examples: bacteria, viruses, fungi etc.But there are some antigens who are earlier exogenous in nature but later become endogenous in nature,these types of antigen known as intracellular antigen.

2. Endogenous Antigen: They are produced inside the cell as a result of normal cell metabolism or because of any bacterial cell infection. The fragments are then present on the surface of the infected cells in the complex with MHC class 1 molecules.

3. Autoantigens: They are self protein or nucleic acid that is attacked by the host immune system, and they cause autoimmune disease. They are formed mainly due to genetic and environmental factors.

4. Tumor Antigens: These are presented by MHC molecules on the surface of tumor cells. They result in tumor-specific mutation during transformation of normal cells into cancer cells.

5. Native Antigens: It is an antigen that is not processed by an APC to smaller parts. T cells cannot bind to native antigen, whereas B cells can be activated by native ones.

B. On the Basis of Immune Response:

1. Immunogen: They are protein or polysaccharides and can generate an immune response on their own.

2. Hapten: These are non-protein, foreign substances that require carrier molecules to induce an immune response.

Diagram of Antigen Versus Antibody:

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Properties of Antigens:

Antigens have following features:

1. They have stable intramolecular structure and complex chemical bonding.

2. They have large stretches which are not composed of long repeating units.

3. They are mainly proteins and polysaccharides.

4. Their molecular weight is at least 8000 to 10000 Da .

5. They can undergo processing by the immune system.

6. Peptide antigens should contain immunogenic regions with at least 30% of amino acids such as lysine, glutamine, arginine, glutamic acid, asparagine and aspartic acid.

 

Structure of Antigens:

The antigenic determinants are the components of antigen and every antigen has several epitopes. Whereas every antibody has at least two binding sites and that site binds to specific antigens and every antigen has several epitopes. The antigen combines with antibodies like lock and key mechanism. The ability of the body to fight against antigen is termed an immunity. This immunity may be inborn or acquired by indirect mode like vaccination known as innate immunity.

 

Types of Immunity:

1.Innate Immunity- It is also called as the natural immunity, as it is inherited from the parents to their young ones and protects from birth throughout the life period.

2. Acquired Immunity- It is also called as the adaptive immunity, as it is acquired after the birth in the form of vaccination, antibiotics, Immunization, etc.

Interaction of Antigen with Antibody:

Interaction between antigen and antibody is very specific as they are totally dependent on hydrogen bonding, electrostatic force and van der waals forces.there interaction is of weak, non covalent nature but some interaction of antigen and antibody are strong in nature. In case either they are multivalent in nature or they have multiple copies of epitopes, under these cases they form stabilized complexes, thus reducing possibility for binding. All antigen binding process is a reversible process and they follow the basic thermodynamic principle of any reversible bimolecular interaction reaction: where KA is the affinity constant, (Ab-Ag) is the molar concentration of the antibody-antigen complex and (Ab) and (Ag) are the molar concentration.

Nature of Antigen-Antibody Bond:

The Binding site of an antibody is located in the F(ab) portion of the antibody molecule and is assembled to heavy and light chains. These binding sites have following features:

1. Bond nature antigen to combine sites of any antibody are non covalent in nature and because of this they are reversible in nature.

2. Bonds formed during the process may be hydrogen bonds, electrostatic bonds or van der waals forces.

3. These types of forces help them to overcome their hydration energies and allow excretion of water molecules.

4. When epitopes come in contact with paratopes they are attracted to each other by ionic and hydrophobic forces.

5. For interaction antigen-antibody has to overcome repulsion between two molecules.

Factors Affecting Antigen-Antibody:

There are several factors which dete
rmine rate of antigen-antibody reaction, these factors are given below:

1. Temperature: chemical nature of antigen-antibody determines optimum temperature for their interaction and the type of interaction they possess are hydrogen bond, van der waals attraction, etc. 

2. pH: The effective pH range for antigen-antibody interaction is 6.5 to 8.4 and above this pH binding is inhibited.

3. Ionic Strength: Ionic strength plays a crucial role in antigen-antibody binding as they are particularly important in blood group serology.