Antigen & Antibody reaction document pdf

Notes for DMLT STUDENTS

1. ANTIGEN & ANTIBODY

2. ANTIGEN An antigen is a substance which, when introduced into a body produce immune response.

3. TYPES OF ANTIGEN A. Complete Antigen These are substances which can induce antibody formation by themselves and can react specifically with these antibodies.

4. B. Haptens Haptens are substances unable to induce antibody formation on its own but can become immunogenic (capable of inducing antibodies) when covalently linked to proteins, called carrier proteins.

5. Heterophile Antigens The same or closely related antigens present in different tissues of more than one species are known as heterophile antigens.

6. BIOLOGICAL CLASSES OF ANTIGENS 1. T-cell Dependent (TD) Antigens These are antigens which require T cells participation to stimulate B-cells to produce antibodies. These antigens are called TD antigens.

7. 2. T-cell Independent (TI) Antigens • These antigens do not require the help of T-cells for stimulation of B-cells. These antigens are called TI antigens. • TI antigens do not require initial antigen processing by antigen-presenting cells (e.g. macrophages) unlike TD antigens.

8. SUPERANTIGENS • Superantigens are molecules that can interact with antigen-presenting cells (APCs) and T-lymphocytes in a nonspecific manner. • Interaction of superantigens with MHC class II molecules of the APC and the Vβ domain of the T-lymphocytes receptor activates a large number of T-lymphocytes than conventional antigens. This leads to massive cytokine expression and immunomodulation

9. ANTIBODIES Antibodies or immunoglobulins are specialised proteins which are formed by immune system in response to an antigen, and react with that antigen specifically. Chemical nature of antibodies is globulin and they are named as immunoglobulins. An antibody molecule consists of two identical heavy and two identical light chains

10. IMMUNOGLOBULIN CLASSES A. Immunoglobulin G (IgG) ( i ) IgG is the major serum immunoglobulin (about 80% of the total amount) (ii) It is the only immunoglobulin that can cross placenta. (iii) IgG appears late but persists for longer period

11. B. Immunoglobulin A (IgA) ( i ) IgA is the second major serum immunoglobulin (about 10-13% of serum immunoglobulins). (ii) IgA occurs in two forms, serum IgA (monomeric form) and secretory IgA (dimeric form). (iii) IgA protects the mucous membranes against microorganisms. (iv) IgA does not fix complement but can activate alternative complement pathway

12. C. Immunoglobulin M (IgM) (Fig. 6.3.4) ( i ) IgM is a pentamer consisting of 5 immunoglobulin subunits and one molecule of J chain. (ii) It constitutes about 5–8 percent of total serum immunoglobulins. (iii) It is heavy molecule (19S) with a molecular weight 900,000 to 1,000,000 hence also called the ‘millionaire molecule’. (iv) It is the first antibody to be produced in response to infection (acute infection) before IgG. (v) It can not cross the placenta, presence of IgM antibody in serum of newborn indicates congenital infections.

13. E. Immunoglobulin E ( IgE ) ( i ) IgE is mainly produced in the linings of respiratory and intestinal tracts. (ii) It is also referred to as reagins . (iii) It resembles IgG in structure. (iv) It is the only heat labile (inactivated at 56°C in one hour) immunoglobulin. (v) It has affinity for surface of tissue cells, particularly mast cells of the same species ( homocytotropism ).

14. D. Immunoglobulin D ( IgD ) ( i ) IgD resembles IgG structurally. (ii) IgD is also present on the surface of unstimulated B lymphocytes in blood and acts as recognition receptors for antigens.

15. IgG – protects the body fluids IgA – protects the body surfaces IgM – protects the blood stream IgE – mediates reaginic hypersensitivity IgD – recognition molecule on the surface of B lymphocytes

16. Monoclonal Antibodies Monoclonal antibodies are antibodies which are derived from a single clone of plasma cell and are produced against a single antigenic determinant (epitope) of an antigen

17. APPLICATIONS ( i ) Diagnostic Use: Many commercial diagnostic systems use monoclonal antibodies for identification of bacterial, viral and other antigens. (ii) Pure antibody: Large amount of pure antibody of defined class can be prepared. (iii) Passive immunity: Immunoglobulins against hepatitis B, rabies and tetanus are used for post-exposure prophylaxis. (iv) Therapeutic use: Treatment of cancer.

18. THANK YOU