Antigens and antibodies Lecture №3.pptx.

Antigen

1. Antigens and antibodies.   Lecture № 3 Discipline GENERAL AND CLINICAL IMMUNOLOGY Senior teacher : Akylbubu Alymkulova

2. Molecules that can be recognized by the immunoglobulin receptor of B cells or by the T-cell receptor when complexed with major histocompatibility complex (MHC) are called antigens . The word antigen is a shortened form of the words “ antibody generator .” Learning objectives for Ag • Definition of antigen • Factors influencing immunogenicity • Biological classes of antigens

3. ANTIGEN • Defined as any substance that satisfies two distinct immunologic properties Immunogenicity Antigenicity.

4. Immunogenicity • Ability of an antigen to induce immune response in body (both humoral and/or cell mediated). B cells + antigen → effector B cells (plasma cell) + memory B cells T cells + antigen → effector T cells (helper T cell or cytotoxic T cell) + memory T cells • Antigens are substances that react with antibodies, while immunogens are molecules that induce an immune response.

5. Antigenicity (immunological reactivity) • Ability of an antigen to combine specifically with final products antibodies and/or T cell-surface receptors. • All molecules having immunogenicity property, also show antigenicity, but the reverse is not true

6. Epitope or antigenic determinant • Smallest unit of antigenicity. • Definition – Small area present on Ag comprising of few (4-5) amino acids or monosaccharide residues, that is capable of sensitizing T and B cells and reacting with specific site of T cell receptor or an antibody . • Specific site of an Ab that reacts with the corresponding epitope of an antigen is called as paratope .

7. A number of factors have been identified that make a substance immunogenic . Some of the important determinants of antigenicity include : 1. Molecular size 2. Foreignness 3. Chemical-structural complexity 4. Stability 5. Other factors

8. Molecular Size In general, protein molecules with large molecular weight are highly antigenic. Substances with molecular weights of about 100,000 Da and more are highly munogenic , while substances with molecular weights of less than 5000 Da are generally not immunogenic. • Larger size; more potent is molecule as an immunogen. • Molecules of > 10,000 Dalton molecular weight only can induce immune response (e.g. hemoglobin).

9. Substances with low molecular weight may be made antigenic by adsorbing these on carrier particles, such as bentonite , kaolin , and other inert particles . HAPTENS The antigens that are not immunogenic but can take part in immune reactions are termed as haptens .

10. • Low molecular weight molecules that – lack immunogenicity (cannot induce immune response) – but retain antigenicity or immunological reactivity (i.e. can bind to their specific antibody or T cell receptor). • Haptens - become immunogenic when combined with a larger protein molecule called ‘carrier’. • Hapten -carrier complex is capable of inducing immune response in body.

11. Haptens - Classification • Complex haptens : Contain two or more epitopes. • Simple haptens : Contain only one epitope (univalent ).

12. Self or auto antigens • Belong to the host itself - not immunogenic . • Hosts do not react to their own antigens by exhibiting a mechanism called immunological tolerance . • Sometimes, the self-antigens are biologically altered (e.g. as in cancer cells) and can become immunogenic. ANTIGEN AND HOST RELATIONSHIP • Based on the antigen-host relationship, antigens can be grouped into two groups: Self or auto antigens Non-self or foreign antigens

13. Non-self or foreign antigens- • Immunogenic • 3 types based on their phylogenetic distance to host. Alloantigens - species specific. Isoantigens –Ag present only in subsets of a species, e.g. blood group antigens and histocompatibility antigens Heteroantigens – Ag belonging to 2 different species e.g . antigens of plant or animal or microorganisms etc . A heterophile antigen is a type of hetero antigen that exists in unrelated species.

14. Isoantigens Isoantigens are antigens found in some, but not all, members of a species. A species may be grouped depending on the presence of different isoantigens in its members . These are genetically determined . Human erythrocyte antigens, based on which individuals are classified into different blood groups, are the best examples of isoantigens in humans. The blood groups are of primary importance in: ■ Transfusion of blood and blood products, ■ Isoimmunization during pregnancy, and Providing valuable evidence in paternity disputes, the results of which are supplemented by m ore recent DNA fingerprinting tests.

15. Histocompatibility Antigens Histocompatibility antigens are the cellular determinants specific for each individual of a species . These antigens are associated with the plasma membrane of tissue cells. Human leukocyte antigen (HLA) is the major histocompatibility antigen that determines the homograft rejection. Therefore, HLA typing is absolutely essential before carrying out transplantation of tissue or organ from one individual to another.

16. FACTORS INFLUENCING IMMUNOGENICITY • Size of the antigen • Chemical nature of the antigen • Susceptibility of antigen to tissue enzymes • Structural complexity • Foreignness to the host • Genetic factor • Optimal dose of antigen • Route of antigen administration: • Repeated Number of doses of antigens • Multiple antigens: • Effect of prior administration of antibody:

17. Chemical nature of the antigen • Immunogenity Order • Proteins > carbohydrates > lipid > nucleic acids. Route of antigen administration • Immune response is better induced following parenteral administration of an antigen . • Depends on the type of antibody produced. • Site of injection may influence immunogenicity

18. Adjuvant • Any substance that enhances the immunogenicity of an antigen. • Added to vaccines to increase the immunogenicity of the vaccine antigen .

19. Adjuvants Alum (aluminium hydroxide or phosphate) Mineral oil (liquid paraffin) Freund's incomplete adjuvant- It is a water-in-oil emulsion containing a protein antigen in the aqueous phase . Freund's complete adjuvant is the mixture of Freund's incomplete adjuvant & suspension of killed tubercle bacilli in the oil phase. Lipopolysaccharide (LPS) fraction of Gram-negative bacilli Other bacteria or their products- Mycobacterium bovis Toxoid (diphtheria toxoid and tetanus toxoid act as adjuvant for Haemophilus influenzae -b vaccine) Nonbacterial products: Silica particles, beryllium sulfate , squalene, and thimerosal.

20. Mechanism of adjuvant action 1 . Delaying the release of antigen 2 . By activating phagocytosis 3 . By activating TH cells 4 . By granuloma formation BIOLOGICAL CLASSES OF ANTIGENS •Depending on the mechanisms of inducing antibody formation, antigens are classified as: T cell dependent (TD) antigens. T cell independent (TI) antigens.

21. T-dependent (TD) Antigens • Most of the normal antigens are T cell dependent, they are processed and presented by antigen-presenting cells (APCs) to T cells which leads to T cell activation. • Activated T cells secrete cytokines that in turn stimulate the B cells to produce antibodies . T-independent (TI) Antigens • Eg . bacterial capsule, flagella and LPS do not need help of T cells and APCs . • Directly bind to Ig receptors present on B cells & stimulate B cells polyclonally . • Leads to increased secretion of non- specific Ab

22. Superantigens • activate T cells directly without being processed by antigen presenting cells (APCs) • variable β region of T cell receptor (vβ of TCR) appears to be the receptor for superantigens .

23. Superantigens • Directly bridge non-specifically between major histocompatibility complex (MHC)-II of APCs and T cells. • Non-specific activation of T cells leads to massive release of cytokines which can activate B cell polyclonally , which leads to increased secretion of non- specific antibodies ( hypergammaglobulinemia )

24. Bacterial superantigen : Staphylococcal toxin- Toxic shock syndrome toxin-1(TSST-1); Exfoliative toxin ; Enterotoxins Streptococcal toxin- Streptococcal pyrogenic exotoxin (SPE)-A and C Mycoplasma arthritidis mitogen-I Yersinia enterocolitica Yersinia pseudotuberculosis Viral superantigen : Epstein-Barr virus associated superantigen Cytomegalovirus associated superantigen Rabies nucleocapsid HIV encoded superantigen ( nef - negative regulatory factor)

25. Disease associated with superantigens • Conditions associated with staphylococcal toxins are as follow so Toxic shock syndrome Food poisoning Scalded skin syndrome o Rare conditions such as- Atopic dermatitis , Kawasaki syndrome, psoriasis, acute disseminated encephalomyelitis.

26. Antibodies are globulin proteins (immunoglobulins) that are synthesized in serum and tissue fluids, which react specifically with the antigen that stimulated their production. Three types of globulins are present in the blood: alpha, beta, and gamma. The antibodies are the gamma globulins. Antibodies are one of the major plasma proteins, and against infection often referred to as “first line of defense”. The most important function of antibodies is to confer protection against microbial pathogens .

27. Antibodies confer protection in the following ways: 1. They prevent attachment of microbes to mucosal surfaces of the host. 2. They reduce virulence of microbes by neutralizing toxins and viruses. 3. They facilitate phagocytosis by opsonization of microbes. 4. They activate complement, leading to complement-mediated activities against microbes.

28. Learning objectives Immunoglobulin classes Structure of antibody Antibody or immunoglobulin Specialized glycoprotein, produced from activated B cells (plasma cells) in response to an antigen . Capable of combining with the antigen that triggered its production.

29. Antibody or immunoglobulin • constitutes 20-25 % of total serum proteins . • 5 classes (or isotypes) of immunoglobulins – IgG, IgA, IgM, IgD and IgE . STRUCTURE OF ANTIBODY • ‘Y-shaped’ heterodimer; • 5 polypeptide chains . – 2 identical light (L) chains , 25,000 Da each • 2 types- Either kappa ( κ) or lambda ( λ), 2 identical heavy (H) chains each 50,000 Da

30. H & L chain domains • Light chain contains one variable domain (VL ) and one constant domain (CL ). • Heavy chains possess one variable domain (VH ) and 3 or 4 numbers of constant domains (CH )- – Heavy chains γ, α and δ have 3 constant domains-CH1, CH2 and CH3. – Heavy chains µ and ε have 4 constant domains- CH1 to CH 4.

31. Light chains An immunoglobulin molecule has two light chains. Each light chain is made up of 220–240 amino acids. Light chain is attached to the heavy chain by a disulfide bond. The light chains are structurally and chemically similar in all classes of immunoglobulins. They are of two types: kappa () and lambda (). These two types differ in their amino acids present in constant regions . Each immunoglobulin has either two or two chains but never both.

32. Ig A (immunoglobulin A) – Exists in two forms: serum and secretory . Half-life is 6 days. Provides protection of mucous membranes from infection . Makes up 10-15% of all immunoglobulinsIt protects the mucous membranes against microbial pathogens. It serves an important effector function at mucousmembrane surfaces, which are the main entry sites for mostpathogenic organisms. Because it is polymeric , secretory IgA can cross-link large antigens with multiple epitopes.

33. ■ Binding of secretory IgA to bacterial and viral surface antigens prevents attachment of the pathogens to the mucosal cells, thus inhibiting viral infection and bacterial colonization. Complexes of secretory IgA and antigen are easily entrapped in mucus and then eliminated by the ciliated epithelial cells of the respiratory tract or by peristalsis of the gut. ■ Breast milk contains secretory IgA and many other molecules that protect the newborns against infection during the first month of life. Because the immune system of infants is not fully functional, breast-feeding plays an important role in maintaining the health of newborns.

34. Ig M (immunoglobulin M ) is a macroglobulin, pentamer , half-life 5-8 days, synthesized in the early stages of the immune response, effectively agglutinates antigens, makes up 10% of all immunoglobulins . IgM shows the following biological activities: ■ Pentameric IgM, because of its high valency , is more efficient than other isotypes in binding antigens with many repeating epitopes, such as viral particles and red blood cells. ■ It is more efficient than IgG in activating complement. Complement activation requires two Fc regions in close proximity, and the pentameric structure of a single molecule of IgM fulfills this requirement .

35. ■ IgM is the first immunoglobulin produced in a primary response to an antigen. The immunoglobulin confers protection against invasion of blood by microbial pathogens. ■ IgM antibodies are short lived and disappear early as compared to IgG. The presence of IgM antibody in serum, therefore, indicates recent infection. ■ IgM is not transported across the placenta ; The detection of IgM antibodies in serum, therefore, is useful for the diagnosis of congenital infections, such as syphilis, rubella , toxoplasmosis , etc.

36. Ig G (immunoglobulin G) – with a molecular weight of 150,000 Da. IgG is the most abundant class of immunoglobulins in the serum, has 4 subclasses: IgG 1, IgG 2, IgG 3, IgG 4, half-life of 24 days, provides protection against microorganisms and toxins, activates complement components, penetrates the placenta, makes up 75% of all immunoglobulins . In response to infection, IgG antibodies appear late after appearance of IgM antibodies, but persists for a longer period . ■ It confers protection against the icroorganisms that are present in the blood and tissues. It is distributed equally in the intra- and extravascular compartments.

37. ■ It is the only immunoglobulin that crosses the placenta; hence, it confers natural passive immunity to the newborns . ■ It takes part in precipitation, complement fixation, and neutralization of toxins and viruses. ■ It binds to microorganisms and facilitates the process of phagocytosis of microorganisms.

38. Ig E (immunoglobulin E) – Reagin , half-life – 2-5 days. The level in the blood increases in allergic diseases. IgE shows the following biological activities: ■ IgE is also known as reaginic antibody that mediates the type I immediate hypersensitivity (atopy) reactions . ■ IgE is responsible for the symptoms of hay fever, asthma, and anaphylactic shock. IgE binds to Fc receptors on the membranes of blood basophils and tissue mast cells .

39. Cross-linkage of receptor bound IgE molecules by antigen (allergen) induces basophils and mast cells to translocate their granules to the plasma membrane and release their contents to the extracellular environment—a process known as degranulation. As a result, varieties of pharmacologically active mediators are released and give rise to allergic manifestations. ■ Localized mast-cell degranulation induced by IgE may also release mediators that facilitate a buildup of various cells necessary for antiparasitic defense .

40. Ig D (immunoglobulin D) – the majority is associated with the surface membrane of B-lymphocytes, increases sharply during pregnancy and myeloma. IgD is present on the surface of B lymphocytes and both IgD and IgM serve as recognition receptors for antigens. The role of IgD in immunity continues to remain elusive.

41. Thank you!