NAJRAN UNIVERSITY College of Medicine

1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 18 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

2. Mechanisms of protective immunity • Protective immunity against bacteria: • 1- Antibody-mediated immunity • This occurs through the following mechanisms: • Toxin neutralization: Antitoxin neutralizes the toxin produced by some species of bacteria e.g. C. tetani, C. botulinum, C. diphtheriae. • Inhibition of bacterial adherence to target cells. • Opsonizationand enhancement of phagocytosis. • Complement activation leading to bacterial lysis.

3. 2- Cell-mediated immunity • T‑cells mediate a variety of reactions including : • Cytotoxicdestruction of intracellular bacteria. • Activation of macrophages. • Cell mediated reaction is mediated against intracellular micro­organisms, e.g. M tuberculosis, M leprae, brucellosis and salmonellosis.

4. Protective immunity against viruses: • 1- Antibody-mediated immunity • This occurs through the following mechanisms: • Virus neutralization: The neutralizing antibodies; IgG, IgM and IgA prevent virus attachment to receptor sites of host cells. • Antibodies may stabilize the viral capsid and prevent the release of viral genome. • Antibodies may destroy free virus particles through activation of complement or by virus aggregation and enhancement of phagocytosis.

5. 2- Cell-mediated immunity • This is mediated through: • Cytotoxic T‑cells; kill virus infected cells by inserting perforins and granzyme and by apoptosis. • NK cells destroy cells infected with the virus. Their activity is enhanced by IL2 and  interferon. • T‑helper cells, are stimulated to produce IL2 and  interferon which activate TC, NK and macrophages. • Antibody dependent cytotoxicity(ADCC); when specific antibodies coat the virus infected cells, they are lysed by NK cells or macrophages. • Th1 subset of CD4 cells and macrophages. Important lymphokines for these reactions include  interferon, macrophage activating factor and macrophage migration inhibition factor. CD4 cells produce the interleukins and macrophages are the ultimate effectors of delayed hypersensitivity.

6. 3- Interferons They are produced early after viral infection before the appearance of antibodies. They inhibit intra‑cellular virus replication. They are species specific. By contrast, they are not specific in their action, i.e. they inhibit the intra‑cellular replication of not only the inducer virus, but also any other virus. Large quantities of IFNs have been produced by recombinant technology.

8. TYPES OF ACQUIRED IMMANITY Acquired immunity results either from exposure to the organism “active immunity” or from receiving performed antibody or T-­lymphocytes "passive immunity". 1‑ Active Acquired Immunity : The body makes its own immune response as a result of antigenic stimulation. The response develops slowly and persists for long periods. It may occur naturally (following natural infections) or artificially after immunization.

9. A‑ Natural Active Acquired Immunity : • It follows infections whether clinical (i.e. diseases) or subclinical infections. It leaves the individual with long‑lasting immunity (e.g. measles, mumps, small pox and poliomyelitis) or with immunity of short duration (e.g. influenza virus). • B‑ Artificial Active Acquired Immunity : • It follows immunization of the individual with one of the following immunizing agents: • Live attenuated vaccines; e.g. BCG (for T.B.), Madrid E (for typhus), Sabin vaccine (for poliomyelitis). • Killed vaccines; e.g. TAB (for typhoid and paratyphoid fever), Cox vaccine (for typhus), and Salk vaccine (for poliomyelitis). • Bacterial products; e.g. toxoids of diphtheria and tetanus.

10. 2- Passive Acquired Immunity : • The individual is given performed antibodies or sensitized T-lymphocytes. • The protective effect of the transferred immunity develops rapidly, but remains for short duration. • The individual is thus protected passively (i.e. his immune system does not have any active role in this protection), without development of immunologic memory. • Passive immunity may be acquired naturally or artificially.

11. A‑Natural PassiveAcquired Immunity : • Transplacental transfer of antibodies (IgG) from mother to her fetus. • Transfer of maternal antibodies (IgA) via the colostrum occurs during lactation. • The protective effect of this immunity remains for 6 months after birth. • And for the whole lactation period . It is well known that the normal level of IgG in the serum is not reached before two years. During this period lactativeIgA is essential for protection of the child. • B‑ Artificial Passive Acquired Immunity : • Administration of immune serum prepared by injecting animals with antigens and using the sera of these animals, e.g. antitoxic serum against tetanus or diphtheria toxins prepared in horses. • Administration of human immunoglobulin and convalescent serum.