NAJRAN UNIVERSITY College of Medicine

1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 11 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

2. Acquired (specific)IMMUNITY Acquired immunity is a specific immunity occurring in certain individuals after exposure to a particular antigen e.g. an organism. It may be humoral (i.e. due to production of antibodies) or cell mediated (i.e. due to activation of T lymphocytes). The main functions of antibodies are: a- To neutralize toxins and viruses. b‑ To enhance the process of phagocytosis.

3. ANTIGEN (IMMUNOGEN) : An antigen is a substance which when introduced into the body of a foreign animal will stimulate a specific immune response. This immune response occurs in two forms: 1. Antibodies (humoral response): These are produced by one type of lymphocytes called B‑lymphocytes. 2. Cell‑mediated immune response which is a function of another type of lymphocytes called T‑lymphocytes.

4. Hapten: ‑ A low molecular weight substance, which cannot stimulate an immune response by itself (if injected alone). ‑ If it is carried on a larger molecule (if bound to a protein carrier), it becomes antigenic and can induce a specific immune response and reacts specifically with it. ‑ e.g. polysaccharide capsule of pneumococci.

5. Antigenic Determinants (Epitopes): ‑ Small chemical groupings (amino acids or oligosaccharides) present on the surface or within the antigen molecules. ‑ They are responsible for the specificity of the immune response because they are the sites with which the antibodies combine. ‑ They are usually multiple and the valency of the antigen equals the number of its epitopes. ‑ Removal or addition of epitopes may alter the antigen.

6. Factors governing antigenicity: 1. Host factors:age, genetic constitution, nutrition and disease states, etc.. 2. Factors related to the antigen : a) Foreignness: The immune system can distinguish self from non‑self components and the response occurs only to foreign (non‑self) components which are not normally present in the body. Response to self components results in autoimmune disease. b) Molecular size: Antigens should be large molecules of high molecular weight (more than 10.000). Substances of low molecular weight may act as haptens.

7. c) Chemical nature: The more the complexity of the molecule, the more is the antigenicity. Proteins are more antigenic than carbohydrates and lipids. d) Antigen dose: Within an appropriate dose, the antigens have optimum antigenicity. Extremely low dose results in a state of low‑zone tolerance and extremely high dose results in a state of high‑zone tolerance. e)Route of administration : Antigens administered orally or through the mesenteric route are less able to stimulate an immune response than those administered by S.C., I.M., I.V. or inhalation.

8. f) Adjuvants Presence: Adjuvants are substances, which can enhance the immunogenicity of antigens without altering their chemical nature, e.g. • • Mineral gels: such as aluminium hydroxide and calcium phosphate. • • Bacterial products such as endotoxin of Gram‑negative organisms, killed tubercle bacilli and killed Bordetellapertussis (built in adjuvants). • • Oil in water emulsion: e.g. Freund's adjuvant, available in 2 forms • Complete Freund's adjuvant: containing killed mycobacteria. • Incomplete Freund's: containing extracted mycobacterial antigens.

9. Cross reactivity : An antibody produced against an antigen can react with another similar or related antigen. This antibody is called cross‑reacting or heterophil antibody and the stimulating antigen is called heterophilantigen Common types ofantigens : 1. Bacterial antigens: e.g. cell wall (somatic or "0") antigens, flagellar (H) antigens, capsular antigens, fimbrial antigens, extracellular antigens (such as exotoxins) and intracellular antigens (e.g. nucleoproteins). 2‑Other microbial antigens such as viral, fungal and parasitic antigens.

10. 3 ‑ Tissue antigens e.g. • Blood group antigens (ABO antigens and Rh antigens). • 4‑ Drugs: Some drugs acts as antigens while others can act as haptens. • 5 ‑Food antigens such as proteins of milk, eggs and other foods.

11. Class of antigen (T‑dependent and T‑independent antigens): 1-Some antigens can elicit a humoral response without the cooperation of T‑cells. These are called T‑independent antigens, e.g. lipopolysaccharides, polymers of D–amino acids, dextran. 2-Other antigens require the aid of T‑helper cells to stimulate B‑lymphocytes to produce the antibody. These are called T‑dependent antigens, e.g. complex proteins (serum proteins). 3-Superantigens