Immunology
Last revised: Wednesday, April 16, 2003
Reading: Ch. 27 in text

Active and Passive Immunity

• Active immunity: due to individual's immune system learning new antigens, acquiring ability to make B- and T-cells specific for this antigen.
• View diagram with overview of roles of B- and T-cells
• Passive immunity: due to acquiring preformed antibodies from another individual.
• Example: unimmunized person gets tetanus shot after stepping on rusty nail. Shot consists of antibodies to the tetanus toxin made in the body of some animal (e.g. horse). Provides immediate protection, but will not last. Passive immunity typically lost after 6 months.
• Newborn children do not yet have active immunity. For first 6 months, don't get many diseases, protected by mother's antibodies passed to blood system of newborn before birth. After 6 months, infant must rely on its immune system to "learn" and acquire immunity to series of diseases.

Antibodies & Antigens

• Antigens are "foreign" substances that induce some kind of immune response. Typically high mol. wt. (> 10,000 daltons).
• Good antigens include proteins, complex polysaccharides. Nucleic acids not typically good antigens, nor are simple polysaccharides (e.g. glycogen). Low molecular weight molecules are not antigenic.
• Antibodies: proteins found in serum (liquid portion of blood after removal of clotting factors from plasma). Technically called "Gamma globulins", because found in the gamma fraction of serum proteins isolated by electrophoresis. Antibodies = gamma globulins = immunoglobulins, all synonymous terms.
• All antibodies are based on the unit "motif" of a Y-shaped molecule made of two light chains and two heavy chains. Some antibody classes (IgM) occur as multimers in which more than one unit is attached together.
• IgG is the most common antibody in blood and lymph, but IgM is the first to appear.
• View structure of an IgG molecule (requires Chime plug-in)
• Antibody structure (IgG): two heavy chains + 2 light chains. Each chain has a constant region and a variable region.
• Antibody + antigen combine by specific 3-D stereospecific weak chemical bonding. View antibody-antigen bonding (requires Chime plug-in)
• Types of Antibodies. View figure showing different antibody classes
  1. IgG; bivalent (2 binding sites), most common antibody, 70-75% of antibodies. Found in blood and lymph. Only antibody that can cross placenta.
  2. IgM; decavalent (10 binding sites), 10% of total antibodies; first antibody made in response to new antigen. Found in blood and lymph. Binds most avidly (more binding sites than other Ab types)
  3. IgA; bivalent (2 binding sites), has "secretory piece" that allows molecule to travel across membranes, be exposed to moist surfaces (naso-pharyngeal cavity, respiratory tract, lining of intestine, vagina, etc.). 15% of total antibodies. Found in secretions: saliva, milk, tears, colostrum, mucous membranes.
  4. IgE; bivalent; accounts for only 0.000005% of total antibodies. Responsible for allergic, hypersensitive reactions.
  5. IgD; bivalent, only traces in blood, but found on surface of B-cells, signal B-cell to start antibody production. How many antibodies? Although only 5 types, potentially millions of different specificities. Differences occur in variable regions; despite similar overall structure, each antibody type has very specific binding site for only one or a very few antigenic sites.
• For further information, explore structure of antibodies, from Duane Sears text (Freeman).

Antibody-Antigen reactions: what happens?

• See roles of antibodies ("protected" image.) Why is this protected?
• Neutraliztion: if antigen is a toxin,
• Opsonization: if antigen is on surface of a bacterium or virus
• Serological tests: many clinical tests are based on antibody-antigen reactions
• See serological tests for examples of how tests can be designed. ("protected" image.) Why is this protected?
• Western blot: separate protein on electrophoresis gel. Transfer to a membrane by blotting gel. Incubate with specific serum, and use secondary antibody to detect first antibody. Instead of seeing all proteins on gel, only see the target protein. See diagram of Western blot. ("protected" image.) Why is this protected?

T-cells and T-cell receptors

• T cells mature in the thymus gland, and differentiate further into several varieties which can be distinguished based on whether they carry surface proteins of type CD4 or CD8.
• CD4 = T-helper cells, T-delayed-type hypersensitivity
• CD8 = T- cytotoxic cells, T- suppressor cells
• T cells don't produce antibodies. Each T cell, however, is different from its fellow T cells in having the capacity to recognize only one very specific antigen by specific T cell receptors (TCRs).
• TCRs have certain similarities to antibodies--constant and variable regions, specific binding sites, millions of different specific varieties.
• An antibody can recognize an antigen (e.g. the surface protein of a bacterium) floating around in blood, in a pool of cell debris, in a test tube, etc. But a TCR can't do this--it can only recognize an antigen if the antigen is bound to a certain type of protein, MHC.

MHC proteins

• MHC protein = "major histocompatibility complex", binds to fragments of an antigen, carries to cell surface, "presents" antigen.
• The only way for an antigen to get to this position is by being partially degraded inside the cell, then carried to the surface and bound to an MHC protein.
• Over 100 diff. alleles of MHC in humans. Each person has only 2 (1 if homozygous). Tissue rejection occurs if MHC proteins on transplanted tissue are different from self (which usually is the case).
• Two major classes of MHC proteins:
  1. Class I MHC proteins (found on the surface of all nucleated cells) present antigens, identify most body cells as "self", detected by T cytotoxic or TC cells.
  2. Class II MHC proteins (found only on B lymphocytes, macrophages, and other cells that present antigens to T cells) don't stimulate antibody production, but are needed for T-cell communication with B-cells and macrophages.
• View MCH molecules (requires Chime plug-in)
• The importance of MHC proteins is that they allow T cells to distinguish self from non-self. In every (nucleated) cell in your body, antigens are constantly being chewed up and "presented" for inspection by passing T cells. Until this happens, other aspects of the immune response (such as antibody formation) cannot happen
• Note that the when class II MHC proteins present antigens, they are detected by a different group of T cells (called T-helper or TH cells) than class I MHC proteins (which are detected by T cytotoxic or TC cells).
• View animations of immune processes (from Malcolm Campbell)

Cell-mediated immunity

• T cells can participate in a variety of functions (see Fig. 20.18). Note that circulating lymphocytes are mostly T cells, of different types (B cells are typically found in lymph nodes).
• Some examples of T-cell functions:
  1. Cytotoxic T cells (TC cells) can recognize "foreign" antigens (e.g. from a cancer, or a virus infection) carried by MCH class I molecules. Once the T-lymphocyte recognizes an infected cell, it produces a set of new proteins that it places on the surface of the other cell. Those proteins then bind to receptors on the infected cell called "death domain receptors" (including the Fas ligand and Trail receptors). This binding triggers a cascade of events in the infected cell that leads to cell suicide, called apoptosis (pronounced A-pahtosis).
  2. T-helper cells (TH cells): required for B-cell activation. View diagram showing activation of TH cells.
  3. T-suppressor cells (TS cells): prevent induction of TH and B cells.
  4. Delayed-type hypersensitivity cells (TDTH cells); mediate delayed reactions. Won't discuss in this course.
• Another example of cellular immunity is the stimulation of phagocytic activity in macrophages by cytokines ( called lymphokines when released from T lymphocytes), specific signals released by one cell population to control activities of other cell populations.
• Cytokines include a variety of proteins:<
  1. Interleukins = proteins that signal between white cells in immune system.
  2. Migration inhibitory factor (MIF). Inhibits migration of macrophages away from site of infection.
  3. Colony-stimulating factors (CSFs). Regulate production of phagocytic cells.
  4. Interferons. Help in developing antiviral properties. Interferons also modulate immune response, cause increases or decreases in B- and T-cell functions.

Clonal selection

• Antibody synthesis occurs by clonal selection, and involves a number of steps:
  1. there must be a wide variety of T and B cells containing different specificities for a wide variety of antigens;
  2. when a particular antigen is present, it becomes exposed on the surface of some cells in the body (e.g., after ingestion and presentation by a B cell or macrophage) in a complex with class II MHC protein;
  3. a TH cell with corresponding specificity for this particular antigen on its TCR binds to the B cell, and is triggered to release interleukins that stimulate further division of the relevant B cell to form many copies of itself (a clone);
• Note: some antigens can stimulate B-cell activation without needing T-cell (= T-independent antigens). Ex: LPS.

Primary and Secondary Immune responses

• Primary response: requires ~ week after exposure before Ab level in blood rises. IgM appears first, followed by IgG.
• Secondary response: activates memory cells, Ab response detectable within a day, much higher Ab levels, higher IgG response.
  
  
  
  

Superantigens

• Superantigens are produced by certain strains of staphylococci and streptococci; a common example is toxic shock syndrome, brought about by a staph toxin.
• Superantigens bind to beta-chain of T cell receptor, at a site outside normal Ag-binding site. Can stimulate up to 10% of all normal T cells in population --- results in simultaneous and enormous amount of cell-mediated response = systemic inflammatory response.

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Vaccines: History

Jenner and cowpox

Pasteur and cholera

Types of Vaccines

Whole microbe vaccines

1. Killed vaccines : heat, formalin, etc. Example: Salk polio vaccine
2. Live vaccines : attenuated strains. Example: Sabin polio vaccine.
   Note: live vaccines generally preferable (one dose vs many), but may cause disease in immunocompromised, so killed vaccines still useful. Also, if vaccine is from a gram-negative bacterium, presence of LPS is toxic, so killed vaccine better. Example: whooping cough vaccine ( Bordetella pertussis ), the “P” in DPT, is a killed vaccine. Live vaccines also infect family members, so one immunized person can effectively immunize others.

Subunit vaccines

1. Since only surface proteins of a virus or bacterium actually stimulate antibody, why not use subunits instead of organisms? Good idea, but in practice may be too expensive; also require multiple doses to achieve protection. But they are in principle totally safe, since there is no risk of exposure to the pathogen’s reproductive potential.
2. Toxoid: Virulence of some pathogens is due especially to exotoxins, proteins capable of causing serious damage or death. Ex: tetanus toxin, diphtheria toxin. In these cases, subunit vaccines were developed that use inactivated toxin as the antigen, produce antibody called toxoid. Both the D and T in DPT vaccine are toxoids.
3. Polysaccharides: some pathogens are protected from immune system by their capsules. Best way to get antibody protection is to use polysaccharide as antigen, usually link to protein to get better stimulation of Ab production.

Vaccine Issues

1. Successes: Smallpox, polio are 2 most successful vaccines. Smallpox is eradicated, only survives in laboratory freezers. Polio only occurs in a few corners of the underdeveloped world. Influenza, Hepatitis, Mumps, Measles vaccines have all had major impact.
2. Adjuvants: some vaccines are less effective than others. E.g., subunit vaccines don’t replicate, don’t continue to stimulate immune system. Adjuvants are a second factor that prolongs stimulation of immune system. In U.S., alum (aluminum salt) is the only licensed adjuvant.
3. Problematic vaccines : Some vaccines don’t work well. Ex. 1: Strep pneumoniae vaccine is a polysaccharide subunit vaccine, not terribly effective. One problem – many antigenic variations in strains of the bacteria, different polysaccharides. Ex. 2: Tuberculosis vaccine : the only vaccine effective so far is BCG (bacillus of Calmetter and Guerin), made from attenuated strain of Mycobacterium bovis (not M. tuberculosis , the actual pathogen). BCG is pretty effective for infants, but only variably effective for children and adults, ranging from 0-80% effective depending on the study.
4. Passive immunization : can immunize people temporarily with pooled antibodies from other people, e.g. pooled “gamma globulin” (IgG fraction from serum banks). Since many people have some antibodies against variety of diseases, this can provide protection, but it wears out over several months and is gone entirely after 6 months or less.
5. Recombinant DNA vaccines : several vaccines currently under trial where coat or surface antigen genes for different pathogens have been genetically engineered into a different, nonpathogenic host. E.g. vaccinia virus has been engineered to express surface protein from HIV, influenza, and others. Promising idea, but has not worked perfectly yet.
6. DNA vaccines : can inject pure DNA, adsorbed to gold particles into muscle tissue. DNA is expressed, produces protein encoded in DNA for few weeks, enough to stimulate Ab response. Can use DNA segments that encode only surface antigens of a pathogen, not the whole organisms, so no risk of real infection.

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