Immune Response: Antigens & Antibodies
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Last revised: Thursday, January 6, 2000
Ch. 30 in Prescott et al, Microbiology, 4th Ed.Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!Copyright 2000. Thomas M. Terry
B. Specific (Acquired) Defenses
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
- IgG; bivalent (2 binding sites), most common antibody, 70-75% of antibodies. Found in blood and lymph. Only antibody that can cross placenta.
- 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)
- 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.
- IgE; bivalent; accounts for only 0.000005% of total antibodies. Responsible for allergic, hypersensitive reactions.
- 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).
Complement and Opsonization
- Complement = complex of 17 proteins present in normal serum. Heat labile, destroyed @ 56 deg. C. Named C1-C9, also Factor B, D, H, I etc. Complement is used up (fixed) in Antibody-antigen reactions, as a result of series of reactions called complement cascade.
- Classical cascade reaction: When Ab-Ag complex forms, base of Ab (constant region) changes shape. This activates Complement C1, which acquires esterase activity. Activated C1 not activates C2 and C4; generates a new activity which activates C3, etc. (Don't need to memorize steps in this course). After several more steps, activate C8 and C9 ---> membrane attack complex, creates pores in membrane of target cell ---> lysis.
- Opsonization = enhanced phagocytic activity. Can be stimulated by Ab-Ag reactions. Also stimulated by complement bound to Ab-Ag targets. Especially important in binding to capsules, triggering effective phagocytic uptake of capsulated bacteria.
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