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Flashcards in Antibody Function Deck (20)
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What are other terms for antibody mediated and T-cell mediated immunity?

Antibody-mediated immunity is also called humoral immunity. T-cell-mediated immunity is also called cellular immunity.


What is an immune complex?

An immune complex is the result of cross linking of antibodies to epitopes on multiple antigens, thus binding them together into an antigen/antibody complex.


What causes agglutination and precipitation, and what is the difference between the two?

Agglutination/precipitation occurs when the quantities of antibody and antigen are roughly equivalent. It is the assembly of large antigen/antibody complexes formed by cross linking antibodies. If the antigen is a molecule, it is termed precipitation, and if the antigen is a cell or cell-sized it is termed agglutination (which is more readily seen, thus agglutination tests are more sensitive).


What is a quantative precipitin test, and how does it work?

A quantative precipitin test measures a constant quantity of antibody against increasing amounts of antigen. Combinations that have large excesses of either antigen or antibody do not produce much precipitate. However, combinations of equivalent amounts produce large quantities of precipitate, nearly 100% in 50/50 mixes.


What is immunodiffusion?

Immunodiffusion is a technique involving antibodies and antigens interacting in a gel plate. Typically, two wells are cut in the gel, one filled with antigen and the other with antibody. As they diffuse towards each other, a line of precipitate appears in the region where their concentrations are equivalent.


How can immunodiffusion be used to quantify Ig production?

A gel prepared with antibody to the Ig in question (typically IgA) has three wells cut into it. One well is filled with highly concentrated Ig, the next with less concentrated Ig, and the third with the patient's serum. The Ig will diffuse and form rings around the well in diameters proportional to their concentrations, thus allowing the quantification of Ig in the patient's serum.


What are the advantages and disadvantages of IgM?

IgM is the first antibody to appear in serum after immunization, is made by fetuses, and some may be secreted.   It is up to 500 times more effective at activating complement than IgG because complement requires two adjacent Fcs and IgM always has five.   However, IgM is so large that if produced in the quantities of IgG (1000 mg/dl) it would make the blood too viscous to flow.   IgM also cannot activate phagocytes, which lack IgM receptors.


What are the advantages and disadvantages of IgG?

IgG is the most abundant type of Ig in the blood, and is the only Ig to pass through the placenta (needs active trans).   IgG lasts 3 weeks, and can activate phagocytic cells (opsonizing - vital for clearance of bacteria) or activate compliment (if the epitopes and IgG are dense enough).


How is IgA made and how and where does it funciton?

IgA is preferentially made by plasma cells in lymphoid tissue near mucous membranes.   Two IgAs are assembled into a dimer with a J chain while still in the plasma cell and then secreted.   They bind to receptors in the epithelial cells and are transported through the cells and secreted onto the mucous membrane.   The receptors remain bound and and are called Secretory Component after the complex is secreted.   Some monomer and dimer IgA is also found in the plasma, where it can activate compliment via the alternate pathway.


Where is IgD found?

Some IgD is found in the plasma, but the important role for IgD is as a B cell receptor.


What is the primary function of IgE and what other issues does it cause?

IgE has Fc ends that bind receptors on mast cells and basophils.   It functions primarily in the defense against parasites by triggering the release of histamines and eosinophil chemotactic factor.   Eosinophils are uniquely effective at killing parasites.   These actions also allow IgE to cause Type 1 immunosensitivity, aka allergies.


What is the classical pathway of compliment?

The classical pathway is activated by IgM or IgG.   C1q binds to bound Ig and initiates a cascade of enzyme activations.   The order of the classical pathway compliment enzymes is 1-4-2-3-5-6-7-8-9.


What is the alternative pathway of compliment activation?

The alternate pathway is activated by bacteria even in the absence of antibody.   C3 naturally breaks down into C3a and C3b and is usually degraded.   However, structures in the cell walls of bacteria may stabilize C3b in a trimer with Db (forming C3bDb3b), which may then activate C5.   Thus, the alternative pathway procedes via 3-5-6-7-8-9.


What is the lectin pathway of compliment activation?

The lectin pathway is mediated via Mannose Binding Protein/Lectin (MBP or MBL).   Lectins are proteins that bind carbohydrates.   MBP is functionally similar to C1q (thanks to MBP Associating Serine Proteases, MASPs, which activate C2 & C4), and the lectin pathway goes MBP-4-2-3-5-6-7-8-9


What is C1-inh?

C1-inh is an inhibitor of C1.   C1 is constantly being activated and much of this needs to be inhibited.   Persons lacking C1-inh are at risk for angioedema, giant hives (urticaria) that can swell up their face or swell their larynx shut.


What is the MAC?

The MAC is the Membrane Attack (Lytic) Complex.   C5 is most strongly activated by the classical pathway, and in turn activates C6, 7, 8, &9.   C8 and C9 form a lesion on the cell membrane that results in a hole, ending the cell's ability to regulate its osmotic pressure.   This lyses the cell.   The MAC is most effective against gonorrhea and menigitis. 


What are the four actions of compliment?

It is lytic, opsonizing, chemtactic, and anaphylatoxic.


What make compliment opsonizing?

The C3b product opsonizes bacteria by sticking to their walls.   This makes the bacteria susceptible to phagocytes.   It is also the first step of the alternative pathway.


What makes compliment chemotactic?

C5a strongly attracts neutrophils, along with other phagocytes.   This causes much of the inflammation in antibody-mediated reactions, and explains why PMNs are the hallmarks of such reactions.


What makes compliment anaphylatoxic?

C3a, C4a, and C5a are all capable of releasing histamine from mast cells of basophils non-specifically.  This increases blood flow and inflammatory cell concentrations in the area.   This may cause the person to break out in hives, causing the immune response to appear to be an allergic reaction.