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    Hey.

    If you have a sample of erythrocytes and you add anti-red blood cell serum containing antibodies to this sample (causing cross-linkage,) what effect would adding complement to this sample have?

    And what effect would it have (if any) if you added it to a sample of only erythrocytes and no antibody?

    It's my understanding that the complement system assists antibodies to remove pathogens, but I'm not sure in what way it'd have an effect on the experiment above?
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    In brief:
    Complement activation by antibodies triggers the classical pathway. Complement will bind to antibody-coated erythrocytes, activating complement cascade in the serum, leading to the formation of the membrane attack complex (MAC). This will eventually result in the formation of a pore in the membrane of the erythrocytes, leading to cell lysis (haemolysis). This can be observed in the experiment, as the erythrocytes will release red pigment into the surrounding solution turning it a pink/reddish colour.

    complement-antibody complex->classical pathway-> MAC-> cell lysis

    In detail:
    The classical pathway is initiated by the binding of antibody to antigen (non-self) of erythrocytes. The antigen/antibody complexes (called immune-complex) activate/attract complement. Upon complement binding to aggregated antibodies on erythrocytes, the complement cascade will be activated, resulting in the assembly of MACs on erythrocytes, which perforates the membrane causing the haemoglobin to leak. The amount of complement activity in a serum sample is determined by adding diluted serum samples to antibody-coated erythrocytes and measuring the cell lysis percentage. The values are expressed as CH50 in U/ml (dilution of serum required to lyse 50% of the erythrocytes in the assay).

    IgM and IgG are the only antibodies capable of binding complement and activating the classical pathway (thus called complement-fixing antibodies). The binding site of the antibody is called the Fab region, which is constructed from the hypervariable regions of the heavy and light chains. The Fab region can bind to many antigens comprising proteins, carbohydrates, nucleic acids or lipids. The Fc portion of antibodies can bind to Fc receptors (FcRs) on cells and C1q, the first component of classical complement.

    To note:
    The erythrocytes and antibodies will usually come from a different species', so that the antigen of erythrocytes will be recognized as foreign by the antibodies.

    This is a good website: http://www.microbiologybook.org/mobile/m.immuno-7.htm


    Clinical Scenario:
    The complement fixation test can be conducted to test for the presence of a specific antigen or antibody.

    The serum of the patient is obtained. As individuals will have differing amounts of complement in their serum, the complement proteins in the serum must be destroyed and replaced by a standardized amount of complement. The serum is heated so that complement proteins are destroyed but not antibodies, which is possible as complement is more heat-sensitive than antibodies. Known amount of standardized complement proteins is added. The specific antigen (of interest) is added. Antibody-coated erythrocytes from different species (e.g. sheep) is added to serum.

    The test is negative if the solution turn pink at/after this point, as the presence of antibodies specific for the given antigen would result in the formation of immune complexes, which would mean that complement will be depleted-so by the time of addition of antibody-coated erythrocytes there will be no complement proteins left in the serum. However, if no antibodies against the antigen of interest are present, the complement will not be depleted and it will react with the antigen-antibody (immune) complexes, lysing the erythrocytes and spilling their contents into the solution, thereby turning the solution pink.

    https://en.wikipedia.org/wiki/Complement_fixation_test
 
 
 
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