because if a person containing antigen a then serum must contain anyibody b, serum could not contain antibody a or your own blood will agglutinate.
but if the inner linning of veins get rough due to extra deposition of cholestrol then blood clot why ? only due to rough surface ?
ns19 the clot u r saying is not a clot it is the blood clot(accumulating) not flowing freely due to the passage size
blood clot occurs due injury .if that clot is big enough it block the vain leads to pain in that region if i block in side the nerves circulation then paralyze some body parts
blood actually clots inside when we are hurt. the vessels get bursted out during injury and causes loss of rbc cells this is are restored by forming clot by platelets.


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Blood dosen't clot inside the body due to the presence of naturally occurring anti-coagulant heparin secreted by basophils and mast cells.If a clot is formed inside the body in the vessels it will cause serious health problems like stroke or thrombosis.It will stop the blood flow and will result in swelling and pain.If a clot is formed in the heart it will cause heart attack.