The antibody must "bump" into the red cell at the corresponding antigen site. The forces that act to hold the antibody and antigen together are partly physical and partly chemical. Physical forces include complementary shape, i.e., a "key in lock" fitting together. The chemical forces are not well understood. They are believed to be relatively weak forces that act to hold the antigen and antibody together chemically. To do so, the antigen and antibody must be very close together. We will now look at factors that affect reactions. Some of them can be manipulated so that the reactions will be enhanced and we will detect them.
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| First Stage (Sensitization) |