Antibody-antigen reactions

Antibody-antigen reactions:
A reaction that take place between antigens and antibodies are used as defence mechanisms:

i) Complement fixation and activation
• used primarily against bacteria and mismatched red blood cells (after transfusion)
• after antibodies binding to target/receptor  change shape  expose complement binding sites  complement fixation  cell lysis
• also promote phagocytosis due to opsonisation

ii) Neutralisation
• antibodies block specific sites on viruses or bacterial toxins  prevent binding to receptor sites  prevents tissue damage  antibody-antigen complex is eventually phagocytosed

iii) Agglutination
• antigen-antibody complexes can cross link  form larger ‘clumps’ (agglutination)
• IgM has 10 antigen binding sites  potent agglutinating agent
• occurs when mismatched blood cells are transfused and forms the basis of the test for blood typing

iv) Precipitation
• soluble molecules, after binding with antigen  settle out of solution  more easily phagocytosed.