Immune complexes (Antigen-Antibody complexes)

Immune complexes in vitro

Immunogens have more than one antigenic determinant per molecule (are multideterminant).
Immunization with antigen, therefore, results in many antibody populations, each directed toward different determinants on the protein.
Since one molecule of Ab (IgG) can react with two molecules of Ag, and one molecule of Ag can react with many molecules of Ab, a lattice or framework consisting of alternating molecules of Ag and Ab is produced which precipitates.
The extent to which a lattice form depends on the relative amounts of Ag and AB present.
As the amount of Ag added increases, the amount of precipitate and Ab in the precipitate increases, until a maximum is reached, and then decreases with the further addition of Ag.
When there is both sufficient Ag and sufficient Ab, the combination of Ag and Ab proceeds until large aggregates are formed, which are insoluble and precipitate (equivalence).
However, in Ab excess or in Ag excess, less lattice formation occurs and more soluble complexes are formed.

Immune complexes in vivo

These reactions occur in vivo during an immune response.
Initially, there is Ag excess as no Ab to the Ag is present at the time of the first contact with the Ag.
Within days, however, plasma cells develop, producing Ab to the Ag which complex with it (Ag excess).
As more Ab is produced, equivalence is reached resulting in large Ag-Ab complexes which are removed by phagocytic cells through interaction with their Fc and complement receptors.
Plasma cells continue to produce Ab during their short life, increasing the Ab concentration in the serum (Ab excess).
However, as Ag has been removed, no further restimulation of B cells occurs and no more plasma cells develop.
Thus, the Ab concentration in the serum begins to decrease as a result of normal catabolism.

Immune complexes and tissue damage:

If the Ag persists (e.g. with some infectious organisms such as Streptococcus or is self Ag, immune complexes are continually formed and may not readily be removed due to an ‘overwhelmed’ phagocytic system.
This can lead to the deposition of immune complexes in tissues resulting in damaging reactions Ill hypersensitivity.
The complexes activate complement and induce an acute inflammatory response.
Direct interaction of the immune complexes with Fc and complement receptors on the neutrophils causes the release of proteolytic enzymes which damage surrounding tissues.

Immune complexes (Antigen-Antibody complexes) – Bioscience Notes