View source for Antibody

== Features ==
=== Hypervariable sections ===
The antigen specifically binds to the '''variable regions of the heavy and light chains'''. These allow immediate contact. These are actually cleats of the chains at their '''N-termini'''. They are the cause of '''spatial complementarity''', reminiscent of the specific bond between the enzyme and the substrate.

=== Number of binding sites ===
Varies by immunoglobulin class:

* IgG has two (''bivalent''),
* secretory IgA four,
* IgM is theoretically ten, but the practical binding is only about half.

The antigen-antibody complex is not bound covalently, but only by '''non-chemical interactions'''. The formation does not correspond to stoichiometric conditions. If a '''precipitate''' is formed, its maximum amount corresponds to the equivalence zone - however, if there is an excess of any of the components, the amount of precipitate already decreases, because the '''optimal spatial network''' is not arranged (see figure). However, in some other types of interactions, immunocomplexes may be soluble.

=== Antigen-antibody complex size ===
It is very different. Some are so '''large''' that we can observe them with an [[electron microscope]]. '''Small complexes''' have a short half-life, do not bind [[Complement system|complement]] and are not pathogenic. '''Medium-sized complexes''' are deposited in tissues and can thus be a characteristic feature of various diseases.

=== Antigen-antibody interactions ===
It can also cause:

* '''clustering''',
* '''lysis''',
* '''immobilization of cells''' ([[red blood cells]]),
* '''complement fixation.'''

In some cases, binding of the appropriate immunoglobulin may partially or even completely '''neutralize the bacterial toxin'''.