Fab (fragment antigen binding) and Fc fragment (constant fragment or crystalline fragment)
If an antibody molecule is treated with a proteolytic enzyme such as pepsin, the hinge region peptide bonds are broken producing 2 identical F ab fragments and one degraded Fc fragment.
|Proteolytic Cleavage by Pepsin|
If an antibody molecule is treated with a proteolytic enzyme such as papain, the hinge region peptide bonds are broken producing 2 identical F ab fragments and one Fc fragment.
|Proteolytic cleavage by Papain|
Fab fragments has the antigen binding site where as Fc fragment is involved in complement fixation, attachment site for various immune cells and for placental transfer.
How antibodies deal with antigens?
Antibodies deal with antigens in different ways:
- Neutralization: Mainly antigens like toxins of bacteria, virus, snake venom etc are neutralized by antigen and make them ineffective. Such types of antibodies are often called as antitoxins. Phagocytes will clear out the neutralised antigen-antibody complexes.
|Opsonisation- enhancing phagocytosis by macrophages|
- Opsonisation: Here we should know 2 terms clearly. First one is opsonin which refers to any substance (eg: an antibody or c3b) that promotes phagocytosis of antigens by binding to them. The process of enhancing phagocytosis is called as opsonisation. In opsonisation by antibodies, antibodies will cover or form a coat over the entire antigen surface (microbe), thereby promoting a stable adhesive contact with an appropriate phagocytic cell and thus making them more susceptible to phagocytosis. Antibodies involved in promoting phagocytosis are called as opsonins.
- Agglutination: Here antigens especially bacteria are made into sticky masses or clumps by the activity of antibody. Thus the antigens are immobilized and phagocytes will dispose of such clumped antigens easily. Antibodies involved in causing agglutination of antigens are often termed as agglutinins
- Precipitation: Here as the term indicates antibodies binds to antigen and form precipitates that are easily ingested by the phagocytes.
- Complement activation: In the previous post, we have discussed in detail the process of complement activation. Antigen-antibody complexes activate complement proteins which may clear the pathogen either by lysis, opsonisation, inflammatory response or by inviting phagocytes to the site of infection.