Immunoglobulins (Ig) (Antibody)
Antibodies (a.k.a. Immunoglobulin, (Ig)) contribute to the fight against pathogens via several mechanisms:
Ig molecules can bind to the surface of bacteria and viruses and prevent their attachment to and invasion of host cells.
Ig molecules (Specially IgG) can bind and neutralize toxins.
Opsonization: When bound on the surface of extracellular pathogens, the Ig Fc region can bind to receptors on macrophages and aid phagocytosis.
When Ig binds bound to pathogens, the Fc
portion can activate the complement system, which forms lytic pores in the surface of the
microorganism. This is referred to as the classical pathway
of complement activation. In addition, complement breakdown products also serve to
opsonize the pathogen, attract more macrophages, and form pores on the surface of the
microorganisms.
Properties of Human Immunoglobulins
| Property | IgM | IgG | IgA | IgE | IgD |
| % of Serum Ig | 10 | 75 | 15 | <0.01 | <0.5 |
| Structure | Pentamer | Monomer | Dimer | Monomer | Monomer |
| Complement Fixation | +++ | + | - | - | - |
| Transplacental Passage | - | + | - | - | - |
| Allergic Response | - | - | - | + | - |
| Mucosal Secretion | - | - | + | - | - |
| Opsonization | +* | +++ | - | - | - |
* Opsonization is via Complement
+
Major Functions of Human Immunoglobulins
| Immunoglobulin | Major Function |
| IgM | Main Ig during Primary Response (Early antibody). Fixes Complement (most effectively). |
| IgG | Main Ig during Secondary Response (late antibody). Opsonization. Fixes Complement. Neutralizes Toxins, Viruses. |
| IgA | Secretory mucosal Ig Prevents invasion from gut mucosa. |
| IgE | Immediate Hypersensitivity. Mast cell and Basophil reactions. Activates Eosinophils in helminth infection. |
| IgD | Function Unknown. Mostly on the Surface of B cells. |
IgM: You can observe that it contains TEN antigen binding sites.
Consequently, it does not necessarily need to have high affinity for the antigen
(analogous to ten weak people holding a rope, collectively they are strong, but
individually they do not need to be). This is true for the case of IgM, since this is the
Ig secreted early in the humoral immune response. Thus B cells have not gone through
affinity maturation to the same extent that they will when they switch isotype and begin
to secrete IgG.
A monomeric form of IgM constitutes the specific Ig receptor on virgin B cells.
IgG: This Ig is secreted later in the Immune response, where B cells have gone through affinity maturation. IgG is also secreted by memory B cells during secondary immune responses. IgG has two antigen binding sites, and they both have very high affinity for the same antigen. Note that each antigen binding site is formed from a light chain variable domain (Lv) and a Heavy chain variable domain (Hv). IgE and IgD also have a similar monomeric structure as IgG.
IgA: This Ig is specifically secreted by B cells that are in the submucosal regions of the intestine and respiratory mucosa. These B cells are activated in the regional lymph nodes (e.g. Peyers patches of the gut), but they migrate to and settle in submucosal regions, and secrete IgA (mechanism unknown). The J chain that holds the two subunits of IgA together is synthesized by the B-cell but a secretory mucosa is added by the mucosal epithelial cells during transport to the gut lumen.