Antibody
From Ganfyd
We now know the principle upon which protection is secured: an active acquired immunity follows upon an attack of a disease of a similar nature. Smallpox and cowpox are closely allied and the substances formed in the blood by the one are resistant to the virus of the other. 
- Wm. Osler 1913
Antibodies are the most specific and obvious of the humoral factors in acquired immunity.
Various sorts of immunoglobulin with different stalks and different numbers of specific attachment sites per operating unit.
- Immunoglobulin G (IgG)
- Immunoglobulin G (IgM)
- Immunoglobulin G (IgA)
- Immunoglobulin G (IgE)
- Immunoglobulin G (IgD)
Contents |
Immunoglobulin G (IgG)
IgG is the so-called "mature" immunoglobulin, produced by higher animals. Medically it is the antibody that is produced some time after first exposure to a new antigen. Four subclasses mainly related to differences in the 'hinge region' are defined serologically and have differing physical properties to the dominant IgG1 subclass:
- IgG1
- Ig gamma-1 chain C region
- Freely flexible hinge
- Disulfide linkage of the carboxyl terminal of the light chain to cysteine at position 220 of the CH1 sequence of the heavy chain compared to position 131 in the other subclasses
- IgG2
- Ig gamma-2 chain C region
- Shortened more rigid hinge region
- Resistant to cleavage by proteolytic enzymes
- IgG3
- Ig gamma-3 chain C region
- Lengthened more rigid hinge region with highest molecular weight
- Molecule overall has greater flexibility as Fab fragments are further away from the Fc fragment
- Susceptible to cleavage by proteolytic enzymes
- IgG4
- Ig gamma-4 chain C region
- Shortened intermediate flexibility hinge region
IgG1
IgG1 deficiencies often result in hypogammaglobulinemia and usually present with chronic and recurrent infections of the lower airways.
IgG2
IgG2 deficiency is associated with impaired response to infections with encapsulated bacteria or immunisation with polysaccharide antigens. Classically recurrent respiratory tract due to Streptococcus pneumoniae and/or Haemophilus influenzae
IgG3
IgG3 deficiency is usually associated with with chronic and recurrent infections of the lower airways and often has IgG1 deficiency at same time
IgG4
IgG4 deficiency has been classically difficult to assess as levels are naturally low in healthy children. However IgG4-related disease is important and often associated with autoimmune pancreatitis.
Immunoglobulin M (IgM)
IgM is the so-called "primitive" immunoglobulin, produced by higher animals and lower creatures. It is usually the first antibody to be produced by the immune system after exposure to a new antigen.
The two facts above are useful to doctors. People who have recently been exposed to measles antigen, for example - whether as the disease, or the vaccine - will first develop measles-specific IgM (from about 7 days after exposure), and then, several weeks after exposure, will develop measles-specific IgG. This is used in the saliva test, in which a person who has had an illness thought, clinically, to be possibly measles. A sample of saliva is tested for the presence of antibodies. as part of the process of infectious disease surveillance and vaccine programme monitoring.
Interpretation of saliva test for measles
| IgG present | IgM present | Interpretation |
|---|---|---|
| Yes | No | Prior immunity: recent disease very unlikely to be measles |
| No | Yes | Recent exposure. In the absence of history of recent vaccination, recent disease very likely to be measles |
| No | No | No immunity. No evidence of recent or prior immunity to measles. Vaccination recommended |
Immunoglobulin A (IgA)
IgA is associated with mucosal immunity. In polio disease, or during vaccination with live oral polio vaccine (OPV), the virus is present in the gut, stimulating IgA production and mucosal immunity. In contrast, injection with inactivated polio vaccine (IPV) bypasses the gut, stimulating IgG production. It is thought that, while IPV protects the recipient from disease caused by the virus, it might not prevent the virus from surviving and replicating in the gut, where it is relatively protected from IgG. If this is the case, it means that people who have been vaccinated with IPV may become infected with wild polio virus, not become ill, but excrete the virus, and therefore potentially pass the disease on to others. For this reason OPV is preferred as a public health intervention in the event of a polio outbreak.
Immunoglobulin E (IgE)
IgE has a role in fighting infection with parasites such as hookworm.
It also plays a part in Anaphylactic conditions, such as asthma.
