IgA : Structure, Subtypes, Properties and Function

   Immunoglobulin A (IgA) or Antibody A is found in blood. But it is predominant in the secretions of the body. For example in the mucus, mother's milk, tears and saliva.

Structure of IgA

Structurally IgA consists of two light chains and two alpha(α) heavy chains.
Each heavy chain contains one variable domain and three constant domains.

Dimer structure of IgA

There are two subclasses of IgA designated as like IgA1 and IgA2.   
- IgA one is mostly found in the serum.
- IgA 2 is predominant in the secretions.

• These two subclasses differ from each other in the hinge region. The hinge region of IgA 1 is almost double of IgA 2.

• Each subclass of IgA can exist in two main forms in the body these are
- Secreted IgA (sIgA) and
- Secretory IgA (SIgA).

Secreted immunoglobulin A (sIgA) :

• It is written as (sIgA), here note that alphabet s is in lowercase.

• It is also known as Serum IgA, it circulates in blood.

• It can be a Monomer, Dimer a rarely Trimer.

• Plolymeric form of IgA is formed when two IgA monomers are held together by a glycoprotein known as J chain. J stands for joining.

• It is also synthesized by plasma cells.

• This J chain binds to the constant region of the IgA monomers through disulfide bonds.

• Secrete immunoglobulin A are  synthesized and released in the circulation by activated B-cells or Plasma cells.

Secretory immunoglobulin A (SIgA) :

• Secretory IgA is written as (SIgA), here S is written in uppercase.

• These secretory IgA are predominant in Secretion at the mucosal surfaces.

• It is also a dimer molecule.

But while entering the external secretions of the body it undergoes some modification.

- The B-cells which are located beneath the mucosal surfaces differentiate into IgA producing plasma cells.

- These plasma cells synthesize and release polymeric IgA antibodies. The J chain is also synthesized inside the plasma cell. So these are secreted IgA antibodies.

- These secreted IgA bind to a receptor expressed by epithelial cells of the mucosal surfaces. This receptor is called poly-Ig receptor (pIgR).

- This receptor(pIgR) recognizes the J chain of secreted IgA and bind to their C-terminal domains.

- Once it bound IgA is internalised into a transport vesicle which moves to the opposite side of the epithelial cell.

- During the process the poly-Ig receptor is cleaved enzymatically.

- The peptide remaining bound to the IgA dimer is called secretory component or piece.

- This IgA dimer with attached secretory component exits the cell. Now the antibody has become secretory IgA.

- This secretory piece thus helps the transportation of the IgA into secretions. It also protects IgA from breakdown by proteolytic enzymes found in the secretions.

Properties of immunoglobulin A (IgA) :

• Immunoglobulin A constitutes about 5 to 15% of the total antibody concentration in the human serum.

• It is the major immunoglobulin in the external secretions of our body, such as saliva, mucus, gastric fluid, tears etc.

• IgA guards or protects the mucosal surfaces of the body.

• Half-life of IgA is about 6 days.

• Generally Immunoglobulin G is the most abundant antibody in the serum, but most abundant antibody in the human body is IgA. If all the production of IgA at mucosal surfaces is taken into account then it is the most abundant immunoglobulin in the human body. In terms of quantity.

• This is obvious since humans have about 400 square meters of mucosal surface, which include respiratory, digestive and reproductive tracts. So the quantity of IgA produced per day in an adult human is more than all other antibodies combined.

The functions of IgA

• IgA is the major antibody found in Collis from of milk and nursing mothers. 

• IgA provides passive immunity to the infant.

• It protects the infant from gastrointestinal infections during the first few weeks after birth.

• It Neutralizes toxins and pathogens. They prevent pathogens adherence to mucosal surfaces and thus blocks them from entering the bloodstream.

• Immunoglobulin A triggers  degranulation of granulocytes such as neutrophils.

• They also play important role in antibody dependent cell-mediated cytotoxicity (ADCC) since they bind to the specific FC receptors on the killer cells.

• They are poor complement fixers and opsonins.