HOW MANY DIFFERENT TYPES OF IMMUNOGLOBULINS ARE KNOWN?

Four different isotypes of immunoglobulins have been described in pigs: IgM, IgG, IgA and IgE. The existence of porcine IgD has not yet been demonstrated.

 None of these immunoglobulins are able to pass through the placenta.

Immunoglobulin transference from the mother to the fetus only takes place through the colostrum. Piglets absorb immunoglobulins in their intestine and later on these immunoglobulins reach the serum.  

It is the first immunoglobulin to be produced after the immune response takes place. IgM is the  predominant isotype in the primary response.  

IgM is the second most abundant immunoglobulin in the serum, after IgG. It represents 10-12% of the total porcine immunoglobulin. Its serum concentration is between 1 and 5 mg/ml; in milk, 0.3 - 0.9 mg/ml and in colostrums 2.5 - 3.2 mg/ml.

No different subclasses of IgM have been described in the pig, although an allotypic variant has been observed in some animals.

Just as the other immunoglobulins, IgM can be found as a part of the BcR of B lymphocytes. If this is the case, it will be either as a monomer of 180 KDa  or  as an antibody, secreted in the body fluids as a polymer, consisting of five monomers of 180 KDa each and with a total molecular weigh of 900 KDa and a sedimentation coefficient of 17.8 S. 

 IgM diagram, with CH segments and J chain.

Different  monomers are linked to each other and form a pentamer. This occurs by means of a small polypeptide chain rich in cysteine and known as J chain. Each IgM monomer has the structure described above for all the immunoglobulins, with two heavy chains mu (m) and two light chains kappa (k) or lambda (l). IgM does not have a hinge region, although it does have a CH4 fragment in its heavy chains, which is the site of complement cascade activation.

The role of the IgM is essential as that of the first immunoglobulin to be produced in the humoral response. Even though its affinity to bind antigens is smaller than that of IgG, its pentameric structure allows it to bind in a multiple way to antigens and so activate the complement cascade. Just one pentameric molecule of IgM, once bound to the antigen, is able to initiate both complement cascade and phagocytosis. IgM is particularly effective against a huge number of gram negative bacteria and it can neutralize viral agents. Due to its big size it is mainly present in the blood serum.   

IgG immunoglobulin.

IgG is the main isotype in the swine species. It represents between 80 to 85% of total immunoglobulins in serum and colostrum. It is the most important antibody in the secondary response.  Its serum concentration is from 17-29 mg/ml, in milk, 1-3 mg/ml and from 30-70 mg/ml in colostrums. At least five subclasses have been described: IgG1, IgG2a,  IgG2b, IgG3 and IgG4. DNA studies have shown however, that there are 8 genes encoding the constant region  Cg.  

The role of this immunoglobulin in the humoral response is essential. 

The structure of IgG is the same both when it is part of the BcR  and when it is in an antibody form.  It consists, in both cases, of a monomer of 180 KDa with two identical heavy chains gamma (g) and two light chains kappa  (k) or lambda (l), which are also identical. It has a hinge region.  

Due to its small size, IgG can readily leave the circulation and go into the tissues, where it has an important role in the defense. It has a high affinity for binding antigens, and can opsonize them to allow phagocytosis (chapter 3), agglutinate or precipitate those antigens. IgG has a great  capability for viral neutralization. It is important for its anti-bacterial activity, and can activate the complement by the two possible ways, as well as participating in ADCC reactions.   

It is the pig's most important immunoglobulin in regard to mucosa immunity and lactation. Its serum concentration goes from 0.5 to 5 mg/ml, in milk, from 3 to 7 mg/ml and in colostrum from 9.5 to 10 mg/ml.

Two different subclasses of IgA have been described in the pig: IgA1 and IgA2 

IgA in its basic structure is a monomer of 150 KDa, even though it is usually secreted as a dimer (with a J chain).  It can be also found forming trimers and sometimes even bigger polymers.It has two identical heavy chains alpha (a) and two light chains kappa or lambda just as other immunoglobulins do.   

IgA activity is essentially related to the mucosa immunity, where it acts at different levels.: It does this by preventing the attachment of antigens in gut walls, and neutralizing the activity of some viruses, both inside and outside epithelium cells. 

Diagram of the different  levels where  IgA acts.

IgE represents less than 0,01% of the total of  circulating immunoglobulins in the swine species. This immunoglobulin has been determined by functional methods and can be induced by viruses, such as the African Swine Fever.

IgE is a monomer with a sedimentation coefficient of 8S. Just as IgM, its heavy chains have four constant domains (CH), with a molecular weight of 190 KDa.It is therefore, bigger than IgG. The Fc fraction of IgE has a fragment which is bound tightly by receptors on mast cells. Here mast cells are triggered to release powerful inflammation mediators contained in the cell granules. Histamine is the most abundant of these substances in the pig, especially in the gut and lungs. These mechanisms of defense are very important against parasitic infections.