Antibodies (Ac) or immunoglobulins (Ig) are glycoproteins synthesized by B lymphocytes stimulated by certain antigens and are able of combining with their specific antigens (Ag). The result of this specific reaction is the activation of  immune functions (complement activation, cytotoxicity, etc.), which ultimately aim is to remove the foreign Ag. The Ac are synthesized and secreted by lymphocytes B after antigenic stimulation by Ag meeting.  Although the synthesis of a certain type of Ac usually involves a previously contact with activating Ag, this cannot be demonstrated in all cases (e.g. natural antibodies that can be detected even from the early days of life without antigenic stimulation, such as Ac for ABO blood groups and explaining why we need transfusions only from a compatible donor).

The Ig base molecule consists of four chains-two heavy chains (H-heavy) and two light chains (L-light) bounded by disulfide bridges; depending on the structure of the heavy chains (H) there are 5 classes of Ig:

1. Immunoglobulin G (IgG)
2. Immunoglobulin M (IgM)
3. Immunoglobulin A (IgA)
4. Immunoglobulin D (IgD)
5. Immunoglobulin E (IgE).

1. Immunoglobulin G (IgG) represents 70-80% of seric immunoglobulins in humans, and the L and H chains are joined by a single disulfide bond. IgG antibodies occur especially during the secondary immune response. They may exert an inhibitory action on the synthesis of IgM antibodies developed during the primary response through a competition mechanism for antigen. An essential function of IgG is that of eliminating antigens, IgG actively participating in the antigens destruction by phagocytic cells, by activating complement, activating macrophages and granulocytes.

2. Immunoglobulin M (IgM) – is the most widespread immunoglobulin in acute infections. It is characterized by a high molecular weight and predominantly occurs during the primary response, the rate of synthesis being controlled by IgG level. IgM does not cross the placenta (due to large molecular weight); therefore, if in the newborn is detected IgM then he presents an acute infection-antibodies cannot come from the mother, unlike IgG-which having the small molecule can cross the placenta and then does not necessarily signify the infection of the fetus. IgM are characteristic for the primary immune response, their place being taken during the secondary response by IgG.

3. Immunoglobulin A (IgA) – is used both in human serum but unlike the rest of the Ig that are normally present only in the serum, IgA is present in various secretions, so two forms of IgA are distinguished:

Serum IgA – presents the classical structure of any Ig; Serum IgA does not cross the placenta and, unlike other Ig classes, does not activate complement.

Secretive IgA – are present abundantly in digestive secretions, saliva, lacrimal secrets, asthma mucus as well as in colostrum and breast milk. The IgA molecule has increased resistance to the action of existing enzymes in most secretions, especially digestive ones. The IgA in colostrum and breast milk are circulated in the digestive tract of the child and plays an important role in antibacterial defence from the mucous membranes, therefore it is recommended to breast-feed the infant in the first months of life.

4. Immunoglobulin D (IgD) – shows a significant increase in pregnant serum only during labor, its ante-and postpartum value being normal.

5. Immunoglobulin E (IgE) – corresponds to antibodies called “Reagin” and has high values in allergic patients (patients with hay disease, asthma asthma, atopic eczema, etc.) and in some parasitic diseases (caused by Ascaris, Toxocara, Echinococcus etc).