Any substance which evokes an immunological reaction is an antigen.
Certain components of the cell membranes act as specific antigens.
They differ from person to person in their chemical composition and there dimensional structure.
The immunocompetent cells can recognize the self from the non-self.
Moreover, the body can recognize one antigen from another leading to specificity of immunological reaction.
Some substances are strongly antigenic, e.
g, dinotrochlorobenzene, while others are weakly antigenic, e.
g, bacterial capsular antigen.
Antibody response is usually selective against certain important regions of the antigen which are termed the antigenic determinant sites.
A hapten is a small molecule which by itself cannot elicit antibody production, but when combined with a carrier molecule, can do so.
The antibodies can combine with the hapten directly, without the help of the carrier.
The presence of haptens in tissue proteins is one of the causes producing auto-immunity.
The main cells involved in immune reactions are the lymphocytes.
They originate from the totipotent cells of the bone marrow.
Some of them pass through the thymus and are then specialized for cells mediated immune reactions and are recognizable by thymus specific antigens on their surface.
These T-lymphocytes are found mainly in the para-cortical areas of lymph nodes and peri-arteriolar sheaths in the spleen.
Upon antigenic stimulation, they are differentiated into plasma cells which synthesize the immunoglobulins and form mediators of humoral immunity.
In peripheral blood 80% of Lymphocytes are T-cells, 15% are B cells and 5% are K cells.
T lymphocytes form spontaneous rosettes with sheep erythrocytes (E rosette) at 4 degrees Celsius.
B lymphocytes are identified either by the rosetter formation with sheep erythrocytes coated with antibodies and complement (EAC rosette) or by demonstration of surface immunoglobulins by fluoresein antibody technique.
Once the antigens enter the body, they are processed by macrophages and then recognized by the antigen sensitive lymphocytes.
These undergo morphological alterations (blastic transofrmation) and after a series of divisions form a clone of cells which react against the specific antigen.
These committed cells are responsible for immunological memory.
Many infections and vaccinations lead to prolonged immunity due to persistence of immunological memory.
Most animals start producing specific antibodies within 10 days of injection of antigen.
These antibodies are mainly of IgM variety.
This is the primary response.
When the same antigen is introduced for the second time, the reaction occurs at a faster and augmented rate and lasts longer.
This time IgG antibodies predominate.
sometimes even large quantities of antigen may not produce immune response.
This is called Tolerance, which again will be specific to that particular antigen.
Tolerance usually fades away with the passage of time.
However, tolerance induced in utero tends to be permanent.