INTRODUCTION OF ANTIGENS
Antigens are any substance which when introduced into the body parenterally can induce the formation of an antibody with which reaction occurs specifically and in observable manner. It can also be defined as any substances that gets identified or recognized by B cell receptors (Ig) and T-cell receptors (along with the major histocompatibility complex).
Antigens are of various types. They are classified as T-cell dependent and T-cell independent antigens on the basis of their ability to induce antibody formation.
WHAT DO YOU MEAN BY T-CELL DEPENDENT ANTIGENS ?
Most proteins that occur naturally are T-dependent antigens. These antigens cannot be responded by B-cells directly. For it, a co-stimulatory signal is required that are produced by TH cells. These antigens structurally are characteristics by having few copies of many different antigenic determinants. These antigens bind to the surface Ig on B cells. After that, they are internalized and further processed to smaller peptides.
Now, they are expressed on the surface of B-cells complexed with MHCII and presented to the T-cells. Secretion of cytokines occurs after these complexes get recognized by TH cells. Cytokines secretion expresses CD40 ligand which interacts with the CD40 on the B-cells. The T-B interaction and the cytokines stimulate the B-cell activation togetherly.
WHAT DO YOU MEAN BY T-CELL INDEPENDENT ANTIGENS ?
Some antigens directly stimulate the production of antibody by B cells. Participation of T cells may not be required in some cases. Such antigens are called TI antigens. These antigens react with BCRs of innate immunity.
T-cell independent antigens can be most of microbial sugars, lipids and certain nucleic acid. These are of two types. One of the type is type I antigen that are generally endotoxin and lipopolysaccharide. These antigens are directly mitogenic for B cells and cause polyclonal B cell activation. Likewise, another type is type 2 antigens. They are generally polymeric compounds like polysaccharides that may be of bacterial cell wall lipopolysaccharide or pneumococcal capsular polysaccharide or flagellar proteins. They activate B-cells with the help of cytokines and complement other cells like macrophages, dendritic cells, mast cells and NK cells so that specific antibodies are generated.
Some of the differences between these antigens are:
|T-cell Dependent antigens
||T-cell Independent antigens
|Involvement of T-cell.
||No involvement of T-cell.
|Involves tertiary complex of T cell receptor, Ag, MHC molecule.
||Involves binary complex of membrane Ig and Ag.
|Antigen processing by macrophages occurs.
||Antigen processing does not occur.
|These antigens are mostly soluble proteins.
||These antigens are lipopolysaccharides, polymeric proteins and polysaccharide.
|These antigens are easily degradable.
||Type II antigens are poorly degradable.
|No complement activation.
||Type II Ag causes complement activation.
|There is isotype switching.
||No isotype switching in type I and limited switching in type II Ag.
|Presence of immunologic memory.
||Absence of immunologic memory.
|No polyclonal activation.
||In type I it occurs, Ag in high doses.
|Effect antibody production fully.
||Limited antibody production i.e., IgM and IgG3.
Image source: microbeonline
Differences between T-cell dependent and T-cell independent antigens