The thymus independent antigens (TI Ags) are of two types Type I and Type II.

(a) TYPE 1:

Type I thymus independent antigens act as mitogens (an agent that triggers mitosis). Some lipopolysaccharides of bacterial cell wall show mitogenic property and act as polyclonal B cell activators regardless of antigenic specificity.

They can activate both mature and immature B cells. Since their activity is independent of T cells, they were considered as true T cell independent antigens.

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T cell independent antigens are usually polymorphic with repeated identical epitopes. They can stimulate B cells directly by cross linking with surface immunoglobulins.

Antibodies produced as a result of interaction of Type I antigens and B cells are generally of IgM type, these antibodies show low affinity due to a little class switching.

Generally Type I antigens induce weaker or no memory cells. The interaction of antigen with B cell surface immunoglobulin receptors induces a cascade of reactions in cell membrane, nucleus and cytoplasm. The cell membrane undergoes depolarization leading to the entry of Ca+ from extra cellular environment to initiate membrane associated pathways to form secondary messenger substances, such as inositol triphosphate (ITP), diacyl glycerol (DAS) and cyclic GMP (cGMP).

The secondary messenger substances activate protein kinases that promote the division of activated B cells. During this stage the B cells express several receptors such as receptors for interleukins (IL’s), Fc receptors, CD19, CD21, CD22, CD23, B cell growth factor receptors (BCGF’s), receptor for B cell differentiation factor (BCDF’s) etc. to respond to several cytokines.

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The binding of cytokines to their respective receptors of B cells help in regulation of activity in B cells, to match with the requirement of the immune system. Even through T cell independent antigens activity suggests their independence of T cells, certain T cells concretions are essential for proper functioning of B cells. In the absence of B cell growth factors (BCGFS) released by T cells, B cells continue to repeat multiplication only. BCGF binding to B cells is necessary to produce antibody secreting plasma cells.

(b) TYPE -2:

Unlike type I thymus independent antigens they are not B cell mitogens. They activate only mature B cells and depend on cytokines derived from Th cells for competent B cell explosion and class switching to isotopes such as IgG, IgA and IgE.

B cell activation by T dependent antigens: Most of the antigens that stimulate humoral immunity are T dependent. Antigens presenting cells and T cells are essential forT dependent antigens to activate B cells.

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Antigen presenting cells engulf and process the antigen to presents it in a suitable form for activation of T cells. The processed antigens are presented to the naive T cells through MHC class II and co stimulatory molecule B7.

The TH cells on interaction with antigen secret different lymphokines that induce activation, differentiation and proliferation of naive B cells. Differentiation and proliferation of B cells leads to the production of a colony that can secrete antibodies for the antigen responsible for its activation.

Some of the cells from the colony become short lived plasma cells that in turn secrete antigen specific antibodies/immunoglobulin and the remaining become memory cells with long life span for future defense against the same antigens.