Question

24. Is there memory in a thymus-independent antibody response? What molecule stimulates the T-Independent response

Answer 1

24 A.Yes, in some cases there will be memory in thymus independent antibody response but in general cases no memory is generated in thymus independent antibody response.

B.Molecules like bacterial lipopolysaccharides , bacteria DNA, cytokinesis like interlukins and interferons stimulate T-independent response

Explanation :- There are two type of immune response- T-dependent and t-independent. The B-cell response to thymus-dependent (TD) antigens requires direct contact with TH cells and Antigens that can activate B cells in the absence of this kind of direct participation by TH cells are known as thymus-independent (TI) antigens. TI antigens are divided into types 1 and 2, and they activate B cells by different mechanisms.

TI-1 antigens activate B-cells via Toll like receptors, which are, in human, expressed on the surface of B lymphocytes after BCR stimulation.In higher concentrations, TI-1 antigens bind to BCR and TLR of various clones of B lymphocytes, which leads to production of multiclonal antibodies. But when the concentration of TI-1 is lower, it can activate only B lymphocytes with specific binding of TI-1 on their BCR, and leads to production of monoclonal antibodies.An example of TI-1 antigen is lipopolysaccharide (LPS) or bacterial DNA. When a specific naïve B cells come in contact with such antigens, these antigenic determinants bind the surface IgM and IgD receptors. This binding leads to clustering of surface immunoglobulins which generates a signal, strong enough to activate the naïve B cells.   This activated B cells produces and releases first immunoglobulin i.e. IgM. Most TI-1 antigens are polyclonal B-cell activators (mitogens); i.e. they are able to activate B cells regardless of their antigenic specificity. At higher concentrations, some TI-1 antigens will stimulate proliferation and antibody secretion by as many as one third of all B cells but in lower concentrations of TI-1 antigens, only those B cells specific for epitopes of the antigen will be activated.

2.TI type 2 activate immune response by cytokins like interleukin and interferon. It consists mainly of highly repetitive surface structures (epitopes) of encapsulated bacteria. The activation of B lymphocytes is caused by cross-linking of a critical number of B cell receptors, which leads to accumulation of BCRs and cross activation of these receptors. It results in proliferation and differentiation of B lymphocytes and production of antibodies. TI-2 antigens can activate only mature B lymphocytes. Hence no memory.

But in some cases where type 2 antigen epitopes cross linked with multiple BCRs for long durations like in conjugate vaccines then B cell activation takes place and switching of BCRs i.e Ig takes places which produces memory.

InEven though the response on TI antigens is not dependent on T lymphocytes, there are some cytokines, produced mainly by T lymphocytes and natural killer (NK) cells, necessary for eliciting reaction against these antigens. The most necessary are interleukin 2 (IL-2), interleukin 3 (IL-3) and interferon γ(IFN-γ). Moreover, additional stimulation by dendritic cells (DC) and macrophages is required.