Antigens which trigger specific T cells are called T-dependent antigens. In the
absence of T cells, these antigens cannot trigger B cells to synthesize antibodies.
T-independent antigens, on the other hand, can stimulate B cells without the aid of
Most antigens are T-dependent. They include microorganisms, proteins, and
haptens on various carriers. T-dependent antigens react either directly with a T
cell, or with a macrophage which processes the information and transfers it to a T
cell. T cells that function in this mechanism are called helper T cells. These
antigens may induce IgG, IgE, IgA, or IgM responses and produce immunological
T-independent antigens are generally large polymers with many repeating units. It
appears that each T-independent antigen carries a specific antigenic signal and a
nonspecific signal which acts as a mitogen. (A mitogen is a substance that
induces mitosis or cell transformation, particularly transformation of lymphocytes.)
This mitogenic signal is directly capable of activating B cells irrespective of their
antigen reactivity. Although T-independent antigens can initiate antibody
production in the absence of T cells, substantial production of antibody does not
`occur. The antibody produced is largely IgM and little or no immunological
memory is produced.
Early evidence about hapten-carrier systems suggests that T cells recognize the
carrier while B cells recognize the hapten. As discussed previously, haptens alone
are not able to induce immune responses. If the hapten is coupled to a carrier,
antibody that reacts with the hapten will be produced. The B cell which binds the
hapten will make antihapten antibody.
Once B cells are stimulated, they become metabolically active and undergo
morphological changes. This process is called blast transformation. B
lymphocytes are small oval cells, but after transformation they become enlarged.
B cells then go through several cell divisions called clonal expansion in order to
increase the number of activated cells. They then differentiate into plasma cells
and memory B cells. Plasma cells secrete antibodies. They are end cells and
survive only about two weeks. Memory cells have the same appearance as small
lymphocytes. Memory cells are responsible for the anamnestic response, the
rapid production of antibody on re-exposure to antigen.