c. The mechanism of macrophage interaction with B and T cells is not
completely understood. A common opinion is that macrophages digest complex
antigens to make them "palatable" for B cells. Another concept is that macrophages
ingest antigens and then manufacture some informational type of ribonucleic acid (RNA)
which is transferred to B lymphocytes and triggers antibody production.
ANTIGEN TRIGGERING OF SPECIFIC T CELLS
a. The triggering of specific T cells occurs with most, but not all, antigens.
Those antigens that are called T-dependent antigens cannot trigger B cells to
synthesize antibodies in the absence of T cells. T-independent antigens, on the other
hand, can stimulate B cells without the aid of T cells.
b. 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 designated as helper T cells. These antigens may
induce IgG, IgE, IgA, or IgM responses and produce immunological memory.
c. 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 (Figure 3-1).
Figure 3-1. Simplified model for humoral immunity.