d. B-Lymphocytes. The B-lymphocyte gets its name from the Bursa of
Fabricius, a lymphoid organ present in birds, which controls the production of
lymphocytes responsible for making humoral antibody. The equivalent organ in man has
yet to be found. Thymus-independent, or B-lymphocytes synthesize and excrete
specific antibodies (surface immunoglobulins) that serve as receptors for antigens.
When triggered by antigen, the B-lymphocytes change to plasma cells, which are
responsible for the excretion of free antibody into the body fluids (for example, humoral
antibody), see figure 1-1. There is much evidence to suggest that macrophages are
required to process antigens for appropriate presentation to lymphocytes before the
humoral response occurs. In addition, many antigens appear to require the cooperation
of both B- and T-lymphocytes. The mechanisms by which T- and B-lymphocytes
interact are complex and far from clear at present. As mentioned previously, it is
humoral antibodies that are dealt with routinely in blood transfusion science, but
possibly cellular reactions will increase in importance in the future.
e. Differentiation of T- and B-Lymphocytes. Approximately 25 percent of
human blood lymphocytes are B cells, 70 percent T cells, and 5 percent have neither T
nor B markers (they are called "null cells"). Immunoglobulins are readily demonstrable
on B, but not T-lymphocytes by immunofluorescence. T- but not B-lymphocytes will
form "spontaneous" rosettes with unsensitized sheep erythrocytes.
PRIMARY AND SECONDARY IMMUNE RESPONSES
a. Following a first exposure to a foreign antigen, specific antibodies can appear
after about five days, rise slowly to a modest level, remain for a variable period, then
gradually decline, eventually becoming undetectable, until further stimulation occurs.
The first antibodies produced in this primary response are usually lgM, but eventually
other immunoglobulins (for example, lgG) may appear. The type of antigen and the
route of administration will influence the pattern observed.
b. After the primary response, a second dose of the same antigen, given days or
even years later, will usually elicit an intense and accelerated secondary (memory)
response. The serum antibody usually begins to rise within two or three days, reaching
a peak in about 10 days. In this secondary response, lgM antibody is often transiently
produced, following a similar pattern to the primary response, but the predominant
antibody produced is lgG, which rises to a much greater concentration than the lgM, and
remains in the plasma much longer. The secondary response is sometimes called an