a. The Process of Antibody Synthesis. When an antigen enters the body, it
may evoke a humoral response, in which antibodies are synthesized by plasma cells
and released into the body fluids (for example, plasma), and/or a cellular response, in
which lymphocytes participate in cell-mediated immunity (for example, rejection of
transplanted tissue and delayed hypersensitivity). That two different responses were
present was originally shown by Chase and Landsteiner in the early 1940s when they
demonstrated that some kinds of immune reaction could be transferred from one animal
to another by the exchange of living cells, whereas others could be transferred by blood
serum. The cells required for the former experiment were lymphocytes. It was not until
the early 1960s that involvement of the lymphocyte was proven.
b. Lymphocyte Populations. Stem cells from the bone marrow are thought to
differentiate to form two distinct lymphocyte populations. The cells that pass through
the thymus become known as T-lymphocytes (T-cells) and the others that are
independent of the thymus B-lymphocytes (B-cells). Although these lymphocytes look
similar by conventional light or electron microscopy, they do look very different by
scanning electron microscopy, and also they can be differentiated by a variety of
surface markers. Their functions are of course different, but there is mounting evidence
for the possibility of cooperation between the two systems.
c. T-Lymphocytes. Once leaving the thymus, where they are known as
thymocytes, the T-lymphocytes are immunocompetent, that is to say, capable of
participating in an immune response. This is the basis of cellular immunity.
T-lymphocytes constitute the greater part of the recirculating pool of small lymphocytes
and have a relatively long half-life. When they encounter an antigen (which may have
to be first processed by a macrophage), they transform to lymphoblasts (See
figure 1-1). These T-lymphoblasts, which have no demonstrable intracellular
immunoglobulin, have several functions:
(1) They divide further into primed antigen-sensitive cells, which provide
immunologic memory because of their long life span.
(2) They release a number of soluble factors (lymphokines) which mediate
(3) They are "killer" cells, which are cytotoxic for cells bearing the
histo-compatibility antigens of a graft or tumor cells.
(4) They may cooperate with the humoral system by triggering