The Animal Self: Molecular Recognition 119“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityonly one type of antibody. Thus, within the entire population of naïve
lymphocytes, only a tiny fraction (perhaps even one cell) actually car-
ries antibodies directed against a particular antigen. Nevertheless, upon
encountering a matching antigen, this single cell starts to proliferate to
form a clone that produces the specific antibody in large quantities. This
phenomenon is called “clonal selection.”
Figure 6.3 shows how clonal selection works. Note that the naïve
B lymphocytes display their preformed antibodies as “receptors” on the
cell surface. Once activated by a foreign antigen, the single expanded
clone secretes the specific antibody whose structure corresponds to the
activated surface receptor.
6.7 MHC protein: A Personalized Molecular Signature
Like B cells, naïve T lymphocytes express surface “receptors” that bind
antigens, but unlike B cells, which release their receptors to become
antibodies, T cell receptors stay on the cell surface and are never
secreted into the blood stream. T cell receptors are as diverse and spe-
cific as those of B cells and are also a result of gene shuffling and clonal
selection. Whereas B cells can recognize antigens directly, T cells rec-
ognize only those antigens that have been processed by “professional”
Fig. 6.3. Clonal selection of B lymphocytes. An immature B cell whose preformed an-
tibody on the cell surface matches a foreign antigen (dark dot on left) starts to proliferate
and secret antibody into the blood stream.