Page 392 - Veterinary Immunology, 10th Edition

secreting multiple cytokines, dividing, and differentiating. As you
VetBooks.ir will learn later, the other antigen-responsive cell populations, B

cells and cytotoxic T cells, cannot respond to antigens unless they
too are stimulated by helper T cells. Because of the central role of

helper T cells, they must be carefully regulated by signals from
other cells and by the activities of many different cytokines.
It is important to point out that the antigen receptors on T cells
are generated randomly. They are not produced in response to

specific foreign antigens. As a result, T cell antigen receptors form a
large diverse repertoire. Any foreign antigen that enters the body
will probably encounter and bind to the receptors on at least one T
cell. Because each T cell has a single receptor specificity, the

repertoire of receptors is, in effect, the repertoire of the T cells. T cell
antigen receptors only recognize antigens associated with MHC
molecules. They do not recognize or respond to free antigen
molecules.

Given the random way in which these receptors are generated,
the strength of binding between an antigen and its receptors (its
affinity) will also vary. Thus an antigen may be bound strongly by
some receptors and weakly by others. If this binding strength is too

weak, this may be insufficient to activate a T cell.
In a newborn animal that has never encountered microbial
antigens, the number of T cells that can bind any specific antigen
may be very low. To increase the probability of an antigen

encountering a T cell with the correct receptor, the T cells are
concentrated in secondary lymphoid organs such as lymph nodes,
where their chances of a successful encounter with antigen-bearing
dendritic cells are maximized. In older animals in which mature T

cells are plentiful, they can migrate into other tissues, where they
will encounter other antigen-presenting cells, such as macrophages
and B cells.

392