Page 451 - Veterinary Immunology, 10th Edition

VetBooks.ir Cellular Responses

The term clonotype is used to describe a clone of B cells expressing a
BCR capable of responding to a single epitope. A newborn animal

with few B cells has available only a limited variety of clonotypes,
but their diversity increases with age as a result of increased use of
alternative sets of V genes and somatic mutation in these V genes
(Chapter 17). This diversity is also increased by the effects of the
intestinal microbiota (Chapter 21). In an adult animal the number of

B cells within a given clonotype depends on their exposure to
different antigens over the animal's lifetime. Thus the most used
clonotypes will increase greatly in number and there may be as
4
many as 10 responsive B cells per clonotype (Box 15.1). Conversely,
the numbers of unused clonotypes will remain very small, and an
individual may have as few as 10 responsive cells in their spleen or
bone marrow.

Box 15.1

Cell Membrane Exchange

It has generally been assumed that individual cells conserve their
major structural components and do not share them with other

cells. In recent years, however, it has become abundantly clear that
different cells may exchange cell surface membranes and their
associated receptors. Thus macrophages may accept fragments of
neutrophil membranes. Activated B cells can also donate their

antigen receptors to nearby bystander cells. This process is
mediated by membrane transfer between adjacent B cells and is
amplified by the interaction of the BCR with specific antigen. The
net effect is to permit a dramatic expansion of the number of

antigen-binding B cells in vivo. The B cells with their newly
+
acquired receptors can act as antigen-presenting cells for CD4 T
cells. This is yet another example of the remarkable efficiency of
the adaptive immune system in responding to a specific antigen

once an animal is primed.

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