Immunology Basics - Immunological Computation: Theory and Applications - page 14

During T cell maturation, they go through a process of selection, which ensures
that they are able to recognize nonself peptides presented by MHC molecules. h is
process has two main phases: positive selection and negative selection (Coutinho,
1980; Paul, 1993).

• Positive selection . In positive selection, T cells are tested for recognition of
MHC molecules expressed on the cortical epithelial cells. If a T cell fails to
recognize any of the MHC molecules, it is discarded; otherwise, it is kept.
• Negative selection . h e purpose of negative selection is to test for tolerant self-
cells. T cells that recognize the combination of MHC and self-peptides fail
this test. h is process can be seen as a fi ltering of a diversity of T cells, in which
only those that do not recognize self-peptides are kept (Kappler et al., 1987).

When a T cell encounters antigens associated with an MHC molecule on a cell,
such a T cell will proliferate and diff erentiate into memory T cells and various
eff ector T cells. Cellular immunity is accomplished by these generated eff ector T
cells. h ere are diff erent types of T cells that interact in a complex way to kill
altered self-cells (for instance, virus-infected cells) or to activate phagocytic cells
(Abbas and Lichtman, 2005; Moss et al., 1992).

1.3.4 B Cell Proliferation: Affi nity Maturation

When receptors on the surface of a B cell bind to an antigen, this B cell gets stimu-
lated to undergo proliferation and diff erentiation. Also, when receptors on the sur-
face of a T cell bind to an antigen, such a T cell proliferates. h is process is called
clonal selection because antigen binding drives a particular cell for clonal expan-
sion. h ereby, B cells that are generated become either memory cells or plasma cells.
Memory cells ensure that subsequent infections by a pathogen receive a more rapid
response, when plasma cells secrete large amounts of antigen-specifi c antibodies.
Figure 1.10 illustrates B cell activation by specifi c antigens.
In early stages of the immune response, the a nity between antibodies and anti-
gens may be low. But as B cells undergo clonal selection, they clone and mutate repeti-
tively to improve the binding a nity between a particular antigen and a B cell type.
h is mutation process is called somatic hypermutation . h en, these activated B cells
mature into plasma cells, which in turn produce antibodies with a high a nity of the
Ag/Ab bonds. h e entire process by which new B cells with high a nity to an antigen
are created (clonal selection somatic hypermutation) is called a nity maturation .

Ultimately, a nity maturation will lead to the production of a pool of
antibody-secreting plasma cells and a pool of memory cells. Plasma cells are
matured B cells that form a large endoplasmic reticulum for massively synthesizing
and secreting specifi c antibodies. In contrast, memory cells are B lymphocytes with
the same specifi city receptors as those on the original activated B cell (Perelson and
Oster, 1979).

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