The Animal Self: Molecular Recognition 117“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitylong-distance systemic effect (thus the term “antibody-mediated
immunity” or “humoral immunity”); the T cells which act by cell con-
tact (cell-mediated immunity) for the destruction of the infected cells.
T cells also secret cytokines for remote regulation of immune function.^9
There are three types of T cells: the cytotoxic T (also called “killer
T”), the helper T, and the regulatory T. All T cells are activated by expo-
sure to the antigen presented to them by antigen-presenting cells, in
particular the dendritic cells. Cytotoxic T destroy virus-infected host
cells. Helper T do not kill cells but instead secrete cytokines to enhance
B cell and macrophage functions. Regulatory T counteracts helper T by
shutting down immune response once infection is over.
6.6 Gene Scrambling Leads to Antibody Diversity
and Specificity
Antibodies are exclusively produced by B lymphocytes. They ward off
infection in the following manners: (1) binding of antibodies to extra-
cellular pathogens can inactivate the invaders; (2) binding of antibodies
to extracellular toxins can neutralize their toxicity; (3) antibodies mark
the invading bacteria for destruction by complements, a family of special
immune proteins; (4) antibodies mark the invading bacteria for phago-
cytosis by macrophages and neutrophils.
For antibodies to be useful, they must be produced in great vari-
eties, yet each type has to be highly specific for binding only one kind
of antigen. How the lymphocytes achieve these two seemingly opposing
goals is a great wonder of nature. Before explaining how antibodies are
made, it will be helpful to clarify the concept of antibody-antigen inter-
action. Antibodies are a special type of protein capable of binding other
molecules, referred to as antigens, usually another macromolecule like
protein or polysaccharide (such as those on a bacterial surface). The part
of an antibody involved in binding is called the “antigen binding site,”
and the part of the antigen that binds to the antibody is called “antigenic
determinant” or “epitope.” The exact fit of the two sites in a “lock and