CELL-MEDIATED IMMUNITY
Cell-mediated immunityinvolves specialized lymphocytes called T cells, also
known as T lymphocytes, to attack foreign organisms rather than using antibod-
ies. T cells are also effective against helminths, fungi, and protozoa. In addition,
T cells regulate aspects of the immune system.
Cell-mediated immunity was explained by Russian biologist, Elie
Metchnikoff, who in the early 1900’s noticed that phagocytic cells were much
more effective in animals that were immunized. This immunity was used in the
mid-twentieth century to protect people against tuberculosis.A Closer Look at Antigens
An antibody identifies its corresponding antigen by one or more regions on the
antigen known as the epitopes, which are also called antigenic determinants. The
epitope must be the right size, shape, and chemical structure for the antibody to
bind to the epitope and then proceed to disable or destroy the antigen.
Antigens tend to have a molecular weight of 10,000 or more yet some foreign
substances might have a lower molecular weight and are not antigens. They are
called haptensand must attach themselves to a large carrier molecule in order to
become antigenic. Antibodies only attack the hapten and not the carrier molecule.
Penicillin is a common hapten. Penicillin does not have an antigen effect in
most people. However, when penicillin attaches to serum proteins an allergic
reaction results in some people. These people are said to be allergic to penicillin.
An allergic reaction is a typical immune response.
Antigens can be proteins, large polysaccharides, lipids, or nucleic acids.
However, antigens that are lipids and nucleic acids must be combined with pro-
teins and polysaccharides; otherwise they are not antigens.A Closer Look at Binding
Antibodies are known as immunoglobulins(Igs), which is a group of soluble
proteins. An antigen can cause the production of different antibodies if the anti-
gen has several epitopes. Epitopes or antigenic determinants,are known as antigen-(^214) CHAPTER 14 Immunity