BSC 215 Lecture Notes - Lecture 47: Clonal Selection, Cell Membrane, Natural Killer Cell
11 Jun 2018
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Specific Defense (The Immune System)
The immune system is the third line of defense. It consists of mechanisms and agents that target
specific antigens (Ags). An antigen is any molecule, usually a protein or polysaccharide, that can
be identified as foreign (nonself) or self (such as MHC antigens described below). It may be a
toxin (injected into the blood by the sting of an insect, for example), a part of the protein coat of
a virus, or a molecule unique to the plasma membranes of bacteria, protozoa, pollen, or other
foreign cells. Once the foreign antigen is recognized, an agent is released that targets that
specific antigen. In the process of mounting a successful defense, the immune system
accomplishes five tasks:
• Recognition. The antigen or cell is recognized as nonself. To differentiate self
from nonself, unique molecules on the plasma membrane of cells called
the major histocompatibility complex (MHC) are used as a means of
identification.
• Lymphocyte selection. The primary defending cells of the immune system are
certain white blood cells called lymphocytes. The immune system potentially
possesses billions of lymphocytes, each equipped to target a different antigen.
When an antigen, or nonself cell, binds to a lymphocyte, the lymphocyte
proliferates, producing numerous daughter cells, all identical copies of the parent
cell. This process is called clonal selection because the lymphocyte to which the
antigen effectively binds is selected and subsequently reproduces to make
clones, or identical copies, of itself.
• Lymphocyte activation. The binding of an antigen or foreign cell to a lymphocyte
may activate the lymphocyte and initiate proliferation. In most cases, however, a
costimulator is required before proliferation begins. Costimulators may be
chemicals or other cells.
• Destruction of the foreign substance. Lymphocytes and antibodies destroy or
immobilize the foreign substance. Nonspecific defense mechanisms
(phagocytes, NK cells) help eliminate the invader.
• Memorization. Long‐lived memory lymphocytes are produced and can quickly
recognize and respond to future exposures to the antigen or foreign cell.
Protecting Your Body
The colossal task of keeping the body safe from outside and inside attacks of bacteria, viruses,
and other nasty critters belongs to the immune system. Your skin and mucous membranes are
the first line of defense from invaders entering through the skin or through openings in the
body. A second line of defense exists inside the body to challenge invaders that make it through
the first line of defense. Sounds more like a war strategy, doesn't it? That's how your body
treats anything that isn't supposed to be there—as an invader that must be destroyed.
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Document Summary
The immune system is the third line of defense. It consists of mechanisms and agents that target specific antigens (ags). An antigen is any molecule, usually a protein or polysaccharide, that can be identified as foreign (nonself) or self (such as mhc antigens described below). Once the foreign antigen is recognized, an agent is released that targets that specific antigen. In the process of mounting a successful defense, the immune system accomplishes five tasks: recognition. The antigen or cell is recognized as nonself. To differentiate self from nonself, unique molecules on the plasma membrane of cells called the major histocompatibility complex (mhc) are used as a means of identification: lymphocyte selection. The primary defending cells of the immune system are certain white blood cells called lymphocytes. The immune system potentially possesses billions of lymphocytes, each equipped to target a different antigen.
