Immune Reponses Help Protect the Body
Against Disease 345
• Physical barriers such as the skin, mucus, and mucous
membranes, as well as chemicals in various secretions, are
the first line of nonspecific innate defenses. The second line
of nonspecific innate defense includes internal defenses
such as antimicrobial substances, macrophages, natural
killer cells, inflammation (as shown at right), and fever. The
lymphatic system also has the ability to adapt to specific
pathogens through adaptive immunity.
• As shown below, adaptive immunity is of two types: cell-
mediated immunity and antibody-mediated immunity. In
cell-mediated immunity, T cells kill infected cells in tissues
and release pathogens for subsequent phagocytosis. In
antibody-mediated immunity, B cells differentiate to form
plasma cells that secrete antibodies, which attack various
pathogens. Antibody-mediated immunity occurs mostly in
blood and fluids.
• In adaptive immunity, various cells present specific antigens
to helper T cells and B cells that contain receptors for those
antigens. These cells undergo clonal activation, where they
proliferate and differentiate into active effector cells and
memory cells. Helper T cells then activate cytotoxic T cells
to undergo clonal selection. Cytotoxic T cells attack and kill
infected cells (cell-mediated immunity). Plasma cells, which
are derived from B cells, secrete antibodies (antibody-
mediated immunity).
• Memory cells help the lymphatic system react more
strongly and quickly when your body is subsequently
reexposed to any given antigen. This immunological
memory is the basis for vaccination.
Inflammation • Figure 12.7c
and increased
^ ^ T is s u e injury
Overview of adaptive immune responses
Mature T cells
Mature B cells
Figure 12.8
■ Active immunity produces antibodies
following exposure to an antigen,
while passive immunity supplies
antibodies from another source
without a full immune response.
Passive immunity is temporary,
but active immunity can last for
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