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Humoral (antibody-mediated)
Immune response
Cell-mediated immune response
Antigen (first exposure)
Engulfed by
Macrophage
Becomes
Antigen-
presenting cell
Free antigens
directly activate
Antigens displayed by
infected cells activate
Stimulates
Stimulates
Stimulates
Helper
T cell
Cytotoxic
T cell
B cell
Memory
helper T cell
Stimulates
Stimulates
Stimulates
Gives rise to
Gives rise to
Antigen (second exposure)
Stimulates
Active
cytotoxic T cell
Plasma
cells
Secrete
Memory
B cell
Memory
T cell
Antibodies
Defend against extracellular pathogens
by binding to antigens and making them easier
targets for phagocytes and complement
Defend against intracellular pathogens
and cancer by binding to and
lysing the infected cells or cancer cells
Figure 1.7
Illustration of humoral and cellular immunity.
secondary immune response, in contrast to the fi rst encounter with the antigen, in
which a slower response, called primary immune response, occurs (see Figure 1.8).
h is augmented antibody response is due to the existence of memory cells, which
rapidly produce plasma cells on antigen stimulation. h us, the immune system learns
from encounters with antigens to improve its response in subsequent encounters, pro-
ducing a so-called immunological memory.
1.3.3
T Cell Maturation
T cells are produced by the bone marrow and are initially inert, that is, they are not
capable of performing their intended functions. To become immune-competent,
they have to go through a maturation process. In B cells, the maturation process
occurs in the bone marrow itself. T cells, instead, have to migrate to the thymus
where they mature.
During maturation, T cells express a unique ABR on their surface called a T
cell receptor (TCR). h e generation of various TCRs is controlled by a random
recombination of diff erent gene segments. Unlike B cells, TCRs can only recog-
nize antigenic peptides that are presented by cell-membrane
(MHC) molecules
(see Figure 1.9).
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