p r o c e ss
d iag ram
B cells can recognize and bind to antigens in lymph,
interstitial fluid, or blood plasma, but T cells only recog-
nize fragments of antigens that are processed and pre-
sented in a certain way. For adaptive immunity to work,
T cells and B cells must be able to distinguish your body’s
own cells and antigens from foreign cells and antigens.
To accomplish this, you have a unique set of proteins
called the major histocompatiblity complex (MHC)
on every cell in your body (except red blood cells). Un-
less you have an identical twin, your MHC proteins are
unique.
When a foreign antigen enters the body, a cell called
an antigen-presenting cell (APC) ingests it and pro-
cesses the foreign substance (Figure 12.9). APCs include
dendritic cells, macrophages, and B cells. They are stra-
tegically located in the skin, mucous membranes, and
lymph nodes. The foreign antigens are bound to the MHC
proteins in the APC and are moved to the APC’s surface,
where they are “presented” to the appropriate lympho-
cytes. Cells that have been infected by intracellular para-
sites can also present antigens in a similar manner.
In cell-mediated immunity, once the APCs present the
antigen to helper T cells, the active helper T cells secrete
interleukin and other proteins that stimulate cytotoxic
T cells to undergo clonal activation. When an infected
cell presents its antigen to the active cytotoxic T cell with
the corresponding antigen receptor, the cytotoxic T cell
destroys the infected cell (Figure 12.10).
Cytotoxic T cells kill infected target cells in one of two
ways. In one method, protein-digesting enzymes called
granzymes trigger apoptosis, the fragmentation of cellular
contents. The cell is destroyed and releases the microbes,
which are then killed by phagocytes. In the second meth-
od, the T cells release two proteins, perforin and granu-
lysin, into infected cells; these proteins enter, destroy the
microbes, and cause cytolysis (cell-bursting).
P rocessing and p re se n ta tio n o f a n tig e n s • Figure 12.9
Antigens enter an antigen-presenting cell (APC), such as a dendritic cell, B cell, or mac-
rophage. After processing, pieces of the antigen combine with major histocompatiblity
complex (MHC) proteins. The antigen-MHC complex is inserted into the surface of the
APC, where it can sensitize and activate a helper T cell. Infected body cells can also
present antigens that have invaded them by going through a similar process using
steps 2-5.
356 CHAPTER 12
The Lymphatic System and Immunity
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