Human Physiology, 14th edition (2016)

510 Chapter 15

T Cell Receptor Proteins

The antigens recognized by B lymphocytes may be either pro-
teins or carbohydrates, but only protein antigens are recognized
by most T lymphocytes. Unlike B cells, T cells do not make
antibodies and thus do not have antibodies on their surfaces to
serve as receptors for antigens. However, T cells do have anti-
gen receptors on their surfaces that are closely related to the
immunoglobulins. The T cell receptors differ from the antibody
receptors on B cells in a very important respect: the T cell recep-
tors cannot bind to free antigens. In order for T lymphocytes to
respond to foreign antigens, the antigens must be presented to
the T cells on the membrane of antigen-presenting cells.
The 2011 Nobel Prize in Physiology or Medicine was
awarded to the discoverer of dendritic cells ( fig. 15.13 ), which
are the chief antigen-presenting cells. Other cells that function
to a lesser degree as antigen-presenting cells are macrophages
and B cells. Both dendritic cells and macrophages can be
derived from monocytes, which are formed in the bone marrow
and migrate through blood and lymph to almost every organ.
Antigen-presenting cells are especially concentrated at poten-
tial sites where antigen-bearing microorganisms might enter,
such as the skin, intestinal mucosa, and lungs.

Figure 15.13 Migration of antigen-presenting dendritic cells to secondary lymphoid organs activates T cells. Once
the T cells have been activated by antigens presented to them by the dendritic cells, the activated cells divide to produce a clone.
Some of these cells then migrate from the lymphoid organ into the blood. Once in the blood, these activated T cells can home in on the
site of the infection because of chemoattractant molecules produced during the inflammation.

Activated

Attracted to T cell

infected site

Dendritic cell

Antigens

Antigen

Antigen

Lymph

vessel

Dendritic

cell

T cell Activated T cell

Lymph

vessel

Lymph

node

Venule

1. Dendritic cell
   processes
   antigens
   2. Dendritic cell
   activates T cell
   3. Activated T cell
   travels through
   lymphatic system


2. Activated T cell
   extravasates to
   combat infection

CLINICAL APPLICATION

Sepsis refers to a systemic, whole-body inflammation that

causes organ dysfunction and can potentially be fatal. Symp-

toms can include high fever, a rapid pulse and respiratory

rate, hypotension (low blood pressure), hypoxemia (low blood

oxygen), oliguria (low urine output) and acidosis due to lactic

acid (chapter 16, section 16.8). Sepsis is usually triggered by

a bacterial infection, and is treated with antibiotics, intrave-

nous fluids, and more aggressive measures when required.

A danger is septic shock, where the blood pressure falls so

low that organs are not adequately perfused.

Endotoxin, more properly termed bacterial lipopoly-

saccharide ( LPS ), is a component of certain bacteria (called

gram-negative bacteria ) and is the most powerful instigator

of sepsis. In localized infections, LPS triggers innate immu-

nity (through toll-like receptors) that helps to clear the infec-

tion. However, when massive amounts of LPS enter the

circulation, it stimulates the widespread release of inflamma-

tory cytokines. Scientists are attempting to develop drugs

to block these effects of LPS and of other molecules from

gram-positive bacteria that play similar roles.