Page 830 - Fundamentals of anatomy physiology

lymph flows through a lymph node, at least 99 percent of the Chapter 22 The Lymphatic System and Immunity 817
antigens in the lymph are removed. Fixed macrophages in the
walls of the lymphatic sinuses engulf debris or pathogens in &T i p s T r i c k s
lymph as it flows past. Antigens removed in this way are then Helper T cells “help” translate the message from the antigen-
processed by the macrophages and “presented” to nearby lym- presenting cells of the nonspecific response to the cells of the
phocytes. Other antigens bind to receptors on the surfaces of specific immune responses.
dendritic cells, where they can stimulate lymphocyte activity.
This process—antigen presentation—is generally the first step in In addition to filtering, lymph nodes function as an early-
the activation of the immune response. warning system. Any infection or other abnormality in a pe-
ripheral tissue puts antigens into the interstitial fluid, and
thus into the lymph leaving the area. These antigens then

Figure 22–7 Lymphoid Nodules.

Pharyngeal
epithelium

Pharyngeal tonsil Germinal centers
Palate within nodules
Pharyngeal tonsil
Palatine tonsil
Lingual tonsil

LM × 40

a The locations of the tonsils 22

Intestinal lumen

Mucous Germinal center
membrane
of intestinal wall

Aggregated
lymphoid nodule
in intestinal mucosa

Underlying
connective tissue

b Diagrammatic view of aggregated Aggregated lymphoid nodules LM × 20
lymphoid nodule