Essentials of Anatomy and Physiology

The stem cells of the thymus produce T lympho-
cytes or T cells; their functions are discussed in the
next section. Thymic hormones are necessary for what
may be called “immunological competence.” To be
competent means to be able to do something well.
The thymic hormones enable the T cells to participate
in the recognition of foreign antigens and to provide
immunity. This capability of T cells is established
early in life and then is perpetuated by the lympho-

cytes themselves. The newborn’s immune system is

not yet fully mature, and infants are more susceptible

to certain infections than are older children and

adults. Usually by the age of 2 years, the immune sys-

tem matures and becomes fully functional. This is why

some vaccines, such as the measles vaccine, are not

recommended for infants younger than 15 to 18

months of age. Their immune systems are not mature

enough to respond strongly to the vaccine, and the

326 The Lymphatic System and Immunity

Afferent lymphatic vessel

Capsule

Cortex

Nodal vein

Nodal artery

Hilus

Valve

Efferent lymphatic

vessel

Bacteria

Lymphocytes

Neutrophil

Plasma cell

Macrophage

Antibody molecule

(enlarged)

Antigen

(enlarged)

A

B

Figure 14–4. Lymph node. (A) Section through a lymph node, showing the flow of

lymph. (B) Microscopic detail of bacteria being destroyed within the lymph node.

QUESTION:What is the function of the plasma cells in a lymph node?