At the molecular level, lymphocytes are characterized by the expression of specific molecules on the plasma membrane known as cluster of differentiation (CD) proteins. CD proteins recognize specific ligands on target cells. Because some CD proteins are present only on specific types of lymphocytes, they are considered specific marker proteins. On the basis of these specific markers, lymphocytes can be classified into three functional cell types.
 T lymphocytes are characterized by the presence of the CD2, CD3, CD5, and CD7 marker proteins and the T-cell receptors (TCRs). These cells have a long life span and are effectors in cell-mediated immunity.
 B lymphocytes are characterized by the presence of CD9, CD19, and CD20 proteins and attached immunoglobulins IgM and IgD. These cells recognize antigen, have a variable life span, and are effectors in antibody-mediated (humoral) immunity.
 NK lymphocytes are non-T, non-B lymphocytes that express the CD16, CD56, and CD94 proteins, not found on other lymphocytes. These cells neither produce immunoglobulins nor express TCR on their surface. Thus, NK lymphocytes are not antigen specific. Similar in action to T lymphocytes, however, they destroy virus-infected cells and some tumor cells by a cytotoxic mechanism.
In response to the presence of antigens, lymphocytes become activated and may divide several times, producing clones of themselves. In addition, clones of B lymphocytes mature into plasma cells. A description of B and T lymphocytes and their functions during immune response reactions is presented in Chapter 14.
Plasma cells are antibody-producing cells derived from B lymphocytes.
Plasma cells are a prominent constituent of loose connective tissue where antigens tend to enter the body (e.g., the gastrointestinal and respiratory tracts). They are also a normal component of salivary glands, lymph nodes, and hematopoietic tissue. Once derived from its precursor, the B lymphocyte, a plasma cell has only limited migratory ability and a somewhat short life span of 10 to 30 days.
The plasma cell is a relatively large, ovoid cell (20 m) with a considerable amount of cytoplasm. The cytoplasm displays strong basophilia because of an extensive rER (Fig. 6.25a). The Golgi apparatus is usually prominent because of its relatively large size and lack of staining. It appears in light microscope preparations as a clear area in contrast to the basophilic cytoplasm.
The nucleus is spherical and typically offset or eccentrically positioned. It is small, not much larger than the nucleus of the lymphocyte. It exhibits large clumps of peripheral heterochromatin alternating with clear areas of euchromatin. This arrangement has traditionally been described as resembling a cartwheel or analog clock face, with the heterochromatin resembling the spokes of the wheel or the numbers on a clock (Fig. 6.25b). The heterochromatic nucleus of the plasma cell is somewhat surprising given the cell’s function in synthesizing large amounts of protein. However, because the cells produce large amounts of only one type of protein—a specific antibody—only a small segment of the genome is exposed for transcription.
Eosinophils, monocytes, and neutrophils are also observed in connective tissue.