The lymphocytic lineage consists of a succession of cells which, starting with the committed stem cell, leads to the production of the small lymphocyte. Morphologically, it comprises the lymphoblast, the large lymphocyte, and the small lymphocyte.
In stained blood smears (Wright or Giemsa staining), lymphoblasts (15-20 μm in size) have a round or oval nucleus. The cytoplasm is sharply delineated, scanty, and basophilic. Large lymphocytes (9-15 μm in size) have a large nucleus, usually eccentric, the cytoplasm is scant, moderately basophil, or a light blue, and contains azurophilic granules (lysosomes). Small lymphocytes (6-9 μm in size) have a round, indented nucleus, and scanty cytoplasm often barely visible.
The maturation and differentiation of lymphocytes diverges at the stage of pre-Lymphoblasts or lymphoblasts into two sub lineages, which are not distinguishable by morphological criteria: part of the lymphocytes differentiates in the thymus (thymus-derived, or T lymphocytes), the other part in the bone marrow (bone-marrow, or B lymphocytes).
T Lymphocytes originate in the fetal liver on about the eleventh day of gestation. These large basophilic blast-like cells gradually accumulate by migration into the thymus and begin to proliferate. Ultra-structurally, these basophilic cells are large and have the typical morphology of lymphoblasts, with copious dense cytoplasm filled with ribosomes but relatively few other organelles and a large nucleus that contains prominent nucleoli.
The proliferation of these cells results in the production of typical small thymic cortical lymphocytes. By other than morphological studies, the intrathymic small lymphocytes can be subdivided into two populations:
A major group accounts for about 90% of the total; they are small, dense, and short lived, located predominantly in the cortex, and express the TL antigen, high levels of Thy-l antigen but low levels of H-2 antigens (these antigens are cell-surface markers which can be detected by serological methods), and are cortisone sensitive.
The minor population which appears to be the functionally active group, is also composed of small lymphocytes, though they are generally larger and less dense than the major group. This minor population, located mainly in the thymic medulla, is cortisone resistant and differs antigenically in being TL negative and expressing high levels of H-2 antigens but low levels of Thy-l antigen.
The high rate of intrathymic mitotic activity and the differentiation of the stem cells are independent of antigenic stimuli and are probably under some as yet poorly understood thymic epithelial cell influence. There are indications that the induction of mitotic activity is in some way due to close contact between lymphocytes and cortical epithelial cells.
B lymphocytes arise in the liver at about 14 days of gestation (in adults in the bone marrow); these cells are larger than normal B lymphocytes, they divide rapidly and synthesize small amounts of monomeric IgM.
These cells, called pre-B cells, contain cytoplasmic IgM but do not bear on their external surface the stable immunoglobulin receptors which characterize B lymphocytes. Pre-B cells lack most of the surface components characteristic of the majority of mature B lymphocytes: functional surface antibody receptors, Fc-receptors for IgG, and receptors for C3, a complement component.
It is also unlikely that pre-B cells will be found to express surface receptors for T helper factors. The absence of these functional receptors serves to protect them from influences exerted by contact with antigens, antigen-antibody complexes, and activated C3.
In this stage, clonal diversification has occurred, i.e., selective expression of immunoglobulin genes present either on the paternal or on the maternal chromosomes, selective expression of either kappa (K) or lambda (λ) light chains, and expression of different sets of genes encoding light- and heavy-chain variable regions (VL and VH )’ The expression of V gene products in pre-B cells implies that the genetic translocation event has occurred by this stage of differentiation.
By this time, each small pre-B cell is ready to become on sIgM+ B lymphocyte, its antibody specificity is determined. Expression of sIgM (secrete IgM) antibodies signals the onset of B lymphocyte differentiation.
The gradual acquisition of receptors for activated C3 and IgG and of other classes of surface Ig appears. Within a given clone of B lymphocytes, all cells are committed to the synthesis of antibodies of identical specificity, but some members become genetically programmed to convert from IgM antibody synthesis to synthesis of IgG, IgA, or IgE antibodies.
At this stage, B lymphocytes express sIgD in addition to either IgM, IgA, or IgE. After mature sIgD+ B lymphocytes are triggered by antigens (or mitogens), sIgD expression is rapidly reduced to low or undetectable levels.
When B cells with all of these receptors are stimulated by the appropriate antigens and T helper cells, they may respond with division giving rise to memory B lymphocytes and with further differentiation into mature plasma cells.
Thus memory cells are generated by antigen-driven expansion of B cell clones. Contrary to red blood cells and platelets (see below) whose entire functional life takes place in the blood, and unlike mature granulocytes, which leave the blood vessels without entering them again, lymphocytes leave the circulation and return to it many times in the course of their life.
They leave the blood stream predominantly in the lymphoid spaces of the tissue, are taken up by the lymphatics, and after traversing one or more lymph nodes return to the blood stream by way of the thoracic duct. The life span is believed to be 10-20 days for nonstimulated lymphocytes; committed lymphocytes may live several months or even years.
The peripheral blood of man contains about 3,000 lymphocytes per mm3, 70%-80% are T lymphocytes and 15%-20% are B lymphocytes, the rest being difficult to classify. About 85% of the lymphocytes in thoracic duct are T lymphocytes, about 80% in lymph nodes, and about 35% of the lymphocytes of the spleen are T cells.
These are the final stage of fully differentiated B lymphocytes. They are usually oval, the nucleus is almost, always situated at one pole. The arrangement of the chromatin is characteristic: the chromocenters from seven to nine large blocks of approximately polygonal outline, resembling a tortoise shell or a “cartwheel” picture.
In stained smear preparations, the cytoplasma is intensely basophilic and its ultramarine color identifies them immediately. Under normal conditions, they are rarely found in the blood. Lymph nodes and particularly their medullary cords are rich in plasmocytes, they are also present in the spleen, bone marrow, and the intestine. Plasma cells develop from stimulated B lymphocytes within 2-3 days and probably die within a few days.