Lymphoctyes are one of many types of white blood cells produced in the bone marrow during hematopoiesis (The other white blood cells or “leukocytes” being neutrophils, monocytes, eosinophils and basophils). Lymphocytes are the central cell in immune function and count for about 24% of the body’s white blood cells. They occur in the blood and lymphoid organs such as the thymus, lymph nodes, spleen, and appendix as well as in the lymph (the colorless fluid in the lymphatic vessels that connect the lymph nodes in the body to each other and to the bloodstream).

Some regions of the vascular endothelium found in various secondary lymphoid organs such as lymph noids (but not the spleen) contain high endothelial venues (HEVs) which also express CAMs as well as vascular addressins (VAs). Recirculating naive lymphocytes have cell surface receptors (called homing receptors) that recognize particular VAs.

There are 2 major populations of lymphocytes; B lymphoctyes (B cells) and T lymphocytes (T cells). B and T lymphocytes that have not interacted with antigen rest in the G0 phase of the cell cycle. These resting lymphocytes have a short life span (from days to weeks) and undergo programmed cell death. Interaction with an antigen in the presence of certain cytokines induces these cells to enter the cell cycle to divide and also become blast cells called lymphoblasts which have a higher cytoplasm and more organellar complexity. These lymphoblasts proliferate and differentiate into effector cells or into memory cells. The effector cells have short life spans whereas the memory cells are long lived cells that reside in the G0 phase of the cell cycle.

T and B cells exhibit fundamental differences in antigen recognition. B cells recognize soluble antigen when it binds to their membrane bound antibody. Because B cells bind antigen that is free in solution, the epitopes they recognize tend to be highly accessible sites on the exposed surface of the immunogen which generally contain hydrophilic amino acids. T cells, on the other hand, recognize processed peptides associated with MHC molecules on the surface of antigen presenting cells and altered self cells.

Different lineages of lymphocytes can be distingushed by their expression of membrane molecules recognized by particular monoclonal antibodies. All of the monoclonal antibodies that react with a particular membrane molecule are grouped together as a cluster of differentiation (CD). Each new monoclonal antibody that recognizes a leukocyte membrane molecule is analyzed for whether it falls within a recognized CD designation. If it does not, it is given a new CD designation reflecting a new membrane molecule.