- The normal adult human body contains on the or-
- der of a trillion (1012) lymphocytes.… Together, the
- thymus and marrow produce approximately 109
- mature lymphocytes each day, which are then
- released into the circulation.
- Tristram G. Parslow60(pp40-41)
Lymphocytes are the cells that carry out antigen-specific immune responses.60-62 The 2 main types are the T lymphocyte and the B lymphocyte, also called the T cell and the B cell. A hyphen does not appear in these terms, unless they are used adjectivally.
Historically, the letters T and B reflected the anatomic sites of maturation of the 2 groups of cells, the thymus and the bursa of Fabricius, respectively. (The bursa of Fabricius is an organ of birds.) Because in human adults B cells mature in the bone marrow, the letter B is sometimes taken as signifying bone marrow.
A third group of lymphocytes is known as natural killer cells, abbreviated NK cells.
In the context of B-lymphocyte development, the prefixes pre- and pro- are encountered; note hyphenation:
B-cell subsets are named in various ways, eg:
CD5+ B cells
B1 B cells
B-cell antigen receptors (BCRs) are membrane complexes of membrane immunoglobulins and the molecules Igα and Igβ (see 15.8.6, Immunoglobulins).
helper T cells: THcells
cytotoxic T cells: TC cells, also called cytotoxic lymphocytes (CTL)
Most helper T cells express the cell marker CD4, and most cytotoxic T cells express the cell marker CD8 (see 15.8.2, CD Cell Markers), giving rise to the following terms:
CD4 cells CD8 cells
When presence or absence of a marker on a T cell is emphasized, superscript plus or minus signs are used. Presence and absence of the CD4 and CD8 markers are often indicated by the terms positive and negative (eg, “double-positive lymphocyte”), as below:
a CD4 cell
a CD8 cell
a CD4 cell
a CD8 cell
a CD4 cell
a CD8 cell
Because other cells, eg, monocytes, may express CD4, authors should use terms more specific than “CD4 cells,” unless context has made clear which cells are referred to, eg:
CD4 lymphocyte count (not CD4 cell count)
Subtypes of helper T cells are as follows:
TH0 TH1 TH2 TH3
The theoretical helper T precursor to these subtypes is abbreviated:
T-cell receptors (TCRs) are protein complexes on the surface of T cells.63 The T-cell receptor-CD3 complex (abbreviated TCR-CD3) is a structure that recognizes antigen. Its subunits, or chains, are designated by Greek letters:
The α and β chains are also referred to as follows:
TCRα and TCRβ
Linked α and β chains and linked γ and δ chains result in these terms:
αβ T cell
γδ T cell
The γ, δ, ε, ζ, and η chains constitute the CD3 complex. The CD3 chains are also referred to individually and as dimers:
There are 2 subtypes of the γ chain:
The TCR protein has variable (V) and constant (C) regions or domains. The gene for TCRα is made up of variable (V), joining (J), and constant (C) segments, as is the β chain, which also has a diversity (D) segment. (These are analogous to the segments of the immunoglobulin genes; see 15.8.6, Immunoglobulins.) These segments may also be referred to as follows:
Vα Vβ Jα Jβ Dβ Cα Cβ
Subgroups (various nonallelic forms) of the V, D, or J segments are specified numerically, eg:
T-cell expression of a particular segment may be indicated by using a superscript plus sign:
T-Cell Receptor Gene Terminology.
Because the V, D, and J gene segments together encode the variable (V) region of the protein, it is unusual to refer to D or J regions of the protein.63
The V, D, and J gene segments are brought together by DNA rearrangement. Descriptive terms include the following:
V/J exon, segment, region, gene, recombination
for α or γ chain genes
V/D/J exon, segment, region, gene, recombination
for β or δ chains
of α and γ or β and δ chains
VDJ, V/D/J,V-D-J, variable-diversity-joining
Official Gene Terminology.
Official gene symbols for specific genes of the types discussed above are presented in 15.6.2, Genetics, Human Gene Nomenclature. The TCR genes begin with TR and use roman letters that correspond to the Greek letters of the TCR component chains, and they contain V, C, D, and J corresponding to the above terms. Like other immune genes, they may contain hyphens:
TRAC TRBC TRBV10-3 TRGC1 TRGJ TRDC