Dictionary Definition
lymphocyte n : an agranulocytic leukocyte that
normally makes up a quarter of the white blood cell count but
increases in the presence of infection [syn: lymph
cell]
User Contributed Dictionary
English
Noun
- cytology immunology A type of white blood cell or leukocyte that occurs in two forms; B-lymphocytes, which produce antibodies in the humoral immune response, and T-lymphocytes, which participates in the cell-mediated immune response.
Derived terms
Translations
type of white blood cell
- Czech: lymfocyt
- Icelandic: eitilfruma
Extensive Definition
A lymphocyte is a type of white
blood cell in the vertebrate immune
system. By their appearance under the light microscope, there
are two broad categories of lymphocytes, namely the large granular
lymphocytes and the small lymphocytes. Functionally distinct
subsets of lymphocytes correlate with their appearance. Most, but
not all large granular lymphocytes are more commonly known as the
natural
killer cells (NK cells). The small lymphocytes are the T cells and
B cells.
Lymphocytes play an important and integral role in the body's
defenses.
Quantity
An average human body contains about 1012 lymphoid cells, and the lymphoid tissue as a whole represents about 2% of the total body weight. It is 20-40% of all leucocytes.Normal reference
range = (1-5)x109 cells per L of blood.
Types of lymphocytes
Natural killer cells
NK cells are a part of innate immune system and play a major role in defending the host from both tumours and virally infected cells. NK cells distinguish infected cells and tumours from normal and uninfected cells by recognizing alterations in levels of a surface molecule called MHC (major histocompatibility complex) class I. NK cells are activated in response to a family of cytokines called interferons. Activated NK cells release cytotoxic (cell-killing) granules which then destroy the altered cells. They were named "natural killer" because of the initial notion that they do not require prior activation in order to kill cells which are missing MHC class I.T cells and B-cells
T cells and B-cells are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies). The function of T cells and B cells is to recognize specific “non-self” antigens, during a process known as antigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In response to pathogens some T cells, called helper T cells produce cytokines that direct the immune response whilst other T cells, called cytotoxic T cells, produce toxic granules that induce the death of pathogen infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal these memory cells will “remember” each specific pathogen encountered, and are able to mount a strong response if the pathogen is detected again.Lymphocyte development
Mammalian stem cells
differentiate
into several kinds of blood cell within the bone marrow.
This process is called haematopoiesis. All
lymphocytes originate, during this process, from a common lymphoid
progenitor before differentiating into their distinct lymphocyte
types. The differentiation of lymphocytes follows various pathways
in a hierarchical fashion as well as in a more plastic fashion. The
formation of lymphocytes is known as lymphopoiesis. B cells
migrate to the spleen and mature into B lymphocytes, while T cells
migrate to and mature in a distinct organ, called the thymus. Following maturation, the
lymphocytes enter the circulation and peripheral lymphoid
organs (e.g the spleen and lymph nodes)
where they survey for invading pathogens and/or tumour
cells.
The lymphocytes involved in adaptive immunity
(i.e. B and T cells) differentiate further after exposure to an
antigen; they form
effector and memory lymphocytes. Effector lymphocytes function to
eliminate the antigen, either by releasing antibodies (in the case
of B cells), cytotoxic granules (cytotoxic
T cells) or by signaling to other cells of the immune system
(helper T
cells). Memory cells
remain in the peripheral tissues and circulation for an extended
time ready to respond to the same antigen upon future
exposure.
They live weeks to several years
| width="15%" | LYMPHOCYTE CLASS | width="40%" |
FUNCTION OF LYMPHOCYTE | width="10%" | PROPORTION | width="35%" |
PHENOTYPIC MARKER(S) |- |NK cells |Lysis
of virally infected cells and tumour cells |7% (2-13%) |CD16 CD56 but not CD3 |- |Helper T
cells |Release cytokines and growth factors that regulate other
immune cells |46% (28-59%) |TCRαβ,
CD3 and
CD4 |-
|Cytotoxic
T cells |Lysis of virally infected cells, tumour cells and
allografts |19% (13-32%) |TCRαβ,
CD3 and
CD8 |-
|γδ
T cells |Immunoregulation and cytotoxicity | |TCRγδ
and CD3 |-
|B
cells''' |Secretion of antibodies |23% (18-47%) |MHC class
II, CD19
and CD21
|}
In the circulatory
system they move from lymph node to
lymph node. This contrasts with macrophages, which are
rather stationary in the nodes.
Lymphocytes and disease
A lymphocyte count is usually part of a peripheral complete blood cell count and is expressed as percentage of lymphocytes to total white blood cells counted. An increase in lymphocytes is usually a sign of a viral infection (in some rare cases, leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person). A general increase in the number of lymphocytes is known as lymphocytosis whereas a decrease is lymphocytopenia.A decrease in lymphocytes occurs when the
human immunodeficiency virus (HIV) hijacks and destroys T cells
(specifically, the CD4+ subgroup of T
lymphocytes). Without the key defense that these T cells provide,
the body becomes susceptible to opportunistic
infections that otherwise would not affect healthy people. The
extent of HIV progression is typically determined by measuring the
percentage of CD4+ T cells in the patient's blood. The effects of
other viruses or
lymphocyte disorders can also often be estimated by counting the
numbers of lymphocytes present in the blood.