Lymphocyte

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Lymphocyte (stained)
Lymphocyte (stained)

Lymphocytes are a type of white blood cell (or "leukocyte"). They serve to help an organism combat infections. Lymphocytes are also known as lymph cells, lymphocysts, and lympholeukocytes.

We find in the blood small lymphocytes, medium lymphocyte, large lymphocyte and Lymphoblast (activated proliferating lymphocyte), which have the respective size of 7-8 um, 9-10 um, 10-12 um and 14-20 um.

The two main categories of lymphocytes are B cells (or "B lymphocytes") and T cells (or "T lymphocytes"). All lymphocyte originate from the bone marrow. B cells (which mature in the bone marrow) relates to the humoral mediated immune response and T cells(which mature in the thymus) mainly relates to the cell-mediated immune response. The humoral mediated immune response deals with extracellular pathogen(bacteria, viruses in plasma) whereas the cell-mediated immune response deals with intracellular pathogen(viruses infected cells, intracellular bacteria).

B cells are responsible for producing antibodies. They express a membrane bound antibody which can bind to a specific antigen. When bound the B cell will ingest the antigen-antibody complex, and express the antigen or part of it on its MHC class II with some complementary receptor. This will then recruit T helper cell to activate it so that the B lymphocyte becomes a short-lived antibody-producing plasma cell.

T cells subdivise into 2 major cell subsets, helper T lymphocyte(CD4+) and cytotoxic T lymphocyte along with Regulatory T cells, natural Killer T cells and γδ T cells. The cytotoxic T cells and γδ T cells can recognize foreign peptide presented through MHC class I and activate cell-mediated killing.

The helper T cells recognize peptide presented through MHC class II and are necessary for efficient activation of most immune cells through receptor binding and cytokines. Helper T cell subdivide into Th1 cell and Th2 cells. Th2 cells activate B cells and are an integrant part of the humoral immune response. Th1 cells mainly activate macrophages to digest intracellular pathogens and are an integrant part of the Th1 response.