A lymphocyte is a type of white blood cell in the vertebrate immune system.There are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes depending on their appearance under microscope.
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
NK cells are a part of innate immune system and play a major role in defending the host from both tumors 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 lymphocytes, or T cells, are Thymus-derived and express the T cell receptor on their surface. They can be separated from other lymphocytes by use of monoclonal antibodies that recognize CD3, a component of the T cell receptor. The majority of circulating lymphocytes in the bloodstream are T cells.
B lymphocytes, or B cells, are Bone marrow-derived antibody-secreting cells that express surface immunoglobulin on their surfaces.
8. Specific immune responses can be differentiated into two major categories based on whether B or T cells are primarily involved.
- Humoral immunity- Immune responses involving antibody that is produced by mature B cells and plasma cells .
- Cellular immunity is mediated by T cells that secrete cytokines and signal effector cells to direct an overall cell-mediated immune response.
B1 and B2 cells?
Both are populations of B cells
- B1-develop earliest during ontogeny. Most express CD5. They are the source of “natural” antibodies are low affinity, IgM, and polyreactive, recognizing both common pathogens and autoantigens.
- B2-develop later in ontogeny and lack CD5 surface marker. Before encountering antigen, mature B2 cells coexpress IgM and IgD antibodies on their surface. With antigen stimulation, they secrete highly specific antibody (IgM, IgG, IgA, or IgE) within the secondary lymphoid tissue.
