Lymphocytes
- Lymphocytes are the major cells of the adaptive immune system.
- Lymphocytes can be divided into three populations:
1) T cells,
2) B cells, and
3) natural killer, or NK, cells.
- B and T lymphocytes differentiate from their respective
lymphoid precursor cells and
- leave the bone marrow in a kind of cellular stasis-not
actively replicating like other somatic cells .
- These cells are said to be naive.
T- lymphocytes or T cells
- Lymphocytes destined to become T lymphocytes or T cells leave the bone marrow and
mature in the thymus gland.
- They can remain in the thymus, circulate in the blood, or reside in lymphoid organs such as the lymph nodes and spleen.
- Naïve T cells require a specific antigen to bind to a specific, membrane bound receptor (the T-cell receptor) to signal the continuation of replication.
- The now "activated" T cells differentiate into effector cells and memory cells.
1. EffectorT cells include the
- T-helper cells (TH) ,
- cytotoxic lymphocytes (CTLs),
- natural killer T cells, or
- T-regulatory cells,
- that respond to a myriad of antigens by producing and secreting cytokines .
- The cytokines secreted from effector T cells control specific responses directing theactions of other host cells.
2. Memory cells are quiescent, only to become activated T cells upon a subsequentexposure to the antigen.
- They provide a faster, heightened response.
B lymphocytes or B cells
- After B lymphocytes or B cells reach maturity within the bone marrow,
- they also circulate in the blood and
- disperse into various lymphoid organs where they await to become activated.
- The activated B cell becomes more ovoid.
- Its nuclear chromatin condenses, and numerous folds of endoplasmic reticulum become more visible.
- A mature, activated B cell is called a plasma cell.
- Plasma cells secrete large quantities of antibodies .
- Activated B cells also produce memory cells that are primed to act upon subsequent exposure to antigen
Natural killer (NK) cells
- Natural killer (NK) cells are a small population of large,
non phagocytic granular lymphocytes that play an important role in innate immunity.
- The major NK cell function is to attack and destroy
- malignant cells and
- cells infected with microorganisms, such as viruses and intracellular bacteria;
- however, NK cells do not recognize antigen.
- They recognize their targets in one of two ways
- First, NK cells survey somatic cells for a specific membrane-bound protein,known as the class I major histocompatibility complex (MHC) protein.
- Normal healthy host cells express class I MHC proteins on the cell surface, andNK cells bear a receptor that recognizes MHC class I.
- When NK cells encounter host cells with altered or missing class I MHC proteins, as sometimes happens during intracellular infection and oncogenesis, the aberrant cells are destroyed.
- NK cells also recognize and eliminate infected and malignant cells, expressing what are called stress, or shock, proteins on their surface.
- Finally, because NK cells have receptors for antibodies, they can also attack cells that are opsonized by antibodies. This process is called antibody dependent cell-mediated cytotoxicity (ADCC) and causes the death of the target cell.
- In all cases, attack by the NK cell results in the release of
pore-forming, perforin proteins and enzymes called granzymes.
- Together the perforins and granzymes induce target cell to commit suicide (apoptosis) .
- Unlike T and B cells, NK cells do not exhibit memory responses to target cells, thus they are considered part of the innate immune response.
