Immunology | Immune system| Immunity

                       Lymphocytes are a kind of agranulocytes (WBCs)of blood. These form the main cells involved in immune system.These are formed by liver and thymus in the foetus and by the bone marrow and lymph node in adults.These form about 30% of leukocytes of blood.

There are two classes of lymphocytes.:

B Lymphocyte

B lymphocytes mature within the bone marrow;when they leave it,each expresses a unique antigen-binding receptor on its membrane.This antigen -binding or B-cell receptor is a membrane-bound antibody molecule.Antibodies are glycoproteins that consist of two identical heavy polypeptide chains and two light polypeptide chains.Each heavy chain is joined with a light chain by disulphide bonds,and additional disulphide bonds hold the two pairs of heavy and light chains form a cleft within which antigen binds. 

               When a naive B cell (one that has not previously encountered antigen)first encounters the antigen that matches its membrane-bound antibody, the binding of the antigen to the antibody causes the cell to divide rapidly;its progeny differentiate into memory B cells and effector B cells called plasma cells.Memory cells have longer life than the naive cells,and they express the same membrane-bound antibody in a form that can be secreted and have little or no membrane bound antibody.Although plasma cells live for only a few days,they secrete enormous amounts of antibody during this time.It has been estimated that a single plasma cell can secrete more than 2000 molecules of antibody per second.Secreted antibodies are major effector molecules of humoral immunity.

A plasma cell

There are thousands of antigen-specific B-cells in the body.Though even the 'capacity' to produce specific antibody even during development and differentiation,but these produce antibody only when stimulated by an antigen.Once B-lymphocytes are stimulated by an antigen by contact,they synthesize RNA,divide rapidly and differentiate into the RER-rich histologically distinctive plasma cells.

A group of plasma cells is known as a clone.These plasma cells produce antibodies at a high rate of about 2,000 molecules per second.These antibodies circulate in the lymph to fight the antigen so forming humoral immune system(humor means body fluid).The B-lymphocytes are short-lived and are replaced by new cells from the bone marrow after every few days.

T Lymphocyte

T lymphocytes also arise in the bone marrow,Unlike B cells,which mature within the bone marrow,T cells migrate to the thymus gland to mature.During its maturation within the thymus,the T cells comes to express a unique antigen-binding molecule,called the T-cell receptor,on its membrane.Unlike membrane-bound antibodies on B cells,which can recognize antigen alone,T-cell receptors can recognize only antigen that is bound to cell-membrane proteins called Major Histo-compatibility Complex(MHC)molecules .MHC molecules that function in this recognition event,which is termed"antigen presentation" are  polymorphic (genetically diverse)glycoproteins found on cell membranes.There are two Major types of MHC molecules: Class I MHC molecules,which are expressed by nearly all nucleated cells of vertebrate species,consist of heavy chain linked to a small invariant protein called Beta 2 microglobulin.Class II MHC molecules,which consist of an alpha and beta glycoprotein chain,are expressed only by antigen-presenting ells.When a naive T cells encounters antigen combined with MHC molecule on a cell,the T cell proliferate and differentiates into memory T cells and various effector T cells.
MODE OF ACTION OF T-CELLS.  

 

Like B-lymphocytes,T cells are also antigen specific,as each T-cell recognizes a specific antigen and different types of t-cells are stimulated by different types of antigens.When a T-cell is stimulated by specific antigen,this immunologically competent T-lymphocyte divides rapidly to form a cloneT-cells,called lymphoblasts.In a clone of T-cells(collectively called committed T-cells),though the cells are morphologically similar but they are divided in three classes(collectively called effector cells)on the basis of their functions:
* Killer T-cells.These migrate to the site of infection and release lymphokines(which attract the macrophages by chemo taxis),lymphotoxins(which kill the foreign cells by secreting cytolysin or perforin which damages the cell membrane of microbes)and interferon(which inactivates the viruses).So the phagocytic activity is greatly enhanced at the site of infection.These also kill the cancer cells.

*Helper T-cells.These are most numerous of T-cells(about 75% of T-cells).These secrete helper factors which stimulate B-cells to produce more antibodies and enhance the activity of killer T-cells.These also produce a variety of factor,called cytokines,which include lymphokines,interleukin,monokines,and interferons.Some of these stimulate the growth and proliferation of B-cells and are called B-cell growth factors.Interleukin-2 cytotoxin stimulates the growth and proliferation of cytotoxic T-cells.

* Suppressor T-cells.These secrete suppressive factors which suppress the whole immune system to protect own body cells from its attack,called immune tolerance.Some of these change into memory or primed cells.These memory cells are stored in spleen and lymph nodes which produce the antibodies spontaneously when the same antigen invade the body.The memory cells also produce more effector cells and memory cells.Some of the memory cells lost for life.These T-cells survive for about 4 or 5 years or even more.