T cell and B cell: The immunity powerhouse

Our immune system protects us from various diseases that could be life threatening. Through immune response immune system distinguishes the invaders/intruders/pathogens that attack our body system and cause disease. Immune system is made with special type of cellular network, protein, tissue and organs that function together to protect against infection/disease. The key organs of the immune system are spleen, lymph nodes, adenoid, tonsils and thymus. Bone marrow also plays a vital role in immunity. Immune system releases the leukocytes lymphocyte and phagocyte. The lymphocytes are B lymphocyte/B cell and T lymphocyte/T cell that fight against foreign invaders called antigens (such as virus, bacteria, cancer cells, body fluid of diseased or sick person). These lymphocytes reside in lymphoid organs and circulate all over the body via lymphatic vessel.

T cells/T lymphocytes are white blood cells (WBCs) bound to TCRs on their surface. These T cells are produced from bone marrow and then mature in thymus by positive and negative selection. Positive selection produces self MHC restricted T cell population, i.e. T cells whose TCRs can recognize self MHC molecules. Negative selection produces self tolerant T cell population. T cells play a key role in immunity, from immune surveillance to immune defense. TCRs recognize antigens bound to major histocompatibility complex (MHCI/MHCII) molecules.

B cells/B lymphocytes are also WBCs, that are produced from hematopoietic stem cells from bone marrow but they are thymus independent (they mature in spleen) and are bound to BCRs (membrane bound immunoglobulin that differentiates B cells from other lymphocytes). Activation of B cells may be in T cell dependent or T cell independent way. B cells produce antibodies (IgA, IgM, IgE, IgD, IgG) that bind with bacterial and viral antigen and kill them.  IgA antibodies are present in nose, digestive tract, ears, and eyes and protect body surfaces that are exposed to foreign substances. IgG antibodies are present in all body fluids and play a vital role against bacterial and viral infection. IgM are present in blood and lymph fluid. IgE antibodies are present in lungs, skin and mucous membranes and help to fight against pollens, fungal spores etc.

The following are the B cell and T cell classifications, along with their functionalities.

Figure 1: B cell classification

Figure 2: T cell classification

So the T cells and B cells are the powerhouse of immunity. Abnormality in any of these cell types results in life threatening immune disorders. In my next post I will describe some of them. 

References

1. Molecular Biology of the Cell. 4th edition: B Cells and Antibodies
2. St. Jude News Release, January 2015: Tumor suppressor protein plays key role in maintaining immune balance