Cells of Lymphoid lineage include T, B, and NK(natural killer)  cells, and among these B and T cells are the part of Adaptive immune response while NK cells are the part of innate immune responses. These cells(B and T) contains a large nucleus and a thin layer of cytoplasm.

B cells

B cells also are known as B lymphocyte. These cells are the type of leucocytes. B cells have B Cell receptors on its surface which can bind to a specific antigen and adaptive response is generated against the antigen. B cell receptors are membrane-bound immunoglobulin. Once the antigen is bound to BCR, B cells differentiate into the plasma cell and the memory cell. Plasma cell secretes soluble immunoglobins. 

Every B cell posses approximately 10⁵  BCR on the plasma membrane. Each of the B cells produces a single type of antibody, each one of them having a unique antigen-binding site. 

Plasma cells

Plasma cells are highly specialized for the secretion of antibodies. A single Plasma cell is capable of secreting hundreds and thousand of antibodies per second. Plasma cells do not divide and have a life span of 1-2 weeks. Plasma cells have large ER (endoplasmic reticulum) and GC (Golgi complex) these cells are only activated once B cell differentiated into plasma cells and this allows plasma cells to produce antibodies for specific antigens. These cells respond to the signals coming from T cells during an attack to pathogen and then continue to produce antibodies.

Memory B cells

The other type of cell in which B cells differentiate into is Memory B cells. These cells have a long life and these cells allow the body to respond rapidly in case of a re-exposed infection.  

T cells

T cell is a type of leucocyte and known as T lymphocytes.  T cells are originated from Bone marrow and become mature in the Thymus.  T cells differentiated and multiply into cytotoxic T cells, and helper T cells. In B cells BCR is present and in the case of T cells TCR is present which is T cell receptor

T cell receptor only recognizes antigens that are bound to the cell membrane proteins known as ad MHC (Major Histocompatibility Complex).  MHC molecules are classified into two classes.

• MHC I (present on all nucleated cells)
• MHC II (present on Antigen-presenting cells such as Macrophage, Dendritic cells, and B Cells).

1. 1. Helper T cells

   • Helper T cell receptor binds to the class II MHC molecule, which is accompanied by a glycoprotein called CD4.
   • Helper T cells release cytokines. The main functions of the helper T cell to help other cells to activate other cells of the immune system. That’s why they are known as T helper cells. T helper cells are further classified into 5 types:

• T_H1 regulates the immune response to intracellular pathogens.
• T_H2 regulates the response to many extracellular pathogens.
• T_H17 secrete IL17, play important role in cell-mediated immunity, and possibly helps in the defense mechanism against fungi.
• T_FH T follicular, have an important role in humoral immunity and regulates B-cell development in germinal centers.
• T_REG regulatory T cells a unique capacity to inhibit an immune response in the case of autoimmune disorders.

1. 2. cytotoxic T Cells

• Cytotoxic T cell receptor binds to the class I MHC molecule, which must be accompanied by a glycoprotein called CD8, which binds to the constant portion of the class I MHC molecule. CD8 are proteins expressed by T lymphocytes.
• cytotoxic t cells release the cytotoxins perforin, granzymes, and granulysin. These cytotoxins trigger the caspase and cascade, which eventually leads to program cell death/apoptosis.
• The ratio of CD4 and CD8 cells is approx. 2:1. Any change in this ratio often indicates the immunodeficiency disease, autoimmune disease, and disorders.

1. References
2. 1. Kuby Immunology by Thomas J. Kindt, Barbara A. Osborne, Richard Goldsby.
3. 2. Brock Biology of Microorganisms, 13th Edition by Madigan, Martinko, Bender, Buckley, Stahl.