Human Immunity and Blood

A T cell or T lymphocyte is a type of lymphocyte that plays a central role in cell-mediated immunity. T cells can be distinguished from other lymphocytes, such as B cells and natural killer cells, by the presence of a T-cell receptor on the cell surface. They are called T cells because they mature in the thymus. Most lymphocytes are short-lived, with an average life span of a week to a few months, but a few live for years, providing a pool of long-lived T cells (4-5 years) and B cells.

A T cell or T lymphocyte is a type of lymphocyte that plays a central role in cell-mediated immunity. T cells can be distinguished from other lymphocytes, such as B cells and natural killer cells, by the presence of a T-cell receptor on the cell surface. Lymphocytes such as B-cells and T-cells are produced in the bone marrow. They are called T- cells because they mature in the thymus.

• A T cell, or T lymphocyte, is a type of lymphocyte (a subtype of white blood cell) that plays a central role in cell-mediated immunity. T cells can be distinguished from other lymphocytes, such as B cells and natural killer cells, by the presence of a T-cell receptor on the cell surface. T-cells are originated from bone marrow and mature in the thymus gland. 
  
• There are 3 main types of T-cells in mammals. They are - cytotoxic cells ( destroy virus-infected cells and tumour cells), helper cells (assist other WBC in immunologic processes including maturation of B cells into plasma cells and memory B cells, and activation of cytotoxic T cells and macrophages and suppressor cells (stop  T cell-mediated immunity toward the end of an immune reaction).

Antigenic determinants (epitopes) are the specific parts of an antigen recognized by the immune system. Both antibodies (secreted or B-cell receptors) and T-cell receptors are capable of binding to these determinants.

• B cell activation occurs in the secondary lymphoid organs, such as the spleen and lymph nodes. 
• After B cells mature in the bone marrow, they migrate through the blood secondary lymphoid organs to which receive a constant supply of antigen through circulating lymph.
•  In birds, the bursa of Fabricius is the site of hematopoiesis, a specialized organ that is necessary for B cell development in birds. 
• Hence, B-lymphocytes differentiate and mature in Bone marrow and Bursa of Fabricius. 
• So, the correct answer is 'Both A and B'.
  

• Perforin is a pore-forming cytolytic protein found in the granules of cytotoxic T lymphocytes (CTL) and Natural Killer cells (NK cells). 
• Upon degranulation, perforin binds to the target cell's plasma membrane, and oligomerises in a Ca2+ dependent manner to form pores on the target cell.
• Hence, Cytotoxic T-cells are similar to natural killer cells in the secretion of Perforins.
•  So, the correct answer is 'Perforins'.

Lymphocytes are kind of white blood cells present in the bone marrow. ADA (adenosine deaminase) is an enzyme that is present in lymphocytes and is very important for the immune system to function. Its primary function in humans is the development and maintenance of the immune system.

T cells are so called because they are predominantly produced in the medullary region of thymus. They recognise  foreign particles (antigen) by a surface expressed, highly variable, T cell receptor (TCR). There are two major types of T cells: the helper T cell and the cytotoxic T cell. As the names suggest helper T cells ‘help’ other cells of the immune system, whilst cytotoxic T cells kill virally infected cells and tumours. T cells with CD4 molecules bind peptides displayed on MHC II molecules. As discussed earlier, such peptides are derived from phagocytized sources, and thus these T cells interact with only with certain special, phagocytic cells. 

Plasma cells are transported by the blood plasma and the lymphatic system. Plasma cells originate at the bone marrow and B cells differentiate into plasma cells that produce antibody molecules closely modeled after the receptors and requires interactions with Helper T cells.

Plasma cells are transported by the blood plasma and the lymphatic system. They originate in the bone marrow: B cells differentiate into plasma cells that produce antibody molecules closely modeled after the receptors of the precursor B cell.

T cells must be activated by interacting with a professional APC presenting an antigen with their T cell receptor recognizes before they can divide and perform their function. For this, they must interact with a macrophage.

The three major types of lymphocyte are T cells, B cells and natural killer (NK) cells. Lymphocytes can be identified by their large nucleus. T helper cells(T_H cells) assist other white blood cells in immunologic processes, including maturation of B cells into plasma cells and memory B cells and activation of cytotoxic  T cells and macrophages These cells are also known as CD4⁺ T cells because they express the CD4  glycoprotein on their surfaces. Helper T cells become activated when they are presented with peptide antigens by MHC class two molecules, which are expressed on the surface of antigen- presenting cells (APCs).

B- cells are the bone marrow cells which are the major components of the immune response. B-cells are produced in the bone marrow, they migrate to the spleen and other secondary lymphoid tissues. B-cells are responsible for generating antibodies to different antigens.