Human Immunity and Blood

WBCs or the white blood cells are the types of blood cells that are main cells of the immune system. The immune system responds to a particular pathogen to give you active or acquired immunity. The surface of every cell is covered with molecules that give it a unique set of characteristics. These molecules are called antigens. Antigens are generally fragments of protein or carbohydrate molecules. There are millions of different antigens and each one has a unique shape that can be recognized by the white blood cells of your immune system. The white blood cells then produce antibodies to match the shape of the antigens. Thus, option B is correct.

Clonal selection is a process to determine how B or T cell recognizes an antigen that enters the body is selected from the per-existing cell pool of differing antigen specificities and its proliferation to generate a clonal cell population that eliminates the antigen.
Thus, the correct answer is option B.

Exogenous antigens are taken up by antigen-presenting cells and presented on MHC class II molecules to helper T-cells (CD4+). MHC class I molecules are typically involved in presenting endogenous antigens to cytotoxic T-cells.

The thymus is the primary lymphoid organ where lymphocytes originate and/or mature. T-lymphocytes originate from lymphoid progenitor cells in thymus via three developmental stages, namely pro, pre and immature T cells. The immature T cells mature in thymus only. B cell originates and matures in red bone marrow, fibrinogen is the clotting factor 1 in blood and phagocytes are the type of WBC. Thus, the correct answer is E.

Phagocytes are the component of the non specific immune response. The encounter of B cell with a specific antigen activates its clonal expansion which in turn gives rise to plasma cell and memory cells.  A plasma cell is a mature B cell that serves in mass production of antibodies against a specific antigen. Pathogens are the disease-causing organisms that carry antigens. Fibrinogen is involved in the clotting process. T cells are responsible for cell-mediated immunity. Thus, the correct answer is A.

MHC class II molecules bind peptides from antigens processed in organelles. These extracellular antigens, such as whole bacteria, engulfed by APCs via phagocytosis or endocytosis are enclosed in an intracellular vesicle. This vesicle containing peptides is fused with another vesicle containing MHC class II proteins. Upon fusion, the peptide is loaded onto MHC class II molecules, and the entire complex migrates to the cell membrane surface, where peptide-specific CD4 T cells recognize it. Thus the correct answer is option A.

Helper T-cells secrete signaling molecules called lymphokines to coordinate the immune response. Killer T-cells (cytotoxic T-cells) secrete perforins, which create pores in the membranes of target cells to induce lysis.

• 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. 
• The types of T-cells are killer cells (cytotoxic cells) which destroy virus-infected cells and tumor cells, and are also implicated in transplant rejection, helper T-cells which  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 and suppressor T-cells which are crucial for the maintenance of immunological tolerance.
  

Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones. Bone marrow is the primary site of new blood cell production or hematopoiesis. Bone marrow is considered as a primary lymphoid organ which is involved in the production and early clonal selection of lymphocyte tissues. Bone marrow is responsible for both the production of T cells and the production and maturation of B cells. 

The thymus is a specialized primary lymphoid organ of the immune system. The T-cells are produced from the bone marrow. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is the largest and most active during the neonatal and pre-adolescent periods.

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.