Tag: immune system, its cells, mediators and antigens

IgD provides the activation signals to the B cells by acting as antigen receptors on the B cell surface. The B cells on being activated begins the immune defense mechanism of the body. So, the correct option is 'IgD'.

B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system by secreting antibodies. BCRs allow the B cell to bind to a specific antigen, against which it will initiate an antibody response. They provide immunity by producing antibodies.

So, the correct option is 'producing antibodies'.

T cells (thymus cells) and B cells ( bone marrow- or bursa-derived cells) are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity, whereas B cells are primarily responsible for humoral immunity (relating to antibodies).

T-lymphocytes are produced in bone marrow but differentiated in thymus glands. T lymphocytes play a central role in cell-mediated immunity. They 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.

Histiocytes destroy bacteria by phagocytosis. A histiocyte is an animal cell that is part of the mononuclear phagocyte system also known as the reticuloendothelial system or lymphoreticular system. The mononuclear phagocytic system is part of the organism's immune system. The histiocyte is a tissue macrophage or a dendritic cell.

WBCs fight against the infections. White blood cells account for only about 1 percent of the blood but their impact is significant. White blood cells, also called leukocytes, are essential for good health and protection against illness and disease. Think of white blood cells as your immunity cells. In a sense, they are continually at war. They flow through your bloodstream to battle viruses, bacteria, and other foreign invaders that threaten your health. When your body is in distress and a particular area is under attack, white blood cells rush in to help destroy the harmful substance and prevent illness. Thus, option A is correct. 

Associated lymphoid organs are composed of lymphoid tissue, and are the sites either of lymphocyte production, or of lymphocyte activation.