Which of the following is NOT true about T cells?
A. Natural killer cells (NK) are exclusively differentiated T cells
B. T cells act in situations where antigens are embedded in cell membranes or are inside host cells and thus are inaccessible to antibodies
C. T cells develop from lymphoid stem cells in bone marrow and mature in the thymus
D. Cell-mediated immunity is primarily carried out by T cells
The correct answer and explanation is :
The correct answer is A. Natural killer cells (NK) are exclusively differentiated T cells.
Explanation:
Natural killer (NK) cells are not differentiated T cells, and thus statement A is incorrect. While both T cells and NK cells are part of the immune system and play crucial roles in immunity, they are distinct in their origin, function, and mechanisms of action.
- Natural Killer Cells (NK):
- NK cells are a type of lymphocyte, but they do not come from the same lineage as T cells. They are classified as part of the innate immune system, and they function to detect and destroy infected or cancerous cells without the need for prior antigen exposure or activation.
- Unlike T cells, which require antigen presentation and recognition, NK cells recognize stressed or infected cells through a different set of receptors, especially recognizing cells that lack “self” markers like MHC class I proteins.
- NK cells are not a result of differentiation from T cells. They are derived from a different progenitor lineage within the bone marrow.
- Other Statements:
- B. T cells act in situations where antigens are embedded in cell membranes or are inside host cells and thus are inaccessible to antibodies:
This statement is true. T cells play a crucial role in recognizing antigens that are not freely circulating in the body but are instead embedded within cell membranes or hidden inside cells. This is why T cells are vital in situations such as viral infections, where antigens are present on infected cells, and antibodies cannot easily reach them. - C. T cells develop from lymphoid stem cells in bone marrow and mature in the thymus:
This statement is true. T cells originate in the bone marrow from lymphoid progenitor cells but mature in the thymus. During their maturation in the thymus, they undergo processes like positive and negative selection to ensure that they can respond to foreign antigens without attacking the body’s own tissues. - D. Cell-mediated immunity is primarily carried out by T cells:
This statement is true. Cell-mediated immunity, which involves the direct attack of infected cells or pathogens, is mainly carried out by T cells. This includes helper T cells (CD4+) that coordinate immune responses and cytotoxic T cells (CD8+) that directly kill infected or cancerous cells.
Thus, the statement that NK cells are differentiated T cells is not true, and A is the correct answer.