Detection introduction
Flow cytometry is currently the standard method for detecting lymphocyte subsets. Under normal circumstances, various lymphocyte subsets: T cells, B cells, and natural killer cells (NK cells) will maintain a certain proportion and quantity for the immune system. When the number of lymphocyte subsets changes, pathological changes will occur, leading to immune-related diseases. Flow cytometry to analyze lymphocyte subsets' expression can help monitor and evaluate the immune status. Hematopoietic stem cells (HSCs) produce mature blood cells in the bone marrow, enhancing the immune system, self-renewal, cell proliferation, and cell differentiation of hematopoietic stem cells regulate the hematopoietic response in the human body.
 
 
Blood stem cells
Hematopoietic Stem Cell (HSCs) is the initial undifferentiated primitive cell of human blood cells, commonly found in bone marrow, liver, and umbilical cord blood, which can differentiate into all types of blood cells. Blood cells include lymphocytes, single Nuclei, red blood cells, platelets, etc. Hematopoietic stem cells will be reduced by factors such as free radicals, cell aging, and gene regulation. Studies have pointed out that appropriately reducing calorie intake and scavenging free radicals can rejuvenate hematopoietic stem cells. When hematopoietic stem cells are low, it will cause blood and immune-related diseases, such as anemia, decreased immune adaptation, autoimmune imbalance, susceptibility to infection, and poor vaccine adaptation.
 
T cells
T cells are essential to the human body's immune cells, which divide into 

• Cytotoxic T cells can directly kill abnormal or infected cells.
• Helper T cells, which can activate other immune cells
• Memory T cells can remember the antigens of bacteria, viruses, and cancer cells. Then quickly differentiate into resistant T cells when stimulated next time.

Low levels of T cells can increase infection risks such as mild flu, malnutrition, autoimmune disease (lupus), congenital immunodeficiency, certain cancers, or steroid and related drug use. The high-level long-term of T cells may refer to more severe diseases, such as acute lymphoma, rheumatoid arthritis, or myasthenia gravis.
 
Natural killer T cells (NKT)
NKT cells are developed from T cells with the characteristics of T cells and NK cells. Like NK cells, they belong to the first line of immune defense. They are activated after exposure to pathogens, then quickly secrete cytokines to strengthen the immune response without needing other cell stimulation and activation.
 
Natural killer (NK) cells
Natural killer (NK) cells serve as the first line of defense against many pathogens. It mainly quickly kills infected cells when pathogens first invade or eliminate tumor cells in the body. NK cells also assist in building adaptive immunity.
The low level of NK cells may refer to the susceptibility to virus infection, which usually occurs in tumors, immunodeficiency diseases, or uses of immunosuppressants. When the NK cell level is too high, it may be due to long-term use of interferon and its inducer or habitual abortion.