B-cells, T-cells and natural killer (NK) cells are the granular lymphocytes that make up the innate immune system. The agranular cells are the second most numerous type of white cell in the body next to neutrophils. More than 50% of leukocytes are neutrophils while up to 35% are lymphocytes. Though they have different capabilities and properties, microscopically, the lymphocytes appear similar to one another.
All of the lymphocytes originate in the bone marrow, and B and NK cells remain in the bone marrow where they mature. T-cells travel to the thymus, where they differentiate and mature. Lymphocytes recognize each other and various pathogens by the antibodies and antigens that lie on the surface of the cell's membrane. Natural killer cells do not have these identifying membrane markings.
The granular lymphocytes known as B-cells attach to T-cells at receptor sites and manufacture plasma cells that secrete antibodies. B-cells neutralize bacteria and viruses or they mark pathogenic cells with antibodies and antigens for destruction and phagocytosis by other cells. Research indicates that B-cells must be present for T-cell activation as the B-cells emit chemical signals that evoke a T-cell response. Scientists believe that B-cells also influence T-cell production and differentiation.
Cytotoxic, helper, and inflammatory are the three distinct types of granular lymphocytes known as T-cells. Cytotoxic T-cells attach to pathogens and emit chemical-containing granules that bore through the membranes of invading organisms. The cells then emit toxic substances into the pathogen, killing the cell. Helper T-cells emit cytokines, histamines, and other chemicals that deactivate the chemical signals secreted by other lymphocytes. This action regulates the inflammatory response and the number of immune defending cells arriving at a particular location.
Inflammatory T-cells produce chemical signals that result in vasodilatation, which increases blood flow and triggers the response of infection fighting lymphocytes. Certain inflammatory T-cells transform into memory cells, which remember and recognize pathogens after a first encounter. T-cells are also responsible for the physiological changes that occur during tissue rejection.
Natural killer cells are the largest of the granular lymphocytes and typically circulate through the blood, lungs, and spleen. Like T-cells, NK cells can secrete membrane perforating chemicals followed by cytotoxic substances. They especially target tumors and viruses. While many believed that these cells acted independently of other immune lymphocytes, research suggests otherwise. Studies indicate that dendritic cells activate an NK cell response by secreting chemicals or by attaching to the NK cell. NK cells can also limit an immune response by secreting cytokines.