Natural killer (NK) cells, which develop from the same precursor cell as B and T cells, are named for their ability to kill certain types of target cells. They constitute about 5% to 10% of circulating lymphocytes. They do not mature in the thymus; however, during their development, they are genetically programmed to recognize transformed cells (i.e., cells infected with a virus or tumor cells). NK cells kill target cells in a similar fashion to that of cytotoxic CD8 T lymphocytes. After recognition of a transformed cell, they release perforins and granzymes (fragmentins), substances that create channels in the cell’s plasma membrane, which induces them to self-destruct (a process known as apoptosis). Their specific markers include CD16a, CD56, and CD94.
and are transported throughout the body, where they are dispersed in the connective tissue.
Lymphocytes undergo antigen-dependent activation in the secondary lymphatic organs.
Immunocompetent lymphocytes (together with plasma cells derived from B lymphocytes and with macrophages) organize around reticular cells and their reticular fibers to form the adult effector lymphatic tissues and organs (i.e., lymphatic nodules, lymph nodes, tonsils, and spleen). Within these secondary (peripheral) lymphatic organs, T and B lymphocytes undergo antigen-dependent activation into effector lymphocytes and memory cells.
Immune Responses to Antigens
Inflammation is the initial response to an antigen.
The initial reaction of the body to invasion by an antigen, either a foreign molecule or a pathogenic organism, is the nonspecific defense known as the inﬂammatory response.
The infammatory response may either sequester the antigen, physically digest it with enzymes secreted by neutrophils, or phagocytose and degrade the antigen in the cytoplasm of macrophages. Degradation of antigens by macrophages may lead to subsequent presentation of a portion of the antigen to immunocompetent lymphocytes to elicit a specifc immune response.
Specific immune responses are either primary or secondary.
When immunocompetent cells encounter a foreign antigen (e.g., antigen associated with pathogenic microorganisms, tissue transplants, or toxins), a specific immune response to the antigen is generated.
A primary immune response refers to the body’s first encounter with an antigen. This response is characterized by a lag period of several days before antibodies (mostly IgM) or specific lymphocytes directed against the invading antigen can be detected in the blood. The initial response to an antigen is initiated by only one or a few B lymphocytes that have been genetically programmed to respond to that specifc antigen. After this initial immune response, a few antigen-specifc B lymphocytes remain in circulation as memory cells.
The secondary immune response is usually more rapid and more intense (characterized by higher levels of  secreted antibodies, usually of the IgG class) than the primary