NK Cells Figure (Figure 17.16). Reprinted from OpenStax Microbiology (p. 706), by N. Parker, M. Shneegurt, A.-h. Thi Tu, B. M. Forester, and P. Lister, 2016, Rice University. Copyright 2016 by Rice University.

NK Cells Figure

One way NK cells can cause cell destruction is by expressing cytotoxic membrane proteins and cytokines that makes the target cell undergo apoptosis. Another way NK cells acts is by undergoing perforin mediated cytotoxicity using perforin and granzymes, which pokes holes in the target cells and enter the cell to stimulate apoptosis, respectively. NK cells have toxic granules but are not considered granulocytes, instead coming from lymphoid (rather than myeloid stem) cells.

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Mechanisms of Natural Killer Cells

Natural killer (NK) cells are large mononuclear lymphocytes that target abnormal cells, including virus-infected and cancerous cells, and lack either T cell receptors (TCRs) or surface immunoglobulin (Ig). Abnormal cells do not express the normal major histocompatibility complex (MHC). Normal MHC markers prevent NK cell activation, so abnormal MHC expression by abnormal cells recruits NK cells (signals cell distress). Binding of NK cells on target cells (and detect abnormal MHC levels) also leads to activation. After activation, NK cells can employ several mechanisms for target cell destruction.

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University of Notre Dame

Michigan State University

The common lymphoid progenitor matures into four mature categories of lymphocytes:

- B lymphocytes
- natural killer (NK) cells
- NK-T cells
- T lymphocytes

Common Lymphoid Progenitors

Some types of agranulocytes include:

- Lymphocytes (Specifically natural killer cells)
- Monocytes

Types of Agranulocytes

Chaplin, D. D. (2010). Overview of the immune response. Journal of Allergy and Clinical Immunology, 125(2), S3-S23.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2923430/
doi: 10.1016/j.jaci.2009.12.980

Overview of the Immune Response

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B lymphocytes, which are commonly called B cells, express surface B cell receptors for antigen immunoglobulins.  B cells are further differentiated based on which kinds of antigens they respond to and which type of antibodies they produce. There are two different types of B cells:
1. Plasma B cells
2. Memory B cells

B Lymphocytes

T lymphocytes, which are commonly called T cells, express T cell receptors (TCRs) on their surface.  These are transmembrane proteins that bind to processed antigens that are presented by antigen presenting cells (ATCs).  
There are three types of T cells, differentiated on the basis of their functions:
1. Cytotoxic T- cells
2. Helper T cells 
3. Regulatory T cells

T Lymphocytes

Natural Killer Cells

Natural Killer (NK)-T cells are lymphocytes that share the characteristics of both NK cells and T cells.  

Natural Killer T Cells

Monocytes are granulocytes that clear microbes via phagocytosis as well as other antigens that have been labeled by immunoglobulin (Ig).  These cells are activated after neutrophils are, and they have been found to present in places with chronic inflammation and infection.  Additionally, monocytes are involved in acute inflammation and causing inflammation in granulomatous conditions, which are chronic infections.  They are able to kill microbes by producing nitric oxide and regulate adaptive immunity by producing cytokines such as IL-12 and interferon (IFN)-γ.  Monocytes can differentiate into macrophages and dendritic cells.

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