I. THE INNATE IMMUNE SYSTEM

D. EARLY INDUCED INNATE IMMUNITY

5. Phagocytosis

a. An Overview of Phagocytic Defense

Fundamental Statements for this Learning Object:

1. Phagocytosis is the primary method used by the body to remove free microorganisms in the blood and tissue fluids.
2. An inflammatory response to injury and/or infection allows phagocytes to leave the bloodstream, enter the tissue, and go to the site of infection or injury.
3. Microorganisms entering lymph nodules found in the respiratory, gastrointestinal, and genitourinary tract can be phagocytosed by fixed macrophages and dendritic cells and presented to B-lymphocytes and T-lymphocytes to initiate adaptive immune responses.
4. Tissue fluid picks up microbes in the tissue, enters the lymph vessels as lymph, and carries the microbes to regional lymph nodes where they are filtered out and phagocytosed by fixed macrophages and dendritic cells and presented to the circulating B-lymphocytes and T-lymphocytes to initiate adaptive immune responses.
5. Dendritic cells located throughout the epithelium of the skin, the respiratory tract, and the gastrointestinal tract phagocytize microbes, enter lymph vessels, and carry the microbes to regional lymph nodes where the dendritic cells present antigens associated with the microbes to the ever changing populations of naive T-lymphocytes.
6. Blood carries microorganisms to the spleen where they are filtered out and phagocytosed by fixed macrophages and dendritic cells and presented to the circulating B-lymphocytes and T-lymphocytes to initiate adaptive immune responses.
7. There are also specialized macrophages and dendritic cells located in the brain (microglia), lungs (alveolar macrophages), liver (Kupffer cells), kidneys (mesangial cells), bones (osteoclasts), and the gastrointestinal tract (peritoneal macrophages.

 

LEARNING OBJECTIVES FOR THIS SECTION

D. Early Induced Innate Immunity

Early induced innate immunity begins 4 - 96 hours after exposure to an infectious agent and involves the recruitment of defense cells as a result of pathogen-associated molecular patterns or PAMPs (def) binding to pattern-recognition receptors or PRRs (def). These recruited defense cells include:

• phagocytic cells: leukocytes such as neutrophils, eosinophils, and monocytes; tissue phagocytic cells in the tissue such as macrophages (def);
  
• cells that release inflammatory mediators: inflammatory cells in the tissue such as macrophages and mast cells (def); leukocytes such as basophils and eosinophils; and
  
• natural killer cells (NK cells (def)).

Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells. These unique microbial molecules are called pathogen-associated molecular patterns or PAMPs (def) and include LPS from the Gram-negative cell wall, peptidoglycan and lipotechoic acids from the Gram-positive cell wall, the sugar mannose (a terminal sugar common in microbial glycolipids and glycoproteins but rare in those of humans), bacterial and viral unmethylated CpG DNA, bacterial flagellin, the amino acid N-formylmethionine found in bacterial proteins, double-stranded and single-stranded RNA from viruses, and glucans from fungal cell walls. In addition, unique molecules displayed on stressed, injured, infected, or transformed human cells also be recognized as a part of innate immunity. These are often referred to as danger-associated molecular patterns or DAMPs.

Most body defense cells have pattern-recognition receptors or PRRs (def) for these common PAMPs (see Fig. 1) enabling an immediate response against the invading microorganism. Pathogen-associated molecular patterns can also be recognized by a series of soluble pattern-recognition receptors in the blood that function as opsonins and initiate the complement pathways. In all, the innate immune system is thought to recognize approximately 10³ of these microbial molecular patterns.

We will now take a closer phagocytosis, beginning with an overview of phagocytic defense.

5. Phagocytosis

a. An Overview of Phagocytic Defense

Phagocytic cells include neutrophils, eosinophils, monocytes, macrophages, dendritic cells, and B-lymphocytes.

Phagocytosis is the primary method used by the body to remove free microorganisms in the blood and tissue fluids. The body's phagocytic cells are able to encounter these microorganisms in a variety of ways:

a. Infection or tissue injury stimulates mast cells, basophils, and other cells to release vasodilators (def) to initiate the inflammatory response (discussed later in Unit 4). Vasodilation results in increased capillary permeability, enabling phagocytic white blood cells such as neutrophils, monocytes, and eosinophils - as well as other leukocytes - to enter the tissue around the injured site. The leukocytes are then chemotactically attracted to the area of infection. In other words, inflammation allows phagocytes to enter the tissue and go to the site of infection. Neutrophils are the first to appear and are later replaced by macrophage.

For More Information: Leukocytes from Unit 5

For More Information: Macrophages and Dendritic Cells from Unit 5

 

b. Lymph nodules are unencapsulated masses of lymphoid tissue containing fixed macrophages and ever changing populations of B-lymphocytes (def) and T-lymphocytes (def). They are located in the respiratory tract, the liver, and the gastrointestinal tract and are collectively referred to as mucosa-associated lymphoid tissue or MALT. Examples include the adenoids and tonsils in the respiratory tract and the Peyer's patches on the small intestines. Organisms entering these systems can be phagocytosed by fixed macrophages (def) and dendritic cells (def) and presented to B-lymphocytes and T-lymphocytes to initiate adaptive immune responses.

For More Information: B-Lymphocytes from Unit 6

For More Information: T4-Lymphocytes from Unit 6

For More Information: T8-Lymphocytes from Unit 6

c. Tissue fluid (def) picks up microbes and then enters the lymph vessels as lymph. Lymph vessels carry the lymph to regional lymph nodes (see Fig. 2). Lymph nodes contain many reticular fibers that support fixed macrophages and dendritic cells as well as ever changing populations of circulating B-lymphocytes (def) and T-lymphocytes (def). Microbes picked up by the lymph vessels are filtered out and phagocytosed in the lymph nodes by these fixed macrophages (def) and dendritic cells (def) and presented to the circulating B-lymphocytes (def) and T-lymphocytes to initiate adaptive immune responses. The lymph eventually enters the circulatory system at the heart to maintain the fluid volume of the circulation.

d. In addition, Langerhans' cells - immature dendritic cells (def) - are located throughout the epithelium of the skin, the respiratory tract, and the gastrointestinal tract where in their immature form they are attached by long cytoplasmic processes. Upon capturing antigens through pinocytosis and phagocytosis and becoming activated by proinflammatory cytokines, the dendritic cells detach from the epithelium, enter lymph vessels, and are carried to regional lymph nodes. By the time they enter the lymph nodes, they have matured and are now able to present antigen to the ever changing populations of naive T-lymphocytes (def) located in the cortex of the lymph nodes.

e. The spleen contains many reticular fibers that support fixed macrophages (def) and dendritic cells (def), as well as ever changing populations of circulating B-lymphocytes and T-lymphocytes. Blood carries microorganisms to the spleen where they are filtered out and phagocytosed by the fixed macrophages and dendritic cells and presented to the circulating B-lymphocytes (def) and T-lymphocytes (def) to initiate adaptive immune responses.

To view a diagram of the lymphatics system, see the Innerbody.com Webpage.

f. There are also specialized macrophages and dendritic cells located in the brain (microglia), lungs (alveolar macrophages), liver (Kupffer cells), kidneys (mesangial cells), bones (osteoclasts), and the gastrointestinal tract (peritoneal macrophages).

 

• QUIZ YOURSELF ON THIS SECTION

Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.