A. Characteristics of Protozoa
Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. Although there are nearly 20,000 species of protozoa, relatively few cause disease. Most inhabit soil and water.
Protozoa reproduce asexually by the following means:
1. fission (def): One cell splits into two.
2. schizogony (def): A form of asexual reproduction characteristic of certain protozoa, including sporozoa, in which daughter cells are produced by multiple fission of the nucleus of the parasite followed by segmentation of the cytoplasm to form separate masses around each smaller nucleus.
3. budding (def): Buds form around a nucleus and pinch off of the parent cell.
Some protozoa also reproduce sexually by fusion of gametes (def) (see Fig. 1: The Life Cycle of Plasmodium).
The vegetative, reproducing, feeding form of a protozoan is called a trophozoite (def). Under certain conditions, some protozoa produce a protective form called a cyst (def) that enable them to survive harsh environments. Cysts allow some pathogens to survive outside their host.
B. The Role of Protozoan Cytoplasmic Membrane Components in Initiating Body Defense
Initiation of Innate Immunity
In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns or PAMPs (def). (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)
Components of protozoa that function as PAMPs include GPI-anchored proteins (GPI = Glycosylphosphatidylinositol) and mannose-rich glycans (short carbohydrate chains with the sugar mannose or fructose as the terminal sugar) that function as PAMPs. These mannose-rich glycans are common in microbial glycoproteins and glycolipids but rare in those of humans. These PAMPs bind to pattern-recognition receptors or PRRs (def) on a variety of defense cells of the body and triggers innate immune defenses such as inflammation (def), fever, and phagocytosis.
by Gary E. Kaiser, Ph.D. Last updated: August, 2019 |
Initiation of adaptive immunity
Proteins associated with protozoa function as antigens and initiate adaptive immunity. An antigen is defined as a substance that reacts with antibody molecules and antigen receptors on lymphocytes. An immunogen is an antigen that is recognized by the body as non-self and stimulates an adaptive immune response.
The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes (def) and T-lymphocytes (def) by means of epitope-specific receptor molecules having a shape complementary to that of the epitope. The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor and is actually an antibody molecule. The receptor on a T-lymphocyte is called a T-cell receptor (TCR). This will be discussed in greater detail in Unit 6.
We will now briefly look at some medically important protozoa classified into phyla based on their motility. Illustrations can be found in your Lab Manual in Lab 20.