The complement proteins are synthesized by hepatocytes but are also produced by monocytes, macrophages, and epithelial cells of the gastrointestinal and genitourinary tracts. These proteins circulate in the blood and extracellular fluid, primarily in an inactivated state. 

An epitope is the smallest part of an antibody to which an antigen molecule binds itself to. 

Apoptosis is one of the changes in the morphology of a host cell caused by viral infections referred to as cytopathic effects. 

The transfer of free DNA or genetic elements from one cell to another is known as transduction. 

Most viral infections of the skin enter the body through hair follicles and ducts of the sweat glands.

A complement-fixation test is an immunological medical test that can be used to identify pathogens like U. urealyticum, one of the smallest bacteria known to be a human pathogen. 

DNA immunization uses a genetically modified plasmid to stimulate immunologic memory.  These vaccines are known as subunit vaccines.

Warts are the result of common viral infections caused by the herpes simplex virus. 

Cytokines are small proteins considered to be chemical messengers that allow for communication between cells of the immune system.

What is one of the major differences between a virus and a bacteria?