[3] Out of the 46 family members in Fijen's study, the 9 who were affected were found to be more susceptible to diseases from the Neisseria genus.

[3] As a protein involved in the function of the immune system, no external changes in physiology or aberrant physical characteristics are expressed by individuals possessing a properdin deficiency.

[3][4] Due to a heightened susceptibility to Neisseria bacterium, individuals with properdin deficiency are far more likely to succumb to bacterial infection such as meningitis, resulting in inflammation of the brain and spinal cord, which causes severe headaches, fevers, and neck stiffness, and may result in further development of other meningococcal diseases and extreme complications such as sepsis.

[5] Individuals with properdin deficiency are also more likely to catch the sexually transmitted disease, gonorrhea, as it is also caused by Neisseria bacterium, resulting in swelling, itching, pain, and formation of pus on the mucous membranes, including, but not limited to, the genitals, mouth, and rectum.

[3] The typical tests for complement deficiencies, such as the measurement of C3 and C4, do not detect low levels of the absence of properdin.

[2] Properdin deficiency is caused by an X-linked recessive allele, meaning that the gene responsible is only present on the X chromosome.

[8] Type I can also be the result of a G-T transversion at position 3511 in exon 7, causing an amino acid substitution from gly271 to valine, which renders the protein non-functional.

[8] One of the mutations is a C-to-T transition at position 2124, located on exon 4, which converted an arginine to a tryptophan, resulting in an improper protein fold.