All patients will receive the investigational medicinal product BP-SCIG 20%, solution for subcutaneous administration, manufactured by BIOPHARMA PLASMA LLC. A loading dose of IMP may be required (at the Investigator's decision): at least 0.2-0.5 g/kg (1.0-2.5 mL/kg) of body weight. This dose is distributed across several days, with a maximum daily dose of 0.1 to 0.15 g/kg. After achieving a steady-state (minimal) level of IgG, maintenance doses are administered at repeated intervals (approximately once a week) to achieve a cumulative monthly dose of 0.4-0.8 g/kg.

The clinical trial will consist of three periods, namely:

screening period: lasting up to 2 months, and involving three screening visits; treatment period (from Cycle 1 visit [the first 4 administrations of the IMP are to be performed in a hospital setting] during the 52 weeks to the last visit of Cycle 13).

follow-up period after completion of the last cycle of the treatment period, with a duration of 1 week.

Inclusion Criteria:

• a signed informed written consent of the patient/patient's parents or legally authorised representative to take part in the study;
• children from 0 to 18 years of age, of both sexes; or adult patients: males and females older than 18 years;
• a diagnosis of primary immunodeficiency with impaired antibody production document-supported according to the ESID criteria, requiring replacement therapy with immunoglobulins;
• patients with a body weight of no less than 9.1 kg;
• patients who received replacement therapy with intravenous immunoglobulin (IVIG) products at intervals from 21 to 28 days at the dose of 0.4-0.8 g/kg, for at least 4 months prior to the start of the study and have total serum IgG levels ≥5 g/L*, which is confirmed by the results of no less than 2 measurements performed immediately before the scheduled IVIG administration after signing the ICF, or patients who received replacement therapy with subcutaneous immunoglobulin (SCIG) products at the dose of 0.4-0.8 g/kg per month, for at least 4 months prior to the start of the study, and have total serum IgG levels ≥5 g/L*, based on the results of no less than 2 measurements performed immediately before the scheduled SCIG administration after signing the ICF; or patients with a newly established diagnosis of PID who have not yet received replacement therapy with IgG products (treatment-naïve patients) and have a serum IgG level ≤ 4 g/L;
• absence of episodes of serious bacterial infections (bacteraemia or sepsis, bacterial meningitis, osteomyelitis/septic arthritis, bacterial pneumonia, visceral abscess) while taking the previous medicinal product and absence of episodes of hospitalisation for at least 3 months prior to inclusion in the study and during the screening period (prior to the first administration of the IMP BP-SCIG 20%);
• a negative pregnancy test (in female patients with childbearing potential); readiness to use reliable methods of contraception throughout the study period;
• patient's ability, in the Investigator's judgement, to comply with all the requirements of the study protocol.

Exclusion Criteria:

• at the request of the patient/patient's parents/patient's legally authorised representative at any time and for any reason;
• patient developing serious and/or unexpected AEs/ARs during the study, which require discontinuation of the product;
• blood transfusions or transfusions of blood components and products, with the exception of the IMP;
• the need to use medicinal products disallowed as part of this study;
• systematic lack of patient compliance with the treatment regimen prescribed by the Investigator;
• systematic lack of patient compliance with the procedures specified in this protocol;
• exclusion based on screening results, including emergence of episodes of serious bacterial infections while receiving a previous IV immunoglobulin product and episodes of hospitalisation during the screening period (before the first administration of the IMP BP-SCIG 20%).
• Receiving the IMP BP-SCIG 20% for less than 4 months;
• Presence of episodes of serious bacterial infections (bacteraemia or sepsis, bacterial meningitis, osteomyelitis/septic arthritis, bacterial pneumonia, visceral abscess) at the time of investigation of PK parameters.