This is a randomized, single-blinded, placebo controlled, single ascending dose phase I study to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) and immunogenicity of KJ103 in healthy subjects.

This single ascending dose (SAD), randomized, single-blinded, placebo controlled study is the first study and it is designed to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of KJ103 in healthy subjects after a single intravenous dose. It will include up to 34 healthy subjects in up to five dose groups.

Inclusion Criteria:

1. Male or female between the ages of 18 and 55 years, inclusive.
2. Male body weight ≥50kg, female body weight ≥45kg, body mass index (BMI) within 18 kg/m2to 35 kg/m2, inclusively.
3. Immunoglobulin (IgG) levels at screening is within the normal range.
4. Considered "healthy" by the investigator. Healthy status is defined by absence of evidence of any active or chronic disease following a detailed medical and surgical history, as well as a complete physical examination including vital signs, 12-lead ECG, hematology, biochemistry, and urinalysis.

Exclusion Criteria:

1. History of or diagnosis at screening of any clinically significant immunodeficiency including but not limited to immunoglobulin A deficiency.
2. History of significant cardiovascular, pulmonary, hematologic, neurological, psychiatric, endocrine, immunologic or dermatologic disease.
3. Any clinically significant illness in the 28 days prior to the first study drug administration.
4. Any history of tuberculosis.
5. Positive screening results to HIV Ag/Ab combo, syphilus, hepatitis A, hepatitis B surface antigen or hepatitis C virus tests.
6. Positive test result for alcohol and/or drugs of abuse at screening or prior to the first drug administration.
7. Current use of tobacco or nicotine-containing products exceeding 10 cigarettes per day or equivalent.
8. Received an investigational drug (or was using an investigational device at the time) within 30 days prior to screening, or at least 5 times the respective elimination half-life (if known), whichever is longer.
9. Female who is lactating.
10. Female who is pregnant according to the pregnancy test at screening or prior to the first study drug administration.