An AE is any untoward medical event that occurs in a participant administered an investigational product, and it does not necessarily indicate only events with clear causal relationship with the relevant investigational product. A TEAE was defined as any event not present prior to administration of the study drug or any event already present that worsened in either severity or frequency following exposure to the study drug. Severity was graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) version 5.0. Severity scale ranges from Grade 1 (Mild) to Grade 5 (Death). Grade 1= Mild, Grade 2= Moderate, Grade 3= Severe, Grade 4= Life-threatening, and Grade 5= Death related to adverse event. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. For Parts B and C, treatment period wise AE data were presented.

Safety analysis set included all participants who received at least 1 dose of M254 or IVIg or placebo.

Number of participants with clinically significant laboratory abnormalities (chemistry, hematology, urinalysis and coagulation) were reported. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A.

Safety analysis set included all participants who received at least 1 dose of M254 or IVIg or placebo.

Number of participants with clinically significant abnormalities in vital signs (blood pressure [systolic blood pressure {SBP} and diastolic blood pressure {DBP}], pulse rate, respiratory rate, and body temperature) were reported. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A.

Safety analysis set included all participants who received at least 1 dose of M254 or IVIg or placebo.

Number of participants with clinically significant abnormalities in ECGs were reported. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A.

Safety analysis set included all participants who received at least 1 dose of M254 or IVIg or placebo.

Rmax is defined as the maximum observed response of M254 on platelet count. Baseline was the pre-dose sample. Data was planned to be collected and analyzed Part C only. For Part C, data were collected and analyzed on the pooled population per the study drug (M254 120 mg/kg and IVIg 1000 mg/kg) which the participants received.

The full analysis set consisted of all participants who received at least 1 dose of M254 or IVIg or placebo and for whom at least 1 post-infusion pharmacodynamics (PD) assessment was completed. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Change from baseline in Rmax of M254 in platelet count was reported. Rmax is defined as the maximum observed response of M254. Baseline was the predose sample. Therapeutic platelet count was defined as >=50*10^9 cells/L. Platelet response of >=20*10^9 cells/L was considered as increase from baseline. Data was planned to be collected and analyzed for Part C only. For Part C, data were collected and analyzed on the pooled population per the study drug (M254 120 mg/kg and IVIg 1000 mg/kg) which the participants received.

The full analysis set consisted of all participants who received at least 1 dose of M254 or IVIg or placebo and for whom at least 1 post-infusion PD assessment was completed. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

AUEC(0-Day 14) is defined as the area under effect curve of the change from baseline in platelet count from Day 0 to Day 14 of M254. Baseline was the predose sample. Data was planned to be collected and analyzed for Part C only. For Part C, data were collected and analyzed on the pooled population per the study drug (M254 120 mg/kg and IVIg 1000 mg/kg) which the participants received.

The full analysis set consisted of all participants who received at least 1 dose of M254 or IVIg or placebo and for whom at least 1 post-infusion PD assessment was completed. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

AUEC(0-Day 28) is defined as the area under effect curve of the change from baseline in platelet count from Day 0 to Day 28 of M254. Baseline was the predose sample. Data was planned to be collected and analyzed for Part C only. For Part C, data were collected and analyzed on the pooled population per the study drug (M254 120 mg/kg and IVIg 1000 mg/kg) the participants received.

The full analysis set consisted of all participants who received at least 1 dose of M254 or IVIg or placebo and for whom at least 1 post-infusion PD assessment was completed. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Number of participants with overall platelet response after M254 administration compared to IVIg were reported. Overall platelet response rate is defined as reaching the therapeutic platelet count. A therapeutic platelet count is defined as greater than or equal to (>=) 50*10^9 cells/liter (L) and an increase from baseline of >=20*10^9 cells/L. Data was planned to be collected and analyzed for Part C only. For Part C, data were collected and analyzed on the pooled population per the study drug (M254 120 mg/kg and IVIg 1000 mg/kg) the participants received.

The full analysis set consisted of all participants who were included in the safety set for whom at least 1 post-infusion PD assessment was completed. Here, N (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Cmax is defined as the maximum observed plasma concentration of M254. Baseline was the predose sample. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

Pharmacokinetic (PK) data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254.

Tmax is defined as the time to reach the maximum observed plasma concentration of M254. Baseline was the predose sample. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254.

AUC(0-Infinity) is defined as area under the plasma concentration-time curve from time 0 to infinite time of M254. Baseline was the predose sample. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

AUC(0-last) is defined as area under the plasma concentration-time curve from time 0 to time of the last quantifiable concentration of M254. AUC(0-last) is calculated by linear-linear trapezoidal summation. Baseline was the predose sample. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254.

t1/2 is defined as the time measured for the plasma concentration to decrease by 1 half to its original concentration of M254. Baseline was the predose sample. Data was planned to be collected and analyzed on pooled population of participants who received placebo in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Vz is defined as volume of distribution of M254 at terminal phase. Baseline was the predose sample. Data for this outcome measure was not planned for pooled placebo arm in Part A and thus no data was presented for placebo arm in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

CL is defined as clearance of M254, calculated as dose/AUC(0-infinity). Baseline was the predose sample. Data for this outcome measure was not planned for pooled placebo arm in Part A and thus no data was presented for placebo arm in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Mean residence time is the average time that drug dose remained in the body. Baseline was the predose sample. Data for this outcome measure was not planned for pooled placebo arm in Part A and thus no data was presented for placebo arm in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, 'N' (number of participants analyzed) indicates number of participants evaluable for this outcome measure.

Percentage of the estimated part for the calculation of AUC(0-infinity) (%AUCextra) of M254 were reported. Baseline was the predose sample. Data for this outcome measure was not planned for pooled placebo arm in Part A and thus no data was presented for placebo arm in Part A. In Part C, combined data of both arms of Group 1 is presented as planned.

PK data analysis set included all randomized participants who received at least 1 dose of M254 or IVIg or placebo with at least 4 evaluable data points adequate to create an evaluable plasma concentration profile of M254. Here, N (number of participants analyzed) indicates number of participants evaluable for this outcome measure.