A Study in Participants With Moderate to Severe Psoriasis

Inclusion Criteria Common to Both Part A:

Participant must have active plaque psoriasis covering at least 10% body surface area and a PASI score of at least 12 at screening and at randomization.
Participant is a candidate for systemic therapy
Participant has a Static Physician's Global Assessment (sPGA) score of at least 3 at screening and at randomization

Inclusion Criterion Specific to Part B

Participant has completed the treatment period for part A (at least through week 20)

Inclusion Criterion Specific to Part C

Participant has completed the treatment period for part B

Exclusion Criteria Common to Part A, B and C:

Participant has pustular, erythrodermic and/or guttate forms of psoriasis
Participant has had a clinically significant flare of psoriasis during the 12 weeks prior to study entry
Participant has recently used any biologic agent/monoclonal antibody within the following washout periods: etanercept >28 days, infliximab or adalimumab >56 days, alefacept >60 days, ustekinumab >8 months, or any other biologic agent/monoclonal antibody >5 half-lives prior to baseline
Participant has received systemic psoriasis therapy (such as psoralen and ultraviolet A [PUVA] light therapy, cyclosporine, corticosteroids, methotrexate, oral retinoids, mycophenolate mofetil, thioguanine, hydroxyurea, sirolimus, azathioprine) or phototherapy (including ultraviolet B or self-treatment with tanning beds) within the previous 4 weeks; or had topical psoriasis treatment within the previous 2 weeks prior to randomization (exception: class 6 [mild, such as desonide] or 7 [least potent, such as hydrocortisone] topical steroids will be permitted for use limited to the face, axilla, and/or genitalia)
Participant has donated more than 500 mL of blood within the last month
Participant has another serious disorder or illness
Participant has suffered a serious bacterial infection (for example, pneumonia, and cellulitis) within the last 3 months
Participant has a history of uncontrolled high blood pressure
Participant has clinical laboratory test results at entry that are outside the normal reference range
Participant is currently participating in or were discontinued within the last 30 days from another clinical trial involving an investigational drug
Participant is a woman who is lactating or breast feeding
If a participant is a woman and could become pregnant during this study, she must talk to the study doctor about the birth control that you will use to avoid getting pregnant during the study
If a participant is post menopausal woman, she must be at least 45 years of age and have not menstruated for the last 12 months
If a participant is a woman between 40-45 years of age, test negative for pregnancy, and have not menstruated during the last 12 months only, she must have an additional blood test to see if you can participate
If the participant is male, he must agree to reduce the risk of female partner becoming pregnant during the study

Exclusion Criteria Specific to B:

If a participant experienced a serious adverse event during Part A considered possibly related to ixekizumab
If a participant experienced an adverse event during Part A that the study doctor believes continued ixekizumab treatment could cause harm to the participant.

Exclusion Criteria Specific to C:

If a participant experienced a Serious Adverse Event during Part B considered possibly related to ixekizumab
If a participant experienced an adverse event during Part B that the study doctor believes continued ixekizumab treatment could cause harm to the participant.