A Study in Participants With Moderate to Severe Psoriasis

The primary purpose for this study is to help answer the following research questions

* The safety of ixekizumab (LY2439821) and any side effects that might be associated with it.
* Whether ixekizumab can help participants with Psoriasis.
* How much ixekizumab should be given to participants.

The study is a Phase 2 study with 3 parts. Part A is a randomized, double-blind, placebo-controlled, parallel-group, dose-ranging design, Part B is an optional, open label extension design and Part C is an additional optional extension period with an open-label design(up to approximately 104 weeks). Approximately 125 participants will be randomized to 1 of 4 ixekizumab groups or to placebo (approximately 25 participants per group) in Part A. Participants will be evaluated for treatment efficacy and the primary endpoint will be evaluated at week 12. Between week 20 and week 32, participants with a less than 75% improvement in their Psoriasis Area and Severity Index (PASI) score compared to baseline will be eligible to begin Part B. Participants in Part B will receive subcutaneous (SC) injections of ixekizumab 120 milligrams (mg) every 4 weeks through week 236. Subsequent to an amendment on May 2012, administration changed to 80 mg every 4 weeks through Week 236. Participants in Part C may receive SC injections of ixekizumab 80 mg every 4 weeks for up to an additional 104 weeks through approximately week 340. Participants who complete Part A, Part B, and Part C will have a total study participation of approximately 344 weeks.

Eligibility Criteria:
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.