Study to Evaluate Adverse Events, Change in Disease Activity, and How Intravenously Infused ABBV-291 Moves Through the Body in Adult Participants With Non-Hodgkin's Lymphoma

Inclusion Criteria:

• For dose escalation (Part 1) only: Participants must have documented diagnosis ofB-cell malignancies including (but not limited to) the following, with histologybased on criteria established by the World Health Organization (WHO), and measurabledisease requiring treatment:

• Mantle cell lymphoma (MCL);

• Marginal zone lymphoma (MZL);

• Waldenstrom macroglobulinemia (WM);

• Diffuse large b-cell lymphoma (DLBCL) (including: germinal center B-cell type,activated B-cell type, primary cutaneous DLBCL [leg type], Epstein-Barrvirus-positive (EBV+) DLBCL [not otherwise specified], DLBCL associated withchronic inflammation, human herpesvirus 8-positive [HHV8+] DLBCL [not otherwisespecified], B cell lymphoma [unclassifiable] with features intermediate betweenDLBCL and classical Hodgkin lymphoma, high-grade B-cell lymphoma [not otherwisespecified], high-grade B-cell lymphoma [with MYC (avian myelocytomatosis viraloncogene homolog) and BCL2 and/or BCL6 rearrangements], DLBCL arising fromfollicular lymphoma [FL] [transformed FL]);

• FL Grades 1 to 3B;

• For dose expansion (Part 2) only: Participants must have documented diagnosis of oneof the following B-cell malignancies, with histology based on criteria establishedby the WHO, and measurable disease requiring treatment:

• Part 2a only: DLBCL (including: germinal center B-cell type, activated B-celltype, primary cutaneous DLBCL [leg type], EBV+ DLBCL [not otherwise specified],DLBCL associated with chronic inflammation, HHV8+ DLBCL [not otherwisespecified], B-cell lymphoma [unclassifiable] with features intermediate betweenDLBCL and classical Hodgkin lymphoma, high-grade B-cell lymphoma [not otherwisespecified], high-grade B-cell lymphoma [with MYC and BCL2 and/or BCL6rearrangements], DLBCL arising from FL [transformed FL]);

• Part 2b only: FL Grades 1 to 3B;

• Part 2c only: Mantle cell lymphoma;

• For all participants (Parts 1 and 2):

• Must be considered relapsed or refractory to, or intolerant of, at least 2 ormore prior lines of therapy known to provide a clinical benefit for theircondition, and for whom there is no appropriate locally available therapy knownto provide clinical benefit (e.g., standard chemotherapy or autologous stemcell transplantation [ASCT]).

• Indolent non-Hodkin's lymphoma (NHL) participants must meet relevant diseasespecific requirements for treatment (e.g., National Comprehensive CancerNetwork [NCCN], Groupe d'Etude des Lymphomes Folliculaires [GELF]).

• History of allogeneic stem cell transplantation must be stable off ofimmunosuppression for at least 3 months.

• For participants enrolled in backfill cohorts or at dose levels previouslycleared, subjects must provide consent to an on-treatment fresh tumor biopsyfrom the same tumor lesion as the baseline tumor tissue. This requirement maybe waived at the discretion of the contract research organization (CRO) MedicalMonitor if collecting a biopsy would place the subject at risk of harm or wouldrequire a technically complicated procedure based on tumor location as assessedby the investigator or could hinder a subject's ability to participate in thestudy.

• Previously treated with a CD79b-targeting therapy (e.g., CD79b monoclonalantibody) a core or excision tumor biopsy subsequent to the most recentCD79b-targeting therapy must be collected. Tumor biopsy requirements may bemodified by Sponsor during the study. This requirement may be waived at thediscretion of the contract research organization (CRO) Medical Monitor ifcollecting a biopsy would place the subject at risk of harm or would require atechnically complicated procedure based on tumor location as assessed by theinvestigator or could hinder a subject's ability to participate in the study.

• CD79b expression status will be assessed in all participants.

• Have an eastern cooperative oncology group (ECOG) Performance Status of 0 or 1.

• Laboratory values meeting the criteria in the protocol within the screening periodprior to the first dose of study drug (if multiple samples are drawn within thescreening period, the sample/result immediately prior to Cycle 1 Day 1 isapplicable).

• Availability of representative baseline tumor tissue (most recent archived tumortissue or fresh biopsy collected during screening phase) suitable forimmunohistochemistry (IHC) testing. This requirement may be waived at the discretionof the CRO Medical Monitor if collecting a biopsy at screening would place theparticipant at risk of harm or would require a technically complicated procedurebased on tumor location as assessed by the investigator or could hinder aparticipant's ability to participate in the study.

Exclusion Criteria:

• History of interstitial lung disease (ILD) or pneumonitis that required treatmentwith systemic steroids, or any evidence of active ILD or pneumonitis.

• Treatment with any of the following:

• Anticancer therapy including chemotherapy, radiotherapy, small molecule,investigational, and biologic agents within 14 days (or at least 5 half-lives,whichever is shorter), prior to the first dose of the study treatment;

• CD79b-directed agents (e.g., CD79b monoclonal antibody therapy) within 4 weeks (or at least 5 half-lives, whichever is shorter) prior to the first dose ofstudy treatment.

• Prior treatment with an antibody drug conjugate that consists of atopoisomerase I inhibitor.