Multimorbidity presents challenges for research and health care. We investigated how immune-proteins influence multiple diseases to identify shared mechanisms and therapeutic opportunities. Using eight large plasma proteome GWAS, we identified cis-acting variants for 151 immune-proteins and applied cis-Mendelian randomization to assess associations with 64 diseases and biomarkers. Protein-disease communities were derived using a knowledge graph integrating multiplicity-corrected associations, druggability, indications, and tissue-trait associations. Replication was sought in independent pQTLs, with colocalization applied to replicated signals. Immune-proteins were frequently implicated in coronary heart disease, venous thromboembolism, atrial fibrillation, type 2 diabetes, and Parkinson's disease. Seven protein communities mapped to pathways including ficolin binding and antimicrobial peptides, enriched for proteins influencing multiple diseases. Several associations replicated with colocalization support, including APOE, CD163, and IL-6R across neurodegenerative, cardiometabolic, and inflammatory traits. We found strong genetic support for druggable immune-proteins with pleiotropic effects across common diseases, highlighting opportunities for indication expansion and therapeutic development.