Crab as a biomarker represents a group of proteins found in crustacean shellfish (crabs, shrimp, lobster). Biochemically, the best-known crab-associated allergens are muscle and structural proteins—tropomyosin, arginine kinase, and related peptides—classified as invertebrate pan-allergens because they are conserved across many arthropods. Tropomyosin was identified decades ago as a common trigger of immune recognition across shellfish and other invertebrates, which is why it is frequently measured in food‑related immune testing. Biologically these proteins serve structural and contractile roles in muscle and cellular scaffolding within the animal. When introduced to the human immune system via ingestion, fragments of these proteins can be recognized by antibody systems, producing measurable IgA and IgG responses that reflect exposure, sensitization patterns, or cross‑reactivity with other invertebrate proteins.

Elevated IgA or IgG reactivity to crab proteins may indicate prior exposure and an active immune recognition pattern to crustacean antigens; IgA elevations often point to mucosal (gut) immune activity while IgG elevations reflect broader systemic antibody responses. High signals can also arise from cross‑reactivity with related invertebrate proteins (for example, other shellfish, dust mites, or insects) rather than exclusive reactivity to crab itself. Low or undetectable crab antibody levels generally indicate no measurable immune response at the time of sampling, which can reflect little recent exposure, immune tolerance, or timing/technical factors. Interpretation is most informative when viewed alongside other Food Zoomer markers, clinical history, and IgA vs IgG patterns rather than in isolation.

Crab is included in the Food Zoomer to capture immune responses to common crustacean proteins and assess where shellfish-related antibodies fit within a person’s broader food‑specific pattern. By measuring crab at the protein and peptide level across IgA and IgG, the panel helps providers detect exposure, potential cross‑reactivity, and whether shellfish reactivity clusters with other dietary antigens—supporting more precise nutrition planning.