Topic overview

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Immune functions are distributed across several plasma proteins. C-reactive protein can bind pathogen-associated phosphocholine, promote opsonization, and activate the classical complement pathway. Complement also supports opsonization, chemotaxis, and pathogen lysis; mannose-binding lectin initiates its lectin pathway. Transferrin and lactoferrin restrict microbial access to iron, while acute-phase proteins participate in inflammatory responses. Immunoglobulins produced by B cells have a variable Fab region for antigen recognition and an Fc region that helps determine effector function. Their roles differ by class: IgM is an early, effective activator of complement; IgG supports long-term immunity and several effector activities; IgA protects mucosal surfaces; IgE participates in responses to parasites and type I hypersensitivity; and IgD acts as a B-cell antigen receptor. Together, these mechanisms connect recognition with pathogen control and clearance.