Which component is a noncellular part of the innate immune system?

Which component is a noncellular part of the innate immune system?

• A. Skin
• B. Phagocyte
• C. Complement system
• D. Antibody

Answer: (C)

The main idea here is recognizing a noncellular part of the innate immune system. The complement system fits this because it’s a group of soluble plasma proteins that circulate without cells and become activated to help fight infections. Once activated, they coat pathogens to make them easier to ingest (opsonization), recruit and activate other immune cells to the site of infection (chemotaxis and inflammation), and, in some cases, assemble a membrane attack complex that can directly lyse certain microbes. These proteins act in a cascade and operate in body fluids, independent of cellular components. In contrast, skin is a physical barrier made up of cells and extracellular tissue; phagocytes are cellular innate responders like neutrophils and macrophages; and antibodies are produced by B cells as part of the adaptive immune response.

The main idea here is recognizing a noncellular part of the innate immune system. The complement system fits this because it’s a group of soluble plasma proteins that circulate without cells and become activated to help fight infections. Once activated, they coat pathogens to make them easier to ingest (opsonization), recruit and activate other immune cells to the site of infection (chemotaxis and inflammation), and, in some cases, assemble a membrane attack complex that can directly lyse certain microbes. These proteins act in a cascade and operate in body fluids, independent of cellular components.

In contrast, skin is a physical barrier made up of cells and extracellular tissue; phagocytes are cellular innate responders like neutrophils and macrophages; and antibodies are produced by B cells as part of the adaptive immune response.