Which of the following is an example of a defense mechanism that belongs to the innate (nonspecific) immune system?

Introduction / Context:
The immune system has two major arms: innate (nonspecific) immunity and adaptive (specific) immunity. This question checks whether you can distinguish an innate immune defense from adaptive immune responses. Recognizing the difference is important for understanding how the body responds quickly to new infections and how it develops long term protection.

Given Data / Assumptions:
• Innate immunity provides a rapid, general defense against many pathogens. • Adaptive immunity is specific to particular antigens and involves B cells and T cells. • The question asks for a defense that clearly belongs to the innate system.

Concept / Approach:
Innate defenses include physical barriers (skin, mucous membranes), chemical barriers (acid, enzymes), and cellular defenses such as phagocytic cells and natural killer cells. These defenses do not require prior exposure to a pathogen and respond quickly. In contrast, adaptive immunity involves B lymphocytes that produce antibodies and T lymphocytes that mediate cellular responses, along with the development of immunological memory. Phagocytosis by neutrophils and macrophages is a classic example of innate immunity, as these cells engulf and destroy pathogens in a nonspecific manner.

Step-by-Step Solution:
Step 1: List innate immune defenses: physical barriers, inflammatory response, phagocytic cells, complement system, and certain cytokines. Step 2: List adaptive immune components: B cells, T cells, antibodies, and memory cells. Step 3: Recognize that phagocytosis is carried out primarily by neutrophils and macrophages and is part of the immediate, nonspecific response. Step 4: Identify the option that mentions phagocytosis by neutrophils and macrophages as an innate defense.

Verification / Alternative check:
Immunology textbooks categorize neutrophils and macrophages as innate immune cells that engulf a broad range of pathogens. They act early in infections, before specific antibodies are produced. On the other hand, B cells and T cells are always placed under adaptive immunity. This classification confirms that the phagocytosis option must be the correct innate defense.

Why Other Options Are Wrong:
Option B: Antibody production by B cells is a classic adaptive, humoral immune response that is specific to particular antigens. Option C: Plasma cells are activated B cells that secrete antibodies, which is also part of adaptive immunity. Option D: Cell mediated immunity via cytotoxic T cells is another branch of adaptive immunity, targeting infected or abnormal cells specifically. Option E: Immunological memory after vaccination is a hallmark of adaptive immunity and is not an innate mechanism.

Common Pitfalls:
Students often confuse fast responses with adaptive immunity because vaccinations and antibodies are frequently discussed. Another pitfall is to assume that any activity involving white blood cells is adaptive. To avoid this, remember that the key difference is specificity and memory. Phagocytic cells of innate immunity attack many different pathogens in the same way, without previous exposure, while adaptive cells are specific and remember particular antigens.

Final Answer:
An example of an innate immune defense is phagocytosis by neutrophils and macrophages.