**Question:** Which of the following is a mature defence mechanism:

A. Humoral immunity
B. Complement system
C. Acquired immunity
D. Innate immunity

**Correct Answer:** D. Innate immunity

**Core Concept:** Innate immunity is the first line of defense against pathogens and is a mature defense mechanism that is present from birth and does not require prior exposure to the pathogen to mount an effective response. It involves a range of non-specific mechanisms that are common to all cells and tissues of an organism, whereas adaptive immunity is a more specific response that develops gradually and requires exposure to a pathogen.

**Why the Correct Answer is Right:** Innate immunity is a mature defense mechanism because it is an immediate, non-specific response to pathogens. It is present in all living organisms, including humans, and is activated as soon as the pathogen is detected. The innate immune system includes components like physical barriers, chemical barriers, cellular responses, and inflammation.

**Why Each Wrong Option is Incorrect:**

A. Humoral immunity (Option A) and cell-mediated immunity (Option C) are components of adaptive immunity, which is a more specific response that develops gradually after an organism is exposed to a pathogen. These options are incorrect because they are part of the adaptive immune response, not a mature defense mechanism.

B. The complement system (Option B) is a part of adaptive immunity and not a mature defense mechanism. It is activated in response to specific antigens and is crucial for the activation of the adaptive immune response.

**Clinical Pearl:** Innate immunity is essential for the survival of an organism and is crucial in protecting the body from pathogens until adaptive immunity develops. The innate immune system can be targeted with medications to treat specific conditions, such as antihistamines for allergic reactions and corticosteroids for inflammation. Understanding the immune system, including innate immunity, is vital for healthcare professionals to effectively diagnose and treat patients with infectious diseases, autoimmune disorders, and other immune-related conditions.