A 24-year-old woman presents with a 3-day history of fever and cough with rust-colored sputum. He had splenectomy 1 year ago. CXR film indicates consolidation of right lower lobe. Blood cultures are positive for alpha hemolytic gram-positive diplococci. Immunity to the causative organism is based on

**Core Concept**
The underlying principle being tested is the immune mechanism against **Streptococcus pneumoniae**, a common cause of community-acquired pneumonia, especially in individuals with compromised immunity such as those post-splenectomy. The patient's condition suggests a pneumococcal infection.

**Why the Correct Answer is Right**
The correct answer is based on the fact that immunity to **S. pneumoniae** is primarily mediated by **capsular polysaccharide antibodies**. These antibodies help in opsonization and subsequent phagocytosis of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Is incorrect because while cell-mediated immunity plays a role in defense against many pathogens, it is not the primary mechanism for protection against **S. pneumoniae**.
**Option B:** Incorrect as it refers to a different immune mechanism not primarily involved in protection against pneumococcal infections.
**Option D:** Incorrect because, although important for overall immune function, it is not specific to the mechanism of immunity against **S. pneumoniae**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that individuals without a spleen are more susceptible to infections with encapsulated organisms like **S. pneumoniae**, making vaccination against these pathogens crucial in such patients.

**Correct Answer:** D. Capsular polysaccharide antibodies

✓ Correct Answer: D. IgG antibodies to a surface acidic polysaccharide