**Core Concept**
Lens antigens are proteins present in the lens of the eye, which can trigger an immune response in susceptible individuals. This is a fundamental concept in the field of immunology and ophthalmology, where the immune system's response to self-antigens is critical.

**Why the Correct Answer is Right**
The correct answer is B. Sequestrated antigen because lens antigens are not exposed to the immune system under normal circumstances. The lens is an immunologically privileged site, meaning it is isolated from the rest of the body by the blood-aqueous and blood-retinal barriers. This sequestration prevents the immune system from encountering lens antigens and mounting an immune response against them. However, in cases of lens injury or trauma, the blood-lens barrier can be disrupted, allowing the immune system to recognize lens antigens as foreign and triggering an immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** Neoantigen is incorrect because neoantigens are new or altered self-antigens that are not normally present in the body. While lens antigens can be altered in certain conditions, they are not typically considered neoantigens.

**Option C:** Altered antigen is incorrect because altered antigens refer to self-antigens that have been modified or altered in some way, making them more immunogenic. While lens antigens can be altered in certain conditions, they are not typically considered altered antigens in the context of immunological privilege.

**Option D:** Cross-reacting antigen is incorrect because cross-reacting antigens are foreign antigens that share similarities with self-antigens, leading to an immune response against both the foreign antigen and the self-antigen. Lens antigens are not typically considered cross-reacting antigens.

**Clinical Pearl / High-Yield Fact**
In the context of immunological privilege, the lens is not the only sequestrated antigen; other examples include the brain and the testes. Understanding the concept of immunological privilege is critical for diagnosing and managing conditions such as uveitis and autoimmune disorders.

**✓ Correct Answer: B. Sequestrated antigen**