**Question:** All are affected in Graft-Versus host reaction except?

A. Lymphocytes
B. Neutrophils
C. Platelets
D. Erythrocytes

**Core Concept:** Graft-versus-host disease (GVHD) is a complication that can occur after allogeneic hematopoietic stem cell transplantation (HSCT) or bone marrow transplantation. It is a condition where the immune system of the recipient recognizes the transplanted donor cells as foreign, leading to tissue damage and inflammation. GVHD affects various organs and systems, including the skin, liver, gastrointestinal tract, and lungs.

**Why the Correct Answer is Right:** Dendritic cells, also known as antigen-presenting cells, are crucial in activating and regulating the immune response. During GVHD, donor dendritic cells present antigens to the recipient's T-cells, leading to an immune response against the recipient's own cells. Erythrocytes (red blood cells) are not involved in this process since they do not express antigens that can be recognized by the donor immune cells.

**Why Each Wrong Option is Incorrect:**

A. Lymphocytes: Although lymphocytes are involved in the immune response against foreign antigens, they are not the primary focus of GVHD. Erythrocytes are more relevant to the correct answer.

B. Neutrophils: Neutrophils are granulocytes involved in the host's immune response against pathogens. They do not play a significant role in GVHD, making them less relevant than erythrocytes.

C. Platelets: Platelets are involved in hemostasis and do not directly contribute to the immune response in GVHD. They are less relevant than erythrocytes.

**Clinical Pearl:** The understanding of Graft-versus-host disease helps in counseling transplant patients regarding potential complications and selecting appropriate prophylactic measures to minimize the risk of GVHD. Awareness of the involved cellular components and their roles in the immune response is crucial for proper diagnosis and management of GVHD.