Blood transfusion is which type of hypersensitivity reaction –

**Core Concept**
Type IV hypersensitivity reactions are cell-mediated immune responses that occur over a longer period, typically 48-72 hours after antigen exposure. In the context of blood transfusion, this reaction is characterized by the activation of T lymphocytes against the transfused blood cells, leading to the release of cytokines and the recruitment of immune cells to the transfusion site.

**Why the Correct Answer is Right**
The correct answer is that blood transfusion is a type of type IV hypersensitivity reaction. This is because the immune system recognizes the transfused blood cells as foreign and mounts a cellular immune response against them. The reaction involves the activation of T lymphocytes, which release cytokines such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), leading to the recruitment of immune cells, such as macrophages and neutrophils, to the transfusion site.

**Why Each Wrong Option is Incorrect**
**Option A:** Type I hypersensitivity reactions are immediate allergic reactions that involve the release of histamine from mast cells, which is not relevant to blood transfusion.

**Option B:** Type II hypersensitivity reactions involve the binding of antibodies to cell surface antigens, leading to cell destruction, which is not the primary mechanism of blood transfusion reactions.

**Option C:** Type III hypersensitivity reactions involve the formation of immune complexes, which deposit in tissues and cause inflammation, which is not the primary mechanism of blood transfusion reactions.

**Clinical Pearl / High-Yield Fact**
It's essential to note that type IV hypersensitivity reactions can be severe and may require prompt medical attention. These reactions can be mediated by the human leukocyte antigen (HLA) system, which is a group of genes that encode proteins responsible for the recognition of self and non-self cells.

**Correct Answer: C. Type IV hypersensitivity reaction.**