What anatomy is labeled as letter A in the image below? 
Definitions:
Agranulocyte
A type of white blood cell without granules in its cytoplasm, which includes lymphocytes and monocytes, important for the immune system's response to infection.
Lymphocyte
A type of white blood cell that is vital to the immune system, responsible for producing antibodies and other responses to pathogens.
Granulocyte
A type of white blood cell characterized by the presence of granules in its cytoplasm, playing a vital role in the body's immune response.
Neutrophil
A type of white blood cell important in the body's defense against infection and capable of engulfing and destroying pathogens.
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