Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) X-linked agammaglobulinemia ?
    ### Response:
    X-linked agammaglobulinemia (XLA) is a condition that affects the immune system and occurs almost exclusively in males. People with XLA have very few B cells, which are specialized white blood cells that help protect the body against infection. B cells can mature into the cells that produce special proteins called antibodies or immunoglobulins.
    