Read the given paragraph and answer the following questions.

Histidine is a semi-essential amino acid that must be obtained from dietary sources for optimal health since the body cannot produce enough on its own. It is a precursor to histamine, an essential neurotransmitter involved in various bodily functions. Histamine plays a role in the immune response, where antigens such as toxins, bacteria, and viruses stimulate antibody formation. Interleukin, a group of naturally occurring proteins, mediates cell communication and promotes the proliferation of white blood cells, aiding immune responses like inflammation. Immunologists classify tissues into lymphoid and non-lymphoid, with lymphocytes produced in primary lymphoid tissues and localizing in secondary lymphoid tissues.