The objective of this chapter is to introduce the lymphoid tissue. Organs in which lymphocytes are produced and present in aggregations are called lymphoid organs. Types of lymphoid tissue are primary lymphoid tissue and secondary lymphoid organs. The lymphocytes are not directly exposed to antigens so that no lymphoid follicle is formed. Lymphocytes are agranulocytes. Being a kind of white blood corpuscles, their aggregation in spleen is termed as white pulp. Sinusoids are filled with blood. Red blood corpuscles form the major component of blood. Hence the sinusoids in spleen are termed as red pulp.