Lymphoid system is a part of major defense in the body. It consists of four parts:I. Lymphatic vesselsII. LymphIII. LymphocytesIV. Lymphoid organsLymphoid tissue is not one of the primary or the basic tissue types of the body but is a variety of connective tissue or, in the case of thymus, epithelial tissue. Lymphoid tissue is divided into central and peripheral types, which are either encapsulated or unencapsulated.

Central lymphoid tissue includes the thymus (encapsulated) and bone marrow (Bursal equivalent), which are the sites where development takes place. In the thymus, T cells; and in the bone marrow, B cells.Peripheral lymphoid tissue includes:1. Diffuse lymphatic tissue: accumulations of unencapsulated lymphocytes in the lamina of various organs, which have a constant close relationship to an epithelium including the tonsils, gut, respiratory tract, etc. 2. Lymph nodeslymphatic vessels; has both efferent and afferent lymphatics3. Lymphatic tissue of the spleenblood circulation