There are three major types of cells in the lymphoid tissues of the body. One category, the , contains immunocompetent cells that are able to destroy or immobilize foreign substances in the body; these foreign substances are called . The two varieties in this group are and . The other class of protective cells are called . These defend the body by engulfing, or , foreign substances. The third cellular group are the , fibroblast-like cells that form the stroma or framework of the lymphoid organs. Lymphoid tissue is specifically classified as a type of connective tissue called . When lymphoid tissue is loosely packed, it is called ; when it is tightly packed into spherical masses, the arrangement is said to be .