Colored scanning electron micrograph (SEM) of a t-cell, a thumus- dervied lymphocyte. Lymphocytes, or common types of white blood cells, are involved in the immune response, acting to protect the body against infection. There are two main types of human lymphocytes: T-cells and B-cells. T-cells (thymus- derived) track down and destroy foreign bodies, while B-cells (bone marrow-derived) identify foreign micro-organisms and make antibodies. Magnification unknown.