<P> They excrete two chemicals that aid in the body's defenses: histamine and heparin . Histamine is responsible for widening blood vessels and increasing the flow of blood to injured tissue . It also makes blood vessels more permeable so neutrophils and clotting proteins can get into connective tissue more easily . Heparin is an anticoagulant that inhibits blood clotting and promotes the movement of white blood cells into an area . Basophils can also release chemical signals that attract eosinophils and neutrophils to an infection site . </P> <P> Lymphocytes are much more common in the lymphatic system than in blood . Lymphocytes are distinguished by having a deeply staining nucleus that may be eccentric in location, and a relatively small amount of cytoplasm . Lymphocytes include: </P> <Ul> <Li> B cells make antibodies that can bind to pathogens, block pathogen invasion, activate the complement system, and enhance pathogen destruction . </Li> <Li> T cells: <Ul> <Li> CD4+ helper T cells: T cells displaying co-receptor CD4 are known as CD4+ T cells . These cells have T - cell receptors and CD4 molecules that, in combination, bind antigenic peptides presented on major histocompatibility complex (MHC) class II molecules on antigen - presenting cells . Helper T cells make cytokines and perform other functions that help coordinate the immune response . In HIV infection, these T cells are the main index to identify the individual's immune system integrity . </Li> <Li> CD8+ cytotoxic T cells: T cells displaying co-receptor CD8 are known as CD8+ T cells . These cells bind antigens presented on MHC I complex of virus - infected or tumour cells and kill them . Nearly all nucleated cells display MHC I . </Li> <Li> γδ T cells possess an alternative T cell receptor (different from the αβ TCR found on conventional CD4+ and CD8+ T cells). Found in tissue more commonly than in blood, γδ T cells share characteristics of helper T cells, cytotoxic T cells, and natural killer cells . </Li> </Ul> </Li> <Li> Natural killer cells are able to kill cells of the body that do not display MHC class I molecules, or display stress markers such as MHC class I polypeptide - related sequence A (MIC - A). Decreased expression of MHC class I and up - regulation of MIC - A can happen when cells are infected by a virus or become cancerous . </Li> </Ul> <Li> B cells make antibodies that can bind to pathogens, block pathogen invasion, activate the complement system, and enhance pathogen destruction . </Li>

Abnormal increase in the number of white blood cells