So named as they were initially found in the Bursa of Fabricius, B cells produce antibodies and are associated with humoral immunity (T cells are part of the cell-mediated immune response), and are an integral part of the adaptive immune system. They represent 20-30% of circulating lymphocytes.

Basophils are derived from the same stem cell line as mast cells and while they are similar to mast cells, they are not identical (they are thought by some to be immature mast cells). They are the least common of all the leukocytes, are a similar size to neutrophils and eosinophils and are characterised by the large number of basophilic staining granules in their cytoplasm. They are present in the circulation but rarely found in tissue.

Mature B cells that undergo stimulation by an antigen undergo class switching, and differentiate into either plasma or memory cells. In the paracortex region of the lymph node binding to MHC II in the presence of IL-4 produced by the CD4+ T cells (TH2 type) causes the B cells to differentiate; most will become plasma cells, however a small number will become memory cells. Follicular dendritic cells present in the germinal centers of peripheral lymphoid organs can absorb intact antigen onto their surface to present to B cells to stimulate differentiation.

Blood cells develop in the bone marrow from a common stem cell in the process known as haematopoiesis. Once mature, cells are divided into groups that reflect their morphological and functional characteristics including the erythrocytes, or red blood cells, the granulocytes, the agranulocytes and the megakaryocytes.

It is important that all aspects of haemostasis can be independently evaluated. This will help to identify the phase affected and to pinpoint what the abnormality is. There are tests available to assess primary haemostasis, secondary haemostasis and fibrinolysis.

Complement is so called because it complements the function of antibody. It is a triggered enzyme cascade and there are more than 20 different proteins in the complement cascades, with most being enzymes or pro-enzymes. It can be activated by both the innate and adaptive immune systems and is one of the main innate protective mechanisms of invertebrates. Due to its destructive potential the complement system is heavily regulated but when activated it works largely by forming pore complexes as well as triggering acute inflammation and by promoting phagocytosis by macrophages and neutrophils.

Eosinophils are a similar size to neutrophils, have a bilobed nucleus and are characterised by the large eosinophilic granules present in their cytoplasm. Produced in the bone marrow they migrate into circulation briefly before moving into tissue where they survive for around six hours. The proportion of eosinophils circulating depends on the state of the animal. Normally numbers are very low but will rise considerably during a parasitic infection or allergic reaction.

Also known as red blood cells (RBCs). Erythrocytes deliver oxygen to, and remove carbon dioxide from tissues. Erythrocytes are derived from the stem cell (CFU-GEMM) and formed in a process known as erythropoiesis.

Erythrocytes contain no nucleus and are thus only produced from stem cells. During the fetal stage production is in both the liver and spleen however production is transferred to the bone marrow (red marrow) in the final stages of gestation. Initially erythropoiesis occurs in all bones, however after puberty production is limited to membranous bones (ribs, vertebrae, pelvic bones etc.) as the long bones contain adipose tissue in place of red marrow.

Transfer of passive immunity in the bovine neonate occurs solely through maternal colostrum. This is in contrast to humans where placental transfer of immunity via specific Fc receptors is the predominant mechanism. However, inadequate transfer of immunity is a relatively commonly diagnosed problem affecting young stock. This is particularly the case in modern Holstein dairy herds, where large milk yields dilute the antibody concentration in the colostrum meaning relatively more must be consumed to have the same immunity transferred. Due to the lower yields and possibly other genetic reasons, this tends to be less of a problem in suckler herds.

Haematopoiesis is also known as haemopoiesis or hemopoiesis and describes the process of blood cell formation. All blood cells are derived from the initial pluripotent stem cell (PPSC) which gives rise to colony forming units (CFUs). These CFUs further differentiate to give rise to their final stage of development where they become the various forms of blood cells or those cells which migrate from the circulation into tissues, such as mast cells and macrophages.

Heterophils are the most abundant granulocyte in most avian species and occur alongside lymphocytes, monocytes, eosinophils and basophils in avian blood. These cells are also found in some reptile and mammalian species.

Lysozyme is one of the major bactericidal agents in secretions and particularly helps to protect vulnerable sites such as the eyes and nasal passages. The lysoszyme exerts bactericidal effects by digesting bacterial cell walls. The complement system is a group of about 30 proteins within the body fluids of all vertebrates and some invertebrates. The main functions of complement are to promote phagocytosis or causes lysis of an invading organism.

IgA is present at low concentrations in plasma, and has minimal function inside the body. However, it is specially adapted for action at mucosal surfaces and as such, is present in high concentrations in mucosal secretions and in colostrum (and milk). In many species (dogs, cats and pigs), it is the major antibody in

IgD is present in ruminants, pigs, dogs and rodents but has not been identified in horses, cats, rabbits and chickens. It is mainly expressed on the surface of B-cells i.e. it is never secreted.

Unlike IgM, IgG and IgA, IgE does not function as a soluble antibody, with binding to Fc? receptors required before it can bind to the target antigen, and is found in low levels in blood plasma. Like IgA, it is produced by plasma cells and is mainly localised to mucosal surfaces.

IgG is the major antibody in blood plasma, and constitutes at least 80% of all antibodies in the body. It is the smallest immunoglobulin, so can readily leave the blood plasma and enter tissues. They can also cross the placenta, providing adaptive immunity to the foetus when the mother is under attack. IgG is also present in breast milk.

IgM is the primordial antibody and, although a monomer, is secreted as a pentamer (five monomers joined by disulphide bonds with two monomers joined by a J chain). This gives it ten identical antigen binding sites although IgM usually has relatively low affinity for its antigen. Its heavy chain is type mu (ľ).

Also called antibodies, Immunoglobulins (Ig) are the soluble form of B cell receptors (BCR) released by plasma cells after they have been activated. Immunoglobulins have to bind to a number of different antigens in a variety of environments and as such there are several different immunoglobulin classes. Each class has an optimum environment of action.

Both the innate and adaptive immune systems use receptors to recognise foreign organisms. The innate immune system uses pattern recognition receptors which acts as an early warning system. The adaptive immune response is highly specific for each organism, as B and T cells have specialist surface immunoglobulin receptors which detect specific antigens on foreign pathogens. The Innate immune system is the body's first barrier of defence to infection. It relies on an older, more generic, and faster acting set of tools than the adaptive system. While the adaptive system is essential for a specific response to infection, it is ultimately the innate system that conquers foreign attackers through means of phagocytosis.