Immune System Development

As it matures, the fetus develops its immune organs. Lymphocytes are generated in the primary lymphoid organs: the bone marrow, thymus, and intestinal Peyer's patches.[2'3] T and B lymphocytes from these tissues then start circulating and eventually localize in peripheral or secondary immune tissues, where adaptive, or acquired, immune responses take place. Effective immune responses require immune cells to be localized in secondary lymphoid organs. The neonate requires time for its immune tissues to become mature. Because of their lack of immune system development, neonates are typically more susceptible than older animals to respiratory or intestinal infections. Probiotics have been developed to assist in maturing the intestinal immune tissues. Cytokines and chemokines serve as lymphoid tissue hormones and help to regulate immune system development and differentiation.[5]

Once a foreign antigen (i.e., an antigen produced from infectious microbes or vaccine preparations) enters the body, it is encountered by an APC a dendritic cell or macrophage and is transported to the local lymphoid bThe VIC IUIS Comparative Immunoglobulin Workshop [CIgW] Committee maintains a website on immunoglobulins, Fc receptors and their genes for veterinary species (

Table 1 Comparison of innate immunity to acquired, or adaptive, immunity

Innate immunity

Acquired (adaptive) immunity

Timing Specificity


Receptor Expression Effector proteins

Effector cells

Immediate maximal response Broad antigen specificity Antigen independent None

Fixed in genome Fixed for cell Antimicrobial peptides Acute phase proteins, complement Cytokines, chemokines Monocytes, macrophages Granulocytes, neutrophils Natural killer (NK) cells

Lag time before maximal response Narrow antigen specificity Antigen dependent Positive immunologic memory Enhanced recall responses

Encoded in germline, but rearrangement required

Regulated for each cell

Cytokines, chemokines

Immunoglobulin (Ig) antibodies

Perforins, granzymes

APC: dendritic cells, macrophages

T and B cells

Regulatory cell subsets organ, the lymph node, spleen, or specialized lymphoid tissues in the gut or respiratory sites. In these secondary lymphoid sites the foreign antigen is presented by APCs to T and B cells and an acquired immune response is initiated. Many immune cells excrete a broad range of cytokines and chemokines, such as the interferons (IFNs) and interleukins (ILs), that activate the immune system and encourage cells to migrate and localize to the area of infection or tumor growth.[5]

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