All the cells involved in immunity originate from a common stem cell. The blood and lymph circulation systems include cellular and noncellular elements that are important for a functional immune system. A variety of leukocytes participate in immune responses. Describe the circulation of a leukocyte from the blood to the lymph and back to the blood. Immunogens are foreign molecules of sufficient size, complexity, and accessibility that are recognized by the immune system.
Epitopes are the individual molecular sites at which immune molecules interact with antigens.
Antibody epitopes are conformational antigen structures, whereas T cells epitopes are generally linear. Distinguish among immunogens, antigens, and haptens. The phagocytes are the first cells involved in protection against invading pathogens after physical and chemical barriers have been breached. They are capable of destroying most, but not all, pathogens. In many cases, phagocytes also process and present molecules to antigen-specific leukocytes.