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A living being, called organism (any living plant or animal) represents a characteristic level of organization; it shows responsiveness and adaptability, it acquires materials and energy; it reproduces, and shows growth, development, ageing and death. These attributes distinguish living from the non-living. In the living, the organizational levels consist of molecules and compounds, living cells, tissues, organ systems, individual (organism), [...]

The clonal selection theory explains the mechanism by which the body's immune system responds to the appearance of antigens present in or released from an agent of infection. The immune system includes many millions of different clones of B and T lymphocytes, each clone consisting of cells that are capable of re­sponding to the same form of antigen. Thus, the [...]

There are two ways in which the immune system re­sponds to infection. In the cellular immune response, there is a direct interaction between lymphocytes and invading fungi, bacteria, or virus-infected cells. In contrast, the humoral immune response acts princi­pally against extracellular phases of infection, that is, through the secretion of proteins called antibodies or immunoglobulin's into the blood plasma, lymph, [...]