Describe, in detail, clonal selection and activation of B cells.

During clonal selection, random mutations during clonal expansion induce the generation of B cells with an extended antibody-binding affinity for their antigens. The clonal selection theory may explain why secondary immune responses efficiently inhibit reinfection by the same pathogen.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.