Final answer:
The immune system primarily defends against invading organisms by generating antibodies that target and neutralize foreign cells, involving B cells and T cells as part of the adaptive immune response.
Step-by-step explanation:
Primary Means of Immune Defense
The primary means by which the immune system defends against invading organisms is to generate antibodies that bind to and neutralize foreign cells. This is part of the adaptive immune response.
Antigens are recognized by immune cells, and some of these cells, known as B cells, respond by producing antibodies. Antibodies bind specifically to antigens, marking pathogens for destruction. An important aspect of the immune system is tumor surveillance. This includes Killer T cells of the adaptive immune system, which identify and destroy tumor cells based on abnormal antigens.
Another element of the immune defense is the lymphatic system which makes leukocytes such as lymphocytes, crucial in the immune response. Lymphocytes recognize and help eradicate specific pathogens. These include T cells, which can become killer T cells that destroy infected or cancerous cells, and helper T cells that manage immune responses by directing other leukocytes. Alternatively, activated B cells turn into plasma cells that secrete antibodies, leading to the targeted destruction of pathogens or infected cells.