"Many dairy operations keep cow’s milk for sale and use formula to raise calves. The farmer noted that calves raised on formula needed to be treated for diarrhea more frequently than calves allowed to nurse." Does this scenario describe active or passive immunity? Is it natural or acquired? How is immunity gained? What is the difference between humoral and cellular response by B lymphocytes and T lymphocytes?

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The situation presented describes passive immunity. Let's look at this in more detail.

Passive immunity happens when antibodies are passed from mother to offspring. This can happen before birth or through breastfeeding. Here, the mother cow seems to have developed an immunity against whatever would cause diarrhea, and she has passed that along to her calf through the milk. The calves not nursing from their mothers are not receiving the same antibodies, and they are, therefore, more prone to illness.

Let's also review the difference between humoral and cellular immunity. The humoral immune response uses B cells that recognize pathogens or antigens that are floating about in the blood. The B cells make plasma cells with antibodies designed to fight whatever is circulating that should not be there. Those antibodies destroy the invaders, and the B cells then make memory cells that create immunity.

The cellular immune response involves T cells, and these respond to pathogens that have invaded specific cells. Those cells show MHC markers, and the T cells create cytotoxic T cells that destroy the invaded cells. The T cells also make helper T cells that activate the B cells to make antibodies.

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