Which immune response is triggered by the presence of an antigen?

The immune response that is triggered by the presence of an antigen is clonal selection. This process is central to the adaptive immune response and involves the activation of specific lymphocytes, such as B cells and T cells, that recognize a particular antigen. When an antigen is introduced into the body, it is processed and presented by antigen-presenting cells. This presentation leads to the selection and proliferation of specific immune cells that can effectively target the antigen.

In clonal selection, the B or T cells that possess receptors complementary to the antigen are selected to undergo clonal expansion. This means they rapidly proliferate to produce many identical cells that can respond to the same antigen. This specificity and amplification of the immune response help the body effectively combat infections.

The other options provided do not directly relate to the immediate response triggered by the presence of an antigen: immune tolerance refers to the ability of the immune system to ignore the body's own molecules, acquired immunity refers to the immunity gained through exposure to antigens, and passive immunity involves transferring antibodies from one organism to another, which does not involve the individual's own immune response activation.