B and T cells are:

A. Killer T cells (also known as cytotoxic T cells) recognize and bind to antigens presented by infected cells. They initiate a rapid immune response to destroy the infected or cancerous cells directly.
B. Regulatory T cells help modulate the immune response by preventing overreaction or autoimmunity, but they do not directly initiate a rapid response to antigens.
C. Memory T cells remember previously encountered antigens and respond quickly upon re-exposure to the same pathogen, but they do not initiate the initial response—they are involved in secondary immune responses.
D. Thrombocytes (also known as platelets) are involved in blood clotting, not in the immune response to antigens.

A. Autoimmunity is a malfunction of the immune system where the body attacks its own tissues — it is not a type of protective immunity acquired through infection or exposure.
B. Species immunity refers to natural resistance to certain diseases that affect other species (e.g., humans not being susceptible to some animal viruses), not immunity acquired through infection.
C. Passive immunity involves receiving antibodies from another source (like from mother to baby or via antibody-containing injections) and does not require the immune system to produce its own response.
D. Active immunity is the correct answer. It develops when the body is exposed to a pathogen or its toxins and responds by producing its own antibodies and memory cells, providing long-term protection.