This is how the HIV virus infects and impairs the immune system. Cytotoxic T cells which kill infected cells and cancer cells. Suppressor T cells activity not well understood, but seem to help suppress the rapid division of new lymphocytes when the infection is over. Typically proteins or polysaccharides. May be found on the surface of a microbe, or pollen grain, but may also materials or toxins produced by microbes, plants and animals, such as the protein in wheat which causes food allergies.
Antibodies are proteins found in blood and lymph.
They constitute the class of proteins called immunoglobulins. The antibodies are classified based on their constant region. Potentially millions of different specificities. So there is one type of antibody for current influenza strain, another for measles virus, etc.