Adaptive immunity - the part of the immune system that acts in an antigen-specific manner. It is slower to respond as it takes time to develop following infection, but is much more effective than innate immunity. Speed for response increases with repeated infection ('memory'). Includes T cells, B cells and antibodies. The target of vaccinations.
Antibody - a protein produced by plasma cells (differentiated B cells) that can recognise and bind
Antigen - anything that can be recognised by an antibody. Each antibody will recognise a specific antigen.
Host - the organism to which the cells belong. For example, if we're talking about your white blood cells protecting you, you're the host. If we're talking about the immune cells of fish and how they function, the host we're referring to is fish in general.
Immunocompromised - a state in which the host's immune response is not fully functional or becomes suppressed. Reasons for this can include stress, fatigue, malnutrition, medically-induced suppression (e.g. following transplantation), or disease (e.g. HIV/AIDS).
Innate immunity - the part of the immune system that acts immediately upon recognising an infection. It includes physical barriers, like the skin or mucus, and cells, like the phagocytes. It is non-specific and will follow the same procedure no matter the type of pathogen.
Pathogen - an organism that can cause disease. This includes any form of life, such as bacteria, virus, fungi, protozoa, etc. An opportunistic pathogen is any organism that will no normally cause a disease, but that can cause a disease if the host becomes immunocompromised in some way.