1. Which of the following is an example of innate immunity?
A. Production of IgG in response to streptococcal infection
B. Destruction of auto-reactive T cells within the thymus
C. Lysis of tumor cells by cytotoxic T cells
D. Phagocytosis of bacteria by neutrophils and macrophages*
E. An allergic reaction to ragweed
Are allergies not innate immunity? They happen right away whereas adaptive immunity takes a few days.
A. Good question! Broadly, we define the innate system as that part of the immune system that is involved right away when a foreign antigen tries to enter the body. It remains the same from birth (it’s not modifiable), and it consists of the epithelium, neutrophils, dendritic cells, macrophages, natural killer cells, and complement.
The adaptive immune system kicks into action a bit later than the innate system. It consists primarily of lymphocytes and their products (including antibodies and cytokines). It changes and adapts as new substances are encountered; it has “memory” for antigens it has seen before.
Allergies are not normal immune responses to antigens. They are over-reactions – hypersensitivity reactions – that occur in some people for reasons that are poorly understood. If you had to put allergies into either the innate or adaptive category, though, you would consider them part of the adaptive immune system because they involve lymphocytes and antibodies (mast cells and eosinophils are involved too – but those cells are not considered part of routine innate or adaptive immunity).
In an allergy, the body encounters an antigen (like the pollen granules in the image above), mounts an immune response to it (makes IgE antibodies against that antigen), and for some reason sticks those antibodies onto mast cells. When the patient encounters that antigen again, the system responds in a new way: the antigen bridges IgE molecules on the mast cell and histamine and other bad stuff is released. The system has “learned” how to do this.
Your point is well taken that the symptoms of an allergy occur seemingly immediately after the allergen is encountered – but in reality, the immune system saw that antigen some time ago (maybe even years ago), learned what it looked like, and raised a little army of IgE molecules to sit and wait on mast cells for the next encounter. So what seems like an immediate response is really a learned, adaptive response that takes some time to develop.