Basic Components of the Immune System

INTRODUCTION

    It is generally believed that the immune system evolved as the host’s defense against infectious agents, and it is well known that patients with deficiencies in the immune system generally succumb to these infectious diseases. However, as we shall see, it may well play a larger role in the elimination of other foreign substances, including tumor antigens or cells and antibodies that attack self.

An immune response may be conveniently divided into two parts: 

1) a specific response to a given antigen and 

2) a more nonspecific augmentation to that response. 

    An important feature of the specific response is that there is a quicker response to the antigen during a second exposure to that antigen. It is the memory of the initial response that provides the booster effect.

For convenience, the specific immune response may be divided into two parts: 

1) the humoral response and 

2) the cellular response to a given antigen. As we shall see, however, both responses are mediated through the lymphocyte. 

    Humeral responses are antibodies produced in response to a given antigen, and these antibodies are proteins, have similar structures, and can be divided into various classes of immunoglobulins. Cellular responses are established by cells and can only be transferred by cells. The separation of human and cellular immunity was further advanced by the study of immunodeficient humans and animals. For example, thymectomized or congenitally athymic animals as well as humans cannot carry out graft rejection, yet they are capable of producing some antibody responses. The reverse is also true. Children (and animals) who have an immune deficit in the humoral response do not make antibodies but can reject grafts and appear to handle viral, fungal, and some bacterial infections quite well. An extraordinary finding by Good and colleagues in studying the cloacal lymphoid organ in chickens revealed that, with removal of the bursa Fabricius, these animals lost their ability to produce antibodies and yet retained the ability to reject grafts.

    Out of these and many other contributions, a clearer picture of the division of efforts by lymphocytes begins to emerge. Since cellular immune responses require an intact thymus, cellular immune responses are mediated through the T lymphocytes (thymus), while antibody-producing cells, which are dependent on the bone marrow (the bursa equivalent), are known as B (bursa) cells.

Several types of molecules play a vital role in the immune response, and we will deal with each in detail. Antigens, both foreign and self, are substances that may or may not provoke an immune response. Both T cells and B cells have receptors that recognize these antigens. In the case of B cells, antibodies on the surface are a major source (but not the only one) of antigen recognition, and once activated, they differentiate into plasma cells that produce large quantities of antibodies that are secreted into blood and body fluids to block the harmful effects of the antigen.

    T cells have similar receptors known as T-cell receptors (TCR), and in the context of the major histocompatibility complex (MHC) molecules provide a means of self-recognition and T-lymphocyte effector functions. Often these effector functions  are carried out by messages transmitted between these cells. These soluble messengers are called interleukins or cytokines.