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Principles of Immunohaematology - Introduction

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1. Introduction

The knowledge of basic concepts of immunology is essential in understanding the principles of immunohaematology which incorporate genetics, immunology, biochemistry and haematology.

An introduction to antigens, antibodies, different type of antigen-antibody reactions and complement system forms a part of the basic understanding for immunohaematological procedures. In addition, study of blood group system with their antigens and corresponding antibodies is also very essential to understand red cells serological techniques.

Antigen

An antigen is any substance either protein or non-protein, which if introduced into an individual causes the production of an antibody that will react specifically with that antigen in an observable way.

In the practice of blood transfusion, red cell antigens are of major clinical importance, besides these platelets, leucocytes and plasma proteins also exhibit strong antigenic systems.
Antibody

Antibodies are serum proteins, more specifically immunoglobulins in nature. Antibodies are recognized by their interaction with antigen. An antibody is usually a product of the immune response. Production of specific antibodies plays a significant role in red cell serology which deals with detection of antigen and antibody reactions.

In blood group serology, the interaction of antigen and antibody is mostly detected by agglutination of red cells.

On electrophoresis, plasma proteins separate into five main fractions, i.e albumins, alpha-globulin, beta-globulin, gamma-globulin and fibrinogen. The antibodies are mainly gamma-globulin in nature. In immunoelectrophoresis the gamma-globulin (immunoglobulin) can be separated into five groups. These 5 classes of immunoglobulins are

IgG, IgM, IgA, IgD, lgE.

Of these IgO, IgM and IgA are the main classes of antibodies involved in serological reactions in

relation to blood transfusion.

Antibody Immune Response

Primary response

On first exposure to an antigen the body initiates a primary response. The type of response is slow to begin with and it may take weeks to months for the antibody formation to occur.
Secondary response Once the individual has been sensitized to a particular antigen, a subsequent exposure to the same antigen causes much faster and a more pronounced immune response known as secondary response.
The primary response leads to production of 12M antibodies which is taken over by secondary response and causes production of IgG antibodies.

Naturally-occurring antibodies

The presence of antibody in serum is not always due to exposure to antigenic stimulus by red cells.
Some foreign substances such as bacteria and plants containing red cells-like substances are capable of
producing antibodies and these antibodies are called naturally-occurring antibodies. These are commonly
IgM in nature and occur in the serum of an individual without any specific antigenic stimulus e.g. anti-
A, anti-B and anti-P

Naturally-occurring antibodies are present in individuals lacking the corresponding antigens. The antibodies usually develop by the age of 6-8 months and are maintained throughout life. A state of immunodeficiency or old age may cause reduction in the levels of naturally-occurring antibodies.

Immune antibodies

These antibodies are produced in response to an immune reaction due to sensitization following pregnancy, previous transfusion, organ transplantation or deliberate injection of immunogenic material, are predominantly IgG in nature. Subclasses IgGi and IgG3 are clinically more significant as compared to IgG2 and IgG4.

Below Figure : Naturally-occurring & immune antibodies