Immunity

Immunity is defined as the capacity of the body to resist pathogenic agents. It is the ability of body to resist the entry of different types of foreign bodies like bacteria, virus, toxic substances, etc.

Immunity is of two types:

I. Innate immunity.

II. Acquired immunity.

INNATE IMMUNITY OR NON-SPECIFIC IMMUNITY

Innate immunity is the inborn capacity of the body to resist pathogens. By chance, if the organisms enter

the body, innate immunity eliminates them before the development of any disease. It is otherwise called the natural or non-specific immunity. This type of immunity represents the first line of defense against any type of pathogens. Therefore, it is also called non-specific immunity.

ACQUIRED IMMUNITY OR SPECIFIC IMMUNITY

Acquired immunity is the resistance developed in the body against any specific foreign body like bacteria,

viruses, toxins, vaccines or transplanted tissues. So, this type of immunity is also known as specific immunity. It is the most powerful immune mechanism that protects the body from the invading organisms or toxic substances. Lymphocytes are responsible for acquired immunity.

Types of Acquired Immunity

Two types of acquired immunity develop in the body:

1. Cellular immunity

2. Humoral immunity.

Lymphocytes are responsible for the development of these two types of immunity.

DEVELOPMENT AND PROCESSING OF LYMPHOCYTES

In fetus, lymphocytes develop from the bone marrow. All lymphocytes are released in the circulation and are differentiated into two categories. The two categories are:

1. T lymphocytes or T cells, which are responsible for the development of cellular immunity

2. B lymphocytes or B cells, which are responsible for humoral immunity.

T LYMPHOCYTES

T lymphocytes are processed in thymus. The processing occurs mostly during the period between just before birth and few months after birth. Thymus secretes a hormone called thymosin, which

plays an important role in immunity. It accelerates the proliferation and activation of lymphocytes in thymus. It also increases the activity of lymphocytes in lymphoid tissues.

Types of T Lymphocytes

During the processing, T lymphocytes are transformed into four types:

1. Helper T cells or inducer T cells. These cells are also called CD4 cells because of the presence of

molecules called CD4 on their surface.

2. Cytotoxic T cells or killer T cells. These cells are also called CD8 cells because of the presence of

molecules called CD8 on their surface.

3. Suppressor T cells.

4. Memory T cells.

Storage of T Lymphocytes

After the transformation, all the types of T lymphocytes leave the thymus and are stored in lymphoid tissues of lymph nodes, spleen, bone marrow and GI tract.

B LYMPHOCYTES

B lymphocytes were first discovered in the bursa of Fabricius in birds, hence the name B lymphocytes.

Bursa of Fabricius is a lymphoid organ situated near the cloaca of birds. Bursa is absent in mammals and the processing of B lymphocytes takes place in liver (during fetal life) and bone marrow (after birth).

Types of B Lymphocytes

After processing, the B lymphocytes are transformed into two types:

1. Plasma cells.

2. Memory cells.

Storage of B Lymphocytes

After transformation, the B lymphocytes are stored in the

lymphoid tissues of lymph nodes, spleen, bone marrow

and the GI tract.

ANTIGENS

DEFINITION AND TYPES

Antigens are the substances which induce specific immune reactions in the body.

Antigens are of two types:

1. Autoantigens or self antigens present on the body’s own cells such as ‘A’ antigen and ‘B’ antigen in

RBCs.

2. Foreign antigen s or non-self antigens that enter the body from outside.

NON-SELF ANTIGENS

Following are non-self antigens:

1. Receptors on the cell membrane of microbial organisms such as bacteria, viruses and fungi.

Types of Non-self Antigens

Non-self antigens are classified into two types, depending upon the response developed against them in the body:

1. Antigens, which induce the development of immunity or production of antibodies (immunogenicity).

2. Antigens, which react with specific antibodies and produce allergic reactions (allergic reactivity).

CHEMICAL NATURE OF THE ANTIGENS

Antigens are mostly the conjugated proteins like lipoproteins, glycoproteins and nucleoproteins.

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