T lymphocytes
Antigen interaction with T lymphocytes precipitates the release of lymphokines, substances that modulate other aspects of the immune response. Examples of lymphokines include the interleukins, granulocyte-monocyte colony-stimulating factor (GM-CSF), and y-interferon. B lymphocytes secrete immunoglobulins, the humoral component of adaptive immunity. Eosinophils, neutrophils, and macrophages are phagocytic cells comprising the cellular component of the natural immune response.

Ans.B. Produce immunoglobulins
Immunoglobulins are secreted from the B lymphocytes.
However, T helper cells help the B cells, by activating it and produce into plasma cells.
These plasma cells secrete the immunoglobulins.
Thus, T helper cells indirectly help in the secretion of immunoglobulins.

Ans. is ‘b’ i.e., CD16, CD56
o NK cells have CD-56 and CD-16 on their surface.

Answer is D (There is antibody induced proliferation of NK cells)

Antibodies do not induce proliferation of NK cells. NK cells express CD3, CDI6 & CD56 and also but less commonly CD2 & CD8.

Null cells (NK cells + K cells): Null cells are a heterogeneous group of lymphocytes (non T and non B) called so because

• They do not form E rosettes (test to identify T lymphocytes is negative).
• They lack surface immunoglobulins (test to identify B lymphocytes is negative).
• They account for 5-10% of circulatory lymphocytes.

NK cells: are also called large granular lymphocytes as they are morphologically larger & contain azurophilic granules (which are absent in both T or B cells).

They are first line defence against:

1. Neoplastic cells
2. Virus laden cells

– NK cells are unique in the sense, that, they cause direct cell lysis which is:

– Not mediated by an immune response.

– Does not involve antigen antibody interaction.

• They are therefore capable of getting rid of non sensitized organisms i.e. those that have never been exposed before.
• NK cells are activated to ‘lymphocytes activated killer cells’ (LAK) in presence of IL 2 but not by antibody.
• NK cells are identified by detection of CD57.

Killer cells: participate in cell destruction with aid of antibody e.g. antibody dependent cell mediated cytotoxicity.

• Mechanism: antibody binds to antigenic determinant on the target cell.
• K cells then attach to target cells by means their Fc receptors and cell lysis results. Even here cell lysis proceeds without phagocytosis. 
• They are CD16 positive. 

Remember that many authors do not believe in the distinction of Null cells into Killer cells and NK cells and attribute both these functions to NK cells (Null cells).

Answer is C (Effective against virus infected cells) :

Common to both cytotoxic T cells and NK cells is affectivity against virus infected cells.

Natural Killer cells:

• Are first line of defense against virus laden cells and neoplastic cells.
• Cytotoxicity is not MHC restricted.

Cytotoxic T cells:

• They are antigen specific T cells whose function is to produce antigen specific lysis of target cells by direct cell-to­-cell contact.
• Are usually CD + class I MHC restricted T cells.
• Are important in defense against viral infections of tissues.

Thus the feature common to both cytotoxic T cells and NK cells is affectivity against virus infected cells.

Ans: C i.e. Thymus
Immunologically incompetent cells which go to the thymus are “preprocessed” there (thymus) and become T cells. The letter T comes from the word thymus.

Ans. B i.e. Produce immunoglobulins
B cells divide and transforms into plasma cells which synthesize immunoglobulin. T cells produce activation products (lymphokine) and induce cell mediated response

Ans. is ‘b’ NK cells

Null cells (Large granular lymphocytes)

• Null cells are called so because they lack features of surface markers of both B and T lymphocytes.
• The account for 5 to 10% of peripheral blood lymphocytes.
• They are also called “large granular lymphocytes (LGL)” as they contain large azurophilic cytoplasmic granules.
• Members of this group are:

a) Antibody dependent cytotoxic cells (ADCC)

b) Natural killer cells (NK Cells)